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Title: Aspects of Reproduction and Development in the Prairie Vole (Microtus ochrogaster)
Author: Fitch, Henry S., 1909-2009
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

*** Start of this Doctrine Publishing Corporation Digital Book "Aspects of Reproduction and Development in the Prairie Vole (Microtus ochrogaster)" ***

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    Volume 10, No. 4, pp. 129-161, 8 figs. in text, 6 tables

    December 19, 1957

    Aspects of Reproduction and Development
    in the Prairie Vole (Microtus ochrogaster)




    Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
    Harrison B. Tordoff

    Volume 10, No. 4, pp. 129-161, 8 figs. in text, 6 tables
    Published December 19, 1957

    Lawrence, Kansas


    [Illustration: Look for the Union label!]


Aspects of Reproduction and Development
in the Prairie Vole (Microtus ochrogaster)




The prairie vole is by far the most abundant mammal on the
University of Kansas Natural History Reservation and on grassland
areas throughout northeastern Kansas. This vole therefore affects
the vegetation, perhaps more than any other native vertebrate, and
it is an important food source for most of the vertebrate predators.
Since the Reservation was established, in 1948, more data have been
accumulated concerning this vole than for any other species of
animal there. From February, 1950, to February, 1954, a grid of
live-traps at 50-foot intervals was set for several days each month
in a three-acre field inhabited by voles, and the population of
marked individuals was studied throughout the four-year period. From
November, 1953, to June, 1956, a half-acre trap grid with 20-foot
interval was used on an area adjoining the three-acre field. Other
trap lines in somewhat different habitats were maintained for
shorter periods as a basis for comparison. By June, 1956, a total of
some 3550 voles had been caught and recorded 14,750 times in all.
The present report is a preliminary attempt to analyze, in part,
these extensive data, and is concerned with certain phases of the
species' reproduction and growth that have bearing on the observed
population changes from month to month and from year to year on the

Through the studies of Jameson (1947) and Martin (1956), both made
in the same general area as my own, and several earlier studies, the
life history and ecology of the prairie vole are already well known.
The present report, with much larger amounts of data, further
clarifies certain phases of the ecology; and by using types of data
not available to Jameson and Martin I have dealt with some topics
not included in their reports.

Previous studies of growth in _Microtus_ have been based almost
entirely on weights. However, the weight of an individual vole may
fluctuate widely over a short period, depending on pregnancy and
parturition, length of time in a trap without food, availability of
moisture, and other factors. In the course of my study, in 1954
and 1955, and parts of 1953 and 1956, measurements of total length, in
addition to weights, were recorded for most of the voles

To test the accuracy of measurements, successive readings were
compared in individual voles that were already of large adult size
and that presumably either had stopped growing or were growing so
slowly that the gain was scarcely detectable in the relatively short
periods involved. For 200 such readings 33 per cent were just the
same as previous records for the same animals, 24 per cent deviated
by 1 mm., 22 per cent deviated by 2 mm., 15 per cent by 3 mm., 4.5
per cent by 4 mm., .5 per cent by 5 mm., 1 per cent by 6 mm., and .5
per cent by 7 mm. On the average, successive measurements varied by
1.43 mm., somewhat less than one per cent of the adult vole's total
length. Occasional errors of two to four per cent were easily
eliminated because for the voles used for growth records, series of
measurements were available, with clearly defined trends. The
occasional readings that deviated from the general trend for the
individual were discarded.

Measurements were recorded along with other data in the field at the
point of capture. Obtaining a reasonably accurate measurement on a
live and struggling vole required patience and practice. With the
thumb and forefinger of the left hand, I grasped the vole by loose
skin of the nape, and simultaneously grasped the tail at a point
approximately three-fourths of the distance to the tip. Then, with
gentle but steady pressure, I stretched the vole out to its full
length, meanwhile manipulating a millimeter ruler with the free
fingers, so that the vole was pressed against it, with the nose pad
at the end of the ruler.

The total length measurement is considered the best index to
over-all size. The relative tail-length varies slightly between
individuals, averaging approximately 22 per cent of the total
length. Individuals having broken tails, or having the distal parts
of their tails missing, were not included. The total length can be
measured with greater accuracy than can either the head-and-body
length or the tail-length separately.


As compared with other mammals, voles are tolerant and somewhat
social. That individuals are not mutually exclusive (territorially)
in areas occupied was demonstrated on many occasions when more than
one individual was caught simultaneously in the same live-trap.
Injury of a vole by a trap-mate was a rare occurrence.

Multiple captures often involved a female in oestrus and one or more males,
or a female and her young, but other instances involved various
combinations of sex and age groups. As many as five adults have been
caught in a trap simultaneously at times when the population density
was high. At such times, the meadow habitat is crossed by a maze of
interconnecting surface runways and one runway may be traced
continuously for 100 yards or more. Because each individual vole
normally confines its activity to a small area, only a fraction of
an acre, it is evident that individuals living at different places
overlap in their home ranges, and also in the trailways followed in
foraging. A high degree of tolerance is indicated. Where population
is so sparse that the systems of surface runways comprise separate
and isolated units, trapping experience has shown that one such
system may harbor several or many individuals.

As direct observations on voles under natural conditions are rarely
feasible, because of the animals' timidity, their utilization of
concealing cover, and tendency to crepuscular habits, best evidence
of social habits and underground life is based upon behavior of
captive individuals. Many voles were kept in confinement for varying
lengths of times, either singly or in association with others. Under
such conditions there was sometimes sporadic fighting, but it was
mainly defensive and serious injuries were rare. Two or more voles
caught at a given spot regardless of whether they were found in the
same trap simultaneously, or trapped separately within a short time,
usually were completely tolerant of each other. When at rest in
their container, such voles would huddle together in a corner or in
a nest, if materials were provided, so that collectively they
presented the minimum exposed surface. The intimacy and lack of
antagonism displayed on such occasions, suggested that the voles
were accustomed to living together amicably in the same nest
chamber. In live-trapping, "double" captures in a single trap often
involved the same two individuals. Such trap-mates were often male
and female, and in many instances the female was not in breeding
condition. That the voles are not monogamous in habits was
demonstrated when the same female was often trapped in association
with either of two males. Other trap associates taken together
repeatedly often were two males, or two females. Voles that are nest
mates or "neighbors" may tend to move about together in their
foraging, or one confined in a trap may attract the other
sufficiently to cause it to force an entrance by lifting the heavy
door of a trap.

When a new vole, caught at a different location, is added to a
container in which one or more are already confined, there is mutual
circumspection between the original occupants and the newcomer. At
first, each vole is intimidated by movements of the other, and as a
result, the original occupants huddle in their established corner
while the newcomer cowers in the most remote part of the container.
Gradually the voles become less timid and one may approach another
slowly and cautiously, to sniff at it. The vole approached may react
with a show of hostility which is largely defensive. In the
characteristic posture of threat for defense, the vole crouches, or
rears back on its haunches, with snout elevated and incisors
prominently displayed. If the warning posture is unheeded, or if the
vole is made unusually aggressive by having young to defend, or for
some other reason, it attacks with a sudden forward lunge, striking
the adversary simultaneously with both forefeet and with the
incisors. The lunge is so rapid that when I have observed it, I have
been unable to discern whether the attacker bit its opponent. The
attack serves to force back the other animal, throwing it off
balance and intimidating it. The attacked animal may dodge nimbly to
avoid the lunge, but whether or not it is actually struck, it
usually retreats, avoiding or postponing further hostilities. Voles
that have been kept in containers for periods of hours or days tend
to be more hostile and aggressive toward a newcomer than are those
newly introduced. After series of meetings resulting from the
exploratory behavior of the newcomer and the curiosity or normal
activity of those longer confined, hostility gradually subsides.
Within a few hours a newcomer is usually accepted, and thenceforth
he huddles with other members of the group when at rest, and
hostility is rarely evident.

This ready acceptance on short acquaintance of strange voles into
the family or social group suggests that lack of territoriality
extends even to the use of the nest burrows, and that groups of
voles may share the same nest, huddling together and deriving mutual
benefit from the association, such as warmth in cold weather.
Schmidt (1931: 113), studying this vole in Clark County, Wisconsin,
noted its colonial habits. He found isolated small mounds that were
riddled with burrows, and little sign in intervening areas. At one
mound he trapped two adult males, one adult female, and two young;
at another mound, two adult males, two adult females, and four young
were trapped. My individuals that were released from live-traps were
on many occasions trailed by means of a stiff wire collar with spool
of thread attached, to holes that presumably were their home
burrows. Data obtained in this manner indicated that ordinarily
several or many individuals use the same burrow system. The
histories of individual voles on the study area at the Reservation
indicate shift of home base from time to time, usually for short
distances within the area already included in the home range, but
occasionally to new areas relatively remote from the original home

Severe fighting between adult prairie voles occurs at times.
Occasionally, sharp squeaks accompanied by brisk rustling in the
grass suggesting pursuit or conflict, are heard in their habitat. An
unusually large adult male, long resident on a study area, suddenly
lost weight and deteriorated in condition over a period of several
days, then was found dead in a nest-box attached to a trap.
Dissection revealed numerous punctures in the skin and flesh of the
neck and back, probably made by the incisors of another vole.
Extensive hemorrhage and swelling had occurred, and obviously these
injuries were the cause of death.

Although it was not feasible to study the home life of the voles
underground, clues were gained from those uncovered in runways and
nests beneath large boards and strips of tarpaper, previously
distributed for this purpose. Nests were constructed by the voles
beneath several such pieces of tarpaper and runways appeared beneath
all the pieces that were placed in habitat favorable to the voles.
In summer, however, the high daytime temperatures beneath these
shelters made them uninhabitable to the voles, and they were used
mainly in spring. From February 15 to May 1, 1953, 14 voles were
caught 19 times beneath five of the tarpaper strips, and many other
voles that were seen beneath them escaped. Upon turning one of the
strips I often discovered voles in close proximity. Sometimes two or
more darted from the same nest. The disturbance of repeatedly
raising the strips and exposing the voles' shelters soon caused them
to desert the sites; consequently the information obtained by this
means was limited.


There is sexual activity in every month of the year, but its
incidence varies greatly from one season to another. As has been
indicated by various authors, male voles reach sexual maturity later
than females. It seems that ordinarily the availability of sexually
active males is not a limiting factor, however. While males that are
still well below average adult size produce mature spermatozoa, and
are probably capable of breeding (Jameson, 1947: 145), certain
large old males may sire a disproportionately large percentage of the
litters produced. Observations on males in confinement indicated
that sexual activity tended to be directly proportional to the size
of the testes. Occasional individuals, having much enlarged scrotal
testes were more readily stimulated to sexual activity and more
aggressive toward females than were those in which the testes were
of more nearly typical size or abdominal or were smaller than
normal. The combination of factors controlling size of testes is not
well understood, but males having unusually large testes were caught
most often when food supply was optimum, for instance after a period
of heavy precipitation when an abundant supply of new grass provided
succulent and nutritious food.

In confinement sexual activity was largely inhibited and attempts to
establish a laboratory colony met with failure. Sexual activity was
observed mainly in recently captured males, and their interest was
aroused chiefly by females that had given birth to litters within a
few hours previously. Oestrus is known to follow closely after
parturition. Females found in live-traps with newborn young often
were brought to the laboratory for observation. An apparent instance
of hostility between rival males competing for an oestrus female was
observed on September 2, 1950. The female was found in a trap with
four newborn young, and since the young had not yet attached to her
teats, she was temporarily returned to the trap after recording, to
prevent desertion of the litter. Returning twenty minutes later I
found another adult vole at this trap. It would suddenly emerge from
dense grass nearby, and would move over the trap or around it, with
jerky, halting movements, then would dart back under cover. The
female emerged from the nest box into the trap runway, and sniffed
at the other, and both pressed against the intervening wire barrier.
There was gnawing on the wire by one or both. A third adult vole
appeared. As it moved toward the trap, all three suddenly took alarm
and darted back under cover, the female hiding in the trap nest box.
In a few seconds they again appeared. The two outsiders, presumably
both males, were not individually recognizable, but several times
one was seen to dart at the other, chasing it away momentarily. They
were seldom both in sight at once.

Males confined with post-partum females usually evinced sexual
interest, following them about persistently and nuzzling their
genitalia. The females, however, were often unreceptive perhaps
because they were disturbed by strange surroundings and by the
presence of their litters, so that they usually attempted to escape,
or to rebuff the male's attention. At first the female might flee,
squeaking in protest at the male's pursuit. If he still continued to
follow, she would turn on him, rearing back in the characteristic
threatening pose, and would lunge at him, striking him sharply or
driving him back. After such rebuff, males were usually intimidated
or discouraged so that they temporarily or permanently abandoned
their advances, and small males were more easily rebuffed than were
larger individuals. On several occasions large males having enlarged
testes were not readily rebuffed by females but continued to follow
them. When the female turned upon him, such a male might lunge
against her, throwing her off balance, and causing her to attempt to
escape, and then continuing the pursuit until it ended in copulation
or in more severe fighting. Although not accepted sexually, a
rebuffed male might be readily accepted as a nest-mate, huddling
along with the female and perhaps other individuals of both sexes.
In huddling voles, the most frequently observed type of social
behavior was grooming; one individual would slide its chin or muzzle
through the other's fur with a stroking movement consisting of a
series of rapid forward jerks and the stroking movements might
continue for periods of minutes. The recipient of the grooming
usually made no evident response indicative of either pleasure or
displeasure. Often it seemed to be sleeping while the grooming was
performed. Individuals of both sexes performed this grooming and the
recipient might be of either the same sex or the opposite sex. This
grooming may have some significance as a search for ectoparasites
such as fleas, or mites that often infest the voles. However, after
prolonged grooming by a companion, a vole's fur was of mussed and
disarranged appearance. Although the grooming that occurs between
voles that are resting in nests seems to have no direct significance
as sexual behavior, somewhat similar actions constitute part of the
mating pattern. A sexually aroused male overtaking a receptive
female, slides his chin forward along her back with jerky, stroking
movements. In some observed instances this behavior continued
intermittently for several minutes before actual copulation. In some
other instances it was almost lacking.


In female voles that are sexually quiescent, both those that have
not yet attained breeding maturity, and those that have undergone
regression after attainment of sexual maturity, the vaginal orifice
is not evident. The canal is sealed externally by a membranous
layer of epithelium. Presence of a vaginal orifice indicates that
the individual is in some active stage of the breeding cycle. The
appearance of the orifice varies between different females, and it
changes in the same female from day to day or even from hour to
hour. Presumably these changes in the vaginal orifice are cyclical
and are closely correlated with oestrus, but attempts to trace them
were unsuccessful largely because the normal cycle was rapidly
suppressed in captive voles, which soon became sexually quiescent.
Individual voles living under natural conditions were not trapped
with sufficient regularity to permit tracing the details of changes
in their genitalia.

In those females having the vaginal orifice most developed, the
margins are turgid and slightly inflamed. The circular opening gapes
1.0 to 1.5 mm. in diameter when the tail is raised. A female may
remain in this condition for two days or more. Vaginal smears at
this stage often showed nucleated cells characteristic of oestrus.
Subsequently the margins of the orifice become less prominent and
the opening becomes smaller. The dorsal and ventral walls adhere
until an opening is no longer evident unless the adjacent skin is

In pregnancy the orifice is occasionally sealed, but usually is
evident. It is, however, less prominent than in oestrus, and does
not gape. The margins are less turgid than in oestrus, and the
opening is in the form of a transverse slit through which the
purplish epithelial lining of the dorsal wall of the vagina can be
seen. After parturition, placentae and bloody discharge often are in
evidence in the vaginal canal. Females that have not given birth to
young recently may also have bloody mucous discharge. Its
significance has not been determined. In females that are undergoing
sexual regression, the margins of the vaginal orifice become
shrunken and pale, and the orifice becomes partly or wholly sealed.

Bodenheimer and Sulman (1946:255) concluded from their study of
_Microtus guentheri_ that in this species, as in "the cat," "the
rabbit," "the ferret," and a few other mammals, ovulation is induced
by copulation, and that there is no regular vaginal cycle. Hoyte
(1955:412) disagreed with these conclusions for other species of
_Microtus_, as he trapped individuals of _M. oeconomus_ that had
recently ovulated without copulation (at least no sperm were found
in the genital tracts). In _M. ochrogaster_ oestrus seems to be
controlled largely by the food supply, at least the incidence of
perforate females was found to fluctuate irregularly tending to
follow the trend of rainfall, and, probably in more direct
correlation, the amount of new grass present (see Table 1, and
Martin, 1956:383-384). It therefore seems unlikely that in this
species ovulation is dependent on copulation.

In females that have not yet produced young the teats are minute and
well concealed in the fur, so that they are difficult to find, but
in lactation they become conspicuous. In early lactation the teats
are typically about 1 mm. in diameter and 2.5 mm. in length. As
lactation progresses, they become thickened to nearly twice the
original diameter. After lactation, as inversion occurs, they shrink
to scabrous low prominences, 2 mm. to 3 mm. in diameter, surrounded
by bare skin. There are three pairs of mammae, one pair pectoral and
the other two abdominal. As mentioned by Jameson (1947:146), the
pectoral mammae show little evidence of use in lactating prairie
voles. Probably they are not used at all except in females with more
than the four young in a litter accommodated by the abdominal
mammae. As in various other rodents, the suckling young may cling to
the female's teats and may be dragged over the ground as she moves
about. When the female forages near the nest, she may drag the young
with her instead of leaving them, but she can detach them instantly
if she so desires. On many occasions females found in live-traps had
young that were several days old clinging to their teats. In some
instances young that had their eyes open may have followed the
female into the trap and attached afterward.


In the region of my study the prairie vole breeds the year round, but
the rate of breeding changes continually. There is no regularity in
the trend of the breeding season from year to year. It is obvious that
the species is responsive to environmental changes and is so well
attuned that its breeding is speedily initiated or inhibited by
changes to favorable or unfavorable weather. The incidence of breeding
is highest when temperature is moderate and both water and foods of
preferred sorts are plentiful.

Tables 1 and 2 and Fig. 1, based on 11,109 records representing each
month over a four-year period, show the changing trends from month to
month. The perforate condition recorded in Table 1 may represent any
of several stages in oestrus or pregnancy, but is regarded as a crude
index of rate of breeding, since voles in the anoestrus stage lack the
vaginal orifice. Highest percentages of perforate females occurred in
the months of February, March, April, May, and June, while by far the
lowest percentages were recorded in the drought summers of 1952 and
1953. Even in mid-winter a substantial proportion of the females
trapped were perforate.

  [Illustration: FIG. 1. Average catch per day in a
     three-acre field, in a grid of 100 live-traps, over a four-year
     period. For each year, solid line represents total and dashed
     line represents number of young up to 30 grams in weight. Numbers
     caught are roughly indicative of population density, but many
     variables distort this relationship. Young are never represented
     in the catch in their true ratio to adults, since on the average
     they are less vagile and less attracted to traps.]

 Table 1. Percentages of Adult Females Recorded as Perforate in the
    Monthly Samples From 1950 Through 1953.

        | 1950 | 1951 | 1952 | 1953
  Jan.  | .... | 27.3 | 41.7 | 33.3
  Feb.  | .... | 47.7 | 53.1 | 72.9
  Mar.  | 40.6 | 38.5 | 77   | 50
  Apr.  | 76   | 41.9 | 51.9 | 73
  May   | 84   | 40   | 52   | 58.2
  June  | 67.7 | 41.5 | 19.3 | 16.6
  July  | 57.3 | 45.5 | 12.7 | 15.4
  Aug.  | 43.1 | 52.2 |  5.4 | 31.3
  Sept. | 47   | 56.5 | 51.6 | 56.2
  Oct.  | 44.8 | 48.9 | 43.4 | 60
  Nov.  | 24.4 | 45   | 24.1 | 61.5
  Dec.  | 31.1 | 45   | 37.5 | 41.6

 Table 2. Percentages of Adult Females Recorded to Be in Late
    Pregnancy in the Monthly Samples From 1950 Through 1953.

        | 1950 | 1951 | 1952 | 1953
  Jan.  | .... |  2.3 |  0   |  0
  Feb.  | .... |  0   | 10.4 |  9.1
  Mar.  |  5.8 |  0   | 22.6 | 13.3
  Apr.  |  8   | 19.4 | 22.6 | 27.5
  May   | 21   | 37.1 | 29.5 | 39.4
  June  | 13.3 | 14.9 | 16.5 |  5.5
  July  | 57.3 |  6.7 |  7.9 |  3.8
  Aug.  | 43.8 | 15.2 | 10.8 | 12.5
  Sept. | 40.4 | 15   | 20.3 |  6.2
  Oct.  | 45.2 | 21.9 | 18.9 | 10
  Nov.  |  7   |  8.9 |  3.3 | 23
  Dec.  |  0   |  0   |  0   |  8.3

Usually pregnancy can be recognized only in the last week before birth
of the litter, when the female's abdomen is noticeably distended by
the enlarged fetuses. Palpating to detect embryos was not attempted
because of the danger of injuring them or the female. Because
gestation is of approximately three weeks duration, the figures in
Table 2 represent roughly perhaps one-third, or a little less, of the
adult females actually pregnant. At most times of year a substantial
proportion of adult females (sometimes nearly all) are pregnant. Only
in the winter (including March in 1951) were samples taken in which no
recognizably pregnant females were found. Incidence of pregnancy was
notably high in July, August, September, and October of 1950, May,
1951, May, 1952, and April and May, 1953. A high rate of breeding was
not necessarily followed by an increase in the population. A
relatively low rate of breeding was adequate to maintain the
population level, provided that environmental factors remained
favorable. Fig. 1 shows the average catch per day (with approximately
100 live-traps) over the four-year period, 1950 through 1953. The
young (including all those weighing 30 grams or less, and
corresponding roughly with the part of the population less than two
months old) are shown separately. It is noteworthy that throughout
the entire period the ratio of young to adults tended to be fairly
stable--usually fluctuating between ten and thirty per cent of the
total catch. Ratios of young to adults were notably high in March and
May, 1950; April, June and July, 1952; and April, May and June, 1953.
Ratios of young were notably low in June and December, 1950; January,
February, March, and June through October, 1951; January, February,
and March, 1952; and November, 1953.

In Fig. 1 the catch per day of voles, varying from month to month,
reflects chiefly the changing population density. However, other
factors also have important effects on the catch. For example, bait
acceptance is better in the winter when natural foods, especially
greens, are scarce, with the result that a higher catch can be made
with the same population density. Interference with the trap line by
other animals also affected the catch of voles. In warm weather the
traps were checked in both morning and evening, and the catch was
correspondingly greater than it was in cool weather when the traps
were checked only once daily. The ratios obtained of young to adult
voles cannot be accepted at face value as the true ratios in the
population, either. For the first several days of each trapping
period, the voles caught were mostly adults previously marked and,
presumably, conditioned to the grain bait. Later, young voles not
previously recorded, came to the traps in increasing numbers. The
young, being at first not conditioned to the bait, and also having
relatively small home ranges, would generally be less well represented
in the catch than would the adults.


In other species of _Microtus_, so far as known, a 21-day gestation
period seems to be the rule (Bailey, 1924:528; Hamilton, 1941:13;
Hatfield, 1935:264). _M. ochrogaster_ seems to conform to this
pattern, but the data obtained were meager, because breeding activity
was usually inhibited in voles kept in confinement.

A female live-trapped on July 23, 1951, appeared to be in breeding
condition. When trapped two days later, she had a copulatory plug, and
21 days after this she was found with a newborn litter in a trap. A
female thought to have given birth to a litter between successive
captures on July 20, and July 21, 1951 (on the basis of appearance of
genitalia, and reduction in weight from 53 to 46 grams), appeared to
have just completed parturition when she was examined on August 10. A
female that gave birth to a litter in confinement on May 18, 1954,
bred and was released the same day. She was recorded as pregnant in
the first week of June, but on June 7 was no longer pregnant. If this
pregnancy terminated normally, a gestation of 20 days or less is

Greenwald (1956:221) suggested that in _M. californicus_, oestrus
might occur in the period of lactation, because he found recently
formed corpora lutea in lactating females. In the course of my field
work on _M. ochrogaster_, I obtained precise or approximate dates of
successive litters born at intervals of somewhat more than 21 days
apart. In different females, intervals of 23, 23, 24, 26, and
approximately 27 (between 26 and 28) days were recorded between
successive litters. In four other females intervals between litters
were known only approximately because one of two records was based on
a capture in late pregnancy judged to be within two or three days of
parturition. For these females, intervals of 23, 24, 24, and 26 days
were recorded. From the trend of these records, it seems that females
often became pregnant within a few days after birth of a litter.
Pregnancy from post-partum oestrus would seem to be less frequent than
pregnancies beginning a few days after birth of the previous litter,
and within the period of lactation.


Jameson (1947:146) found an average of 3.4 young per litter in 58
litters of _M. ochrogaster_ from northeastern Kansas, mostly from
Douglas County. Martin (1956:386) recorded a somewhat lower mean of
3.18 ± 0.24 in 65 litters on the Reservation in 1950, 1951, and 1952.
For a total of 82 litters recorded from 1950 through 1956, inclusive,
I obtained an average of 3.37 ± .075 young per litter. Several litters
that were recorded were excluded from this computation as in each
instance there was reason to suspect that they were incomplete. These
included instances of females found in traps with young several days
old, females that may not have completed parturition when they were
released with newborn young, and those litters that might have
sustained losses through cannibalism by the mother or her trap-mates.

Mean numbers of young per litter were found to vary from year to year
and from month to month, as shown by the following lists: 1950, 3.0
(13 litters); 1951, 3.5 (23 litters); 1952, 3.5 (11 litters); 1953,
3.4 (5 litters); 1954, 3.4 (15 litters); 1955, 4.1 (7 litters); 1956,
3.8 (5 litters); January 2.0 (1 litter); February 3.5 (4 litters);
March 4.5 (4 litters); April 3.9 (12 litters); May 3.3 (25 litters);
June 3.0 (9 litters); July 2.7 (4 litters); August 2.9 (7 litters);
September 2.8 (6 litters); October 3.4 (7 litters); November 5.0 (2
litters); December 4.0 (1 litter).

These differences can be logically explained on the basis of changes
in the average age of the breeding females in the population. On the
average, with greater length, weight and age, females produced
progressively larger litters, although individuals did not necessarily
conform to this general trend. For 24 females recorded in 1954-1956
and measured within a few days of birth of their litters, average
length was correlated with number of young as follows: 6 young, 163.5
mm.; 5 young, 158.0 mm.; 4 young, 157.7 mm.; 3 young, 154.6 mm.; 2
young, 160.5 mm.

For 48 other females, recorded in 1950-1953, that were not measured,
but that were mostly assignable to broad age groups on the basis of
their individual histories in the trapping records, the following well
defined trend was demonstrated.

 Table 3. Number of Young per Litter Correlated with Age or Size
    of Female.

                                      |   Number   |     Average
      Age or Size Group of Female     | of females |    number of
                                      |  in sample | young per litter
 More than one year old               |      4     |       4.25
 6 to 12 months old                   |     16     |       3.50
 Large (age indeterminate)            |      9     |       3.44
 2 to 5 months old                    |      9     |       2.90
 Small and medium (age indeterminate) |     10     |       2.80

It seems that the exceptionally high average numbers of young per
litter in March and April result from the breeding females in those
months being nearly all fully mature survivors of the previous year.
In summer, when many females that are only a few weeks old become
pregnant, the average litter declines to less than three young. The
small average litter of 3.0 young for 1950 probably resulted from the
fact that the population on the Reservation was then expanding rapidly
in the newly favorable habitat created by one year's crop of
vegetation after discontinuance of grazing, and had an unusually high
percentage of breeding females that were not fully adult.


In four newborn young, total lengths, in mm., were 47, 45, 45, and 42.
From the length-weight relationships shown in Fig. 2, it seems that a
length of approximately 47 mm. is typical of newborn young of average
weight. Martin (1956:388) found a mean weight of 2.8 ± 0.36 grams in
sixteen newborn prairie voles from the Reservation. For a series of 67
other newborn voles representing 27 different litters in seven
different years, I found an average of 2.9 ± .05 grams. Young ranged
in weight from 3.8 to 2.0 grams. Weights of the newborn voles could
not be correlated with season, size, age of females, or food
conditions. However, a distinct trend toward larger size in those
litters that contained fewer young was evident, as shown in Table 4.

 Table 4. Weight of Newborn Young, Correlated with Number of Young
    per Litter.

               |    Mean    |   Number   |  Number
  Known Young  |   weight   | of litters | of young
  Per Litter   |  in grams  | in sample  | in sample
      2        | 3.1 ± .09 |      7     |     13
      3        | 3.0 ± .17 |     11     |     28
      4        | 2.7 ± .22 |      6     |     17
      5        | 2.6 ± .42 |      3     |      9


Voles less than 100 mm. in total length were seldom captured,
because those less than this size are dependent on the female, and
rarely venture far enough from the nest to be caught in a trap. A
further difficulty in obtaining growth records on the smallest young
is that of making accurate measurements. During their first few days
they partially retain the fetal posture, usually lying on one side,
with the head, body and tail flexed in an arc almost completed by
the tail approximating the muzzle. Straightening the animal by
stretching it and holding it with sufficient firmness to obtain a
measurement might have involved injury to it. Therefore, in most
instances the newborn voles examined were merely weighed or an
approximate measurement was estimated without stretching the young
to its full length.

Newborn voles were obtained when females that were caught in
live-traps produced their litters before they were found and
released. In some instances, females caught while in late pregnancy
were retained in the laboratory for a day or more until parturition
occurred. Many of the newborn voles were marked by toe-clipping,
according to the same system used for adults. Early growth was
measured in some instances by keeping the female with her litter in
confinement, measuring and weighing the young at intervals. In most
instances, the female was released at the point of capture
(presumably near her nest burrow) with the young clinging to her
teats. For the young so released, the incidence of recovery was
remarkably low, seeming to indicate that they were subject to
decimating losses. Perhaps such losses are normal, at least on the
study area where voles are live-trapped regularly. Holding of adults
and partly grown young in live-traps ordinarily has no harmful
effects on them, but the resultant separation of females from newly
born litters may often result in death of the young either from
hunger and exposure, or from attack by other voles and natural

During the first ten days the increase in length from an original 47
mm. is from three to four mm. per day. Figs. 2, 5, and 8 show length
and weights of voles whose ages in days were definitely known
because they were born in the laboratory, or in a live-trap after
the female was caught there. Young voles marked at birth and
released with the female were rarely recovered in the period of
suckling, as they ordinarily remain in the nest burrow when the
female ventures out to forage. Litters retained in the laboratory
therefore have provided most of the records of growth in suckling
young. Growth varied greatly between litters. It was not clearly
correlated with size of female, size of young at birth, or number of
young in litter, but probably was influenced by attentiveness of the
female, her adjustment to captivity, and her productivity of milk.
Within each litter there were usually persistent differences in
development, but these were minor (except for those of occasional
runts) compared with the differences between litters. In several
litters of five young, one was usually smaller than the others at
birth and therefore could not compete successfully with its litter
mates, so that it never gained possession of a teat other than one
of the pectoral pair, and always succumbed within a few days, after
failing to gain weight as its litter mates did. The relatively few
voles marked at birth and recovered after developing under natural
conditions, did not deviate from the trend of those in confinement.


Females in confinement were attentive to young, and, soon after
parturition, licked them clean and huddled over them protectively.
Ordinarily, the newborn young soon attached to a teat, and spent a
large part of its time attached during its early development.
Females found in live-traps with their litters of young less than a
day old, often had some or all of the young clinging to their teats.
Females with newborn litters, when released from live-traps, always
left without attempting to retrieve any young that were unattached.
Such young usually were permanently deserted, but in some instances
disappeared within an hour or less, perhaps rescued by the female
returning for them.

Females with newborn young were made far more aggressive than most
other voles by their tendency to protect their young from possible
danger. In captivity such females usually took the offensive in
attacking or rebuffing any other voles confined with them.
Post-partum females obviously in oestrus were prevented from being
fully receptive by their hostility toward males whose presence might
endanger the young. Such a female has been seen to turn on a
pursuing male and attack him viciously, several times within a few
minutes, before copulation occurred. In captivity, at least, such
attacks would soon discourage a male so that unless he was
exceptionally active sexually, mating was prevented.

Cannibalism, involving destruction of the newborn, is probably an
important factor in the population dynamics of the prairie vole.
Only a small percentage of the young known to have been born on an
area ever survived to be live-trapped; this small percentage was
indirect evidence of decimating losses in the young. Under
unfavorable conditions each of several females killed and ate her
own litter, but the degree of provocation varied greatly among
individuals. Females that gave birth to young in live-traps
occasionally ate one or more of their newborn young, as evidenced by
discarded remnants. Perhaps other instances passed unnoticed because
no remnants were found. That need for food or moisture as well as
psychological stress often motivated such cannibalism was suggested
by the fact that surviving litter mates might be accepted and cared
for by a female that had already eaten one or more of her young.
Although cannibalism is most likely to occur within a few hours
after birth of the young, they may be killed and eaten at any stage
of development. One female that had probably eaten one or more of
her litter, soon after parturition, nursed the two survivors. When
these were two weeks old, all were "pastured out" in a wire mesh
cage in tall brome grass. When the supply of grass had become scarce
(though some was still available), the female killed and partly ate
both her remaining young.

One female was captured with three young attached that were several
days old. The young were detached from the female's teats with great
difficulty. When these young were returned to the female a few
minutes later, after they had been measured, weighed and marked, she
attacked them viciously, and within a few seconds had killed all of
them by biting their heads. In this instance the dead young were not
eaten, although they were temporarily left with the female.

Females with young have ample cause for their circumspective
demeanor toward adult males, which are especially inclined to eat
the newborn. A male engaged in sexual pursuit has been observed to
grasp a young dangling behind the female, pull it from her teat, and
pausing momentarily, nibble its head off, before continuing to
follow the female. Like the genitalia of the post-partum female, the
newborn young seem to have an odor that attracts and excites the

To a lesser degree, adult females also display marked interest in
the newborn young of other individuals, which is liable to result in
cannibalism. The incidence of cannibalism is affected by the
condition, collectively, of the population of voles, and the
availability of nutritious food and moisture. In periods of summer
drought the grass becomes coarse and fibrous, and its protein
content declines. Under such conditions many voles appear to be
undernourished, and some are actually emaciated. Dehydration may be
an important factor at times when dew is unavailable for drinking
and the green vegetation remaining is exceptionally low in moisture
content. Voles caught at such times and brought to the laboratory,
drank avidly, and gained several grams soon after being offered
water or succulence. Cannibalism by adults on newborn young in times
of drought may be motivated by the acute need for moisture and
nutritious food. In times of drought the birth rate is at low ebb.

Adult males have never been observed to display paternal solicitude
toward young, but some individuals, kept with females and their
litters, did not molest the young and were accepted by the females
as members of the family group.

Other things being equal, cannibalism involving the young might be
expected to be greater at times of high population density. Then,
young left in the nest by a female in the course of her foraging
would more often encounter adults and partly grown young, both those
that lived in the same burrow system and exploring intruders from
other areas.

The eyes open at an age of nine or ten days. Then the young enter
upon an exploratory period, when each wanders out of the nest,
emerges from the burrow, and wanders through the adjacent surface
runways in frequent short forays, sometimes following the female and
sometimes alone. Such forays usually cover only a few inches at
first, but as the young vole grows, becomes familiar with its
surroundings, and takes more plant food, its sphere of activity
gradually widens, and family ties are dissolved. Voles reared to an
age of three weeks in the laboratory and then released, survived
just as well if the female was not released with them demonstrating
that they were fully capable of shifting for themselves at this age.
In confinement, however, young voles of greater age continued to
suckle and remained closely associated with the female. Females in
confinement evinced much uneasiness because of their inability to
evade the young when the latter were old enough to walk. The young
then followed the female continually and suckled whenever she
stopped or even while she moved about, unless she paused to remove
them from her teats, but they would not remain detached for more
than a few seconds. When a young followed the female away from the
nest and then attached to a teat, the female after pulling the young
from her teat, would usually carry it, grasped between her incisors,
back to the nest and deposit it there. On one occasion a young vole
caught in a live-trap was partly plucked and eventually killed by
the female on the outside trying to pull it through the wire mesh.

On several occasions, young were successfully transferred from the
mother to another lactating female in confinement, which accepted
them as part of her own litter. Young, up to the time of weaning,
appeared not to differentiate between the mother and other adult
voles. They would follow any larger individual indiscriminately, and
would huddle against it or nuzzle its undersurface searching for a


The following notes are based upon many different litters, and give
some idea of the sequence of events in their early development.

Newborn: The skin is pinkish gray dorsally and pink ventrally. In
profile, sparse and exceedingly fine hairs less than 1 mm. in length
are discernible. The vibrissae are approximately 2 mm. long. The
skin is thin and partly transparent, much wrinkled, with some deeper
folds, notably one between the knee and the heel. The young lie on
their sides making violent convulsive respiratory movements. When
not attached to the female's teats, they may make faint squeaking

One day old: Little changed in appearance or behavior except that
the dorsal surface has become darker because of growth of hair.

Two days old: Covering of fine brown hair readily discernible on
dorsal surface; lower incisors protruding about .5 mm. from the gum;
upper incisors have barely pierced the gum.

Four days old: Pale brown hair averaging about 1 mm. in length over
the dorsal surface gives the young a sleek, seallike appearance. The
young have gained greatly in muscular co-ordination. Part of the
time they may still lie on their sides, but they are able also to
gain an upright sprawling posture. In crawling, they are unsteady
and often topple over on their sides after taking a few halting
steps. They make frequent jerky lateral flexions of the body,
probably to search for a teat. Their eyes and ears still are sealed

Five days old: Young have changed but little in appearance since the
preceding day, but they have become notably more active, with
movements better co-ordinated. When placed on a level surface they
can crawl briskly.

Eight days old: Young are able to stand erect, with bodies held
clear of the ground, and they can even run, but the gait is slow and
clumsy, and the forequarters and hind quarters are poorly
co-ordinated, so that the voles tend to fall on their sides. The fur
averages approximately 3 mm. in length.

Nine days old: At this stage all young have their eyes open or
beginning to open.

Ten days old: All young of this age have their eyes open, but not to
their fullest extent, and the eyes are still slitlike in appearance.
The young have become rather gopherlike in appearance and gait. They
walk briskly but unsteadily, with bodies held high off the ground.
When handled, they struggle vigorously, and try to bite. These young
are similar in size and appearance to the smallest voles caught in
live-traps apart from their mothers.

Thirteen days old: Hair on back has grown to an average length of 8
mm. (shorter on ventral surface, head, and limbs).

Seventeen days old: The young have become alert, and almost as quick
in their movements as adults. They have molariform teeth, and are
taking plant food. When a family group was examined, the young
instantly detached from the female's teats and scattered. The hair
on the back averages 10 mm. long and the vibrissae average 20 mm.

There is intense competition among the young of a litter, especially
if the litter has more than the average number of young. In litters
with more than four young, there is competition for the inguinal
teats, since, in most females at least, the pectoral teats seem to
have an inadequate milk supply. As a result, it is doubtful whether
more than four young to a litter are ever able to survive. From the
time their eyes open, the young compete actively. When litters in
confinement were fed with fresh greens, there was nearly always
quarrelsome squeaking and scuffling, as the young competed for food.
At such times, they have been seen to chase and attack each other.


No individual vole was recaptured with sufficient regularity, from
birth to maturity, to provide a complete growth curve. The curve in
Fig. 7 is a composite based on all available records of voles that
were recorded as making growth in length and were recaptured before
they were fully grown, so that growth rates could be computed. The
figure shows that growth is extremely rapid for the first three
weeks, and thereafter slows gradually but steadily, until in
individuals of adult size, the increment per day is much less than
that in the small young.

Since rate of growth changes rapidly, with a slowing trend, only
those young voles that were recaptured within a few weeks showed the
approximate growth rate for any specific portion of the ontogenetic
curve. Table 5 summarizes the records of 98 such young sorted into
size groups representative of several stages in development. The
slowing trend of growth in voles that are nearing subadult size is
well shown by these records. Throughout the greater part of the
growth curve no difference could be found in rate between the sexes.
It is only after sexual maturity has been attained and growth has
become relatively slow that males become noticeably larger than
females. This tendency for continued growth in the adult males
results in a much more marked disparity in size between the sexes in
the oldest voles, as evident in Fig. 2.

  [Illustration: FIG. 2. Size distribution of prairie voles
     in a year-around sample, including all the measurements of voles
     taken over a three-year period. Young are not represented in
     their actual ratio to the total population in this sample,
     because they are less attracted to the bait, and range less
     widely than adults. The higher ratios of males than of females
     in the three largest size groups is well shown, as is the higher
     ratio of females among those voles of small adult size.]

 Table 5. Average Growth (in Over-all Length) in Young Voles of
   Several Sizes. ([M] = Male; [F] = Female)

  Average lengths  |   Average    |  Average  |
     in mm. at     |   length,    | increment |    Total, and
   beginning and   |   in days,   |  per day  |  number of each
   end of growth   |  of growth   |   in mm.  |   sex in sample
      period       |   periods    |           |
    97.0 to 126.6  |   in 16.8    |   1.76    |  5 (1 [M], 4 [F] [F])
   103.3 to 127.3  |   in 14.9    |   1.61    |  9 (3 [M] [M], 6 [F] [F])
   107.5 to 123.4  |   in 11.0    |   1.44    |  8 (5 [M] [M], 3 [F] [F])
   114.0 to 132.3  |   in 17.5    |   1.05    |  6 (5 [M] [M], 1 [F])
   118.5 to 136.0  |   in 19.7    |    .88    |  6 (3 [M] [M], 3 [F] [F])
   122.1 to 135.8  |   in 16.2    |    .85    | 15 (5 [M] [M], 10 [F] [F])
   129.3 to 145.5  |   in 22.8    |    .71    |  4 (all [M] [M])
   130.6 to 146.1  |   in 19.8    |    .78    | 12 (all [F] [F])
   139.8 to 147.5  |   in 29.5    |    .26    | 10 (all [M] [M])
   141.2 to 148.8  |   in 26.2    |    .29    | 23 (all [F] [F])

  [Illustration: FIG. 3. Changing numbers and composition
     (according to size of individual) in a population of voles on
     an area of approximately one half an acre that was intensively
     sampled with live-traps over periods of months. The population
     as a whole and the ratio of young to adults tended to be higher
     in spring and summer, but with little regularity from one year
     to the next. Weather was far more important than season in
     determining the population trend. Many of the voles recorded
     on the half-acre area ranged more or less beyond its boundaries.]

  [Illustration: FIG. 4. Weight in free-living prairie voles
     in a year-around sample from juveniles to large adults (grouped
     in length-classes of 6 mm. range, separate for each sex). In each
     sample mean, standard error, standard deviation, and extremes are
     shown. Note that mean weight is proportional to length, that in
     each size class females average heavier (because of pregnancy
     in some) and have a much wider range of variation in weight.]

Martin (1956:389) stated that growth in young prairie voles was, in
general, most rapid in the period April-May-June and least rapid in
mid-winter. However, his data were based entirely on weights. The
high incidence of pregnancy in the larger young females in spring
and early summer may have caused the trend. Measurements taken by me
of lengths do not bear out the idea of more rapid growth in the
spring and summer, but, indeed, show the opposite. In most
instances, voles of comparable sizes made significantly more rapid
growth in the colder half of the year (mid-October to mid-March)
than in the warmer half. Dividing the young voles in eight size
groups and separating each group into comparable summer and winter
samples, I found more rapid average growth in the summer sample in
only two instances. These deviations from the general trend
probably resulted from inadequately small sizes of some samples. On
the average, the growth rate in summer was 92 per cent of that in

  [Illustration: FIG. 5. Over-all length plotted against
     weight in young prairie voles, from newborn to the minimum
     size at breeding maturity. The range of variation increases
     as development proceeds, especially after the age of weaning
     is attained.]


Greenwald (1956: 220) found that in females of _Microtus
californicus_ some individuals are extremely precocious sexually,
and might, at an age of as little as two weeks, produce corpora
lutea and have sperm in the uterus. Greenwald mentioned one
perforate female which weighed only 10 grams, but most reached a
weight of at least 30 grams before their first pregnancies. The
sterile cycles passed through earlier seemed to represent a
"tuning-up" stage before establishment of the pituitary-gonad

  [Illustration: FIG. 6. Weight plotted against age in young
     voles, from birth up to 25 days. The range is wide at the start
     and increases as development proceeds.]

Although females of _M. ochrogaster_ are much less precocious in
their manifestations of puberty, they may become perforate well
before impregnation can occur, and seem to pass through sterile
cycles before becoming pregnant. The 18 smallest females recognized
as being pregnant were of the following over-all lengths, in mm.:
149, 149, 149, 148, 148, 148, 147, 146, 145, 145, 144, 144, 143,
143, 143, 142, 135, and 134. As pregnancy is ordinarily recognized
only in the last four days the females must have been impregnated
from 20 to 17 days earlier--when they were in most instances 7 to 11
weeks old and 135 to 145 mm. in length. The two smallest
individuals, recorded as pregnant at 135 and 134 mm., must, if they
were of typical size for their age, have become pregnant at an age
of approximately one month, when they were only 119 and 122 mm. in
length. The smallest lactating females (some of them pregnant also)
were recorded at lengths of 149, 148, 148, 147, 147, 146, 144, 144,
143, 143, and 142 mm. Occasionally females of less than 120 mm. were
found to be perforate, and seemingly had begun oestral cycles.
Records of a female of definitely known age, typical of many of the
same size in her development, are cited below:

March 19, 1956 Born in captivity.

April 7, 1956 (19 days old) Released on study area at site of mother's
capture; length 102 mm., weight 11.1 gms.

April 15, 1956 (27 days old) Recaptured; perforate with a copulatory
plug; length 113 mm., weight 13.4 gms.

April 27, 1956 (39 days old) Recaptured; imperforate; length 131 mm.,
weight 24.3 gms.

May 12, 1956 (54 days old) Recaptured; perforate and in late pregnancy;
length 146 mm.

May 25, 1956 (67 days old) Recaptured; imperforate, in an advanced
state of lactation; length 150 mm., weight 33 gms.

  [Illustration: FIG. 7. Growth curve in the prairie vole;
     dots are based on means of series of definitely known age (born
     in captivity); circles are based on mean lengths of recaptured
     marked young whose ages were not precisely known.]

  [Illustration: FIG. 8. Over-all length in young prairie
     voles of definitely known ages, up to 40 days. All were born
     in captivity. Some were released with the female and developed
     under natural conditions, but their growth rate did not differ
     discernibly from that of those kept in the laboratory. Dots
     indicate individual records; circles are means for ages at
     which four or more records were obtained.]

When captured on May 12, at an age of 54 days, this female appeared
to be within two or three days of parturition, and hence must have
become pregnant at an age of approximately 35 or 36 days. Pregnancy
in the more precocious females probably occurs at a length of
approximately 130 mm. and an age of a little less than 40 days. Such
females are still growing so rapidly that by the time their litters
are born, they have grown to more than 140 mm.


Table 6 is a summarization of 73 records of individuals that made
substantial growth as adults, after they were marked and measured.
These records show the slowing trend of growth with advanced age.
Also, they show the wide range of individual variation in growth
rate, and difference between the sexes. With advanced age, growth in
females lags behind that in males to an increasing extent.
Exceptionally large individuals, of either sex, are many months old,
but some individuals live to be a year old or more without growing
much beyond average adult size. The average growth rate of more than
1 mm. per day in young has slowed to less than .1 mm. per day, on
the average, in adults exceeding 160 mm., and has slowed to less
than .05 mm. per day, on the average, in those exceeding 165 mm.

 Table 6. Size Groups (Over-all Length) in Recaptured Voles That
    Were Marked Before Maturity and Therefore Were of Approximately
    Known Ages. ([M] = Male; [F] = Female)

                |        Estimated age, in days        |
   Size Group   +------------+------------+------------+   Number
  Length in mm. |   Average  |   Maximum  |   Minimum  |  in sample
  171 to 175    |  [M]  435  |    .....   |    .....   |     1
                |  [F]  324  |     338    |     310    |     2
                |  All  361  |     435    |     310    |     3
                |            |            |            |
  166 to 170    |  [M]  304  |     523    |     179    |     9
                |  [F]  398  |     597    |     158    |     6
                |  All  346  |     597    |     158    |    15
                |            |            |            |
  161 to 165    |  [M]  227  |     465    |     104    |    15
                |  [F]  257  |     394    |     134    |    18
                |  All  243  |     465    |     104    |    33
                |            |            |            |
  156 to 160    |  [M]  188  |     349    |     107    |    12
                |  [F]  187  |     284    |      93    |    11
                |  All  188  |     349    |      93    |    23


The prairie vole is non-territorial and somewhat social. Several or
many individuals of both sexes and various sizes may use the same
system of surface runways and burrows and even the same nest. In
general, members of such a group are mutually tolerant. A strange
vole may provoke some hostility at first, but may soon be accepted
as a member of a new group. Consequently, there are frequent shifts
from one home base to another. Sexual relations are probably more or
less promiscuous, although a male and female may rest and travel
together in a semi-permanent association. In confinement only those
males having markedly enlarged scrotal testes showed interest in
females that were in oestrus. Post-partum females especially were
eagerly pursued by such males. Anoestrus females are imperforate,
and a vaginal orifice is present only during an active oestral cycle
or in pregnancy. The perforate condition therefore, is a crude index
of breeding activity in the population. In adult females the ratio
of those that were perforate usually fluctuated between one-fourth
and three-fourths of the total. Only in severe summer drought did
the numbers decline below 24 per cent. Normally, breeding continues
the year around, but it is temporarily inhibited in unusually cold
weather or drought. The highest incidence of pregnancy normally is
in late spring and early summer. The ratio of juveniles in the
population from month to month and year to year is far more stable
than the actual population density.

Gestation is 21 days or a little less. The mean litter is 3.37
± .075 young. Three is the most frequent number per litter, with
four, two, and five in that order of frequency. Larger and older
females have more young per litter, on the average. Average size
is greater in those litters having fewer young. At birth, young
are between 40 and 50 mm. in length (typically, 47 mm.), and weigh
2.9 ± .05 grams.

At an age of nine days the young have their eyes open, and they may
be weaned at an age of approximately three weeks. Young suckle
chiefly from the four abdominal teats. The pectoral mammae seem to
be inadequately developed, with the result that in exceptionally
large litters of five, six or seven young, usually no more than four
survive. Until weaning the young spend much of their time attached
to the female's teats. She may even drag them behind as she forages.
Females that have suckling young become much less tolerant of other
voles. Attacks on young, and cannibalism, are common. Adult males,
especially, are liable to eat the newborn young. The acquisition of
cannibalistic habits by individuals, and seasonal lack of adequately
nutritious plant foods may result in the killing off of young in
such numbers that the population level is held down.

In young females sterile oestral cycles often begin at about the
time of weaning. Earliest pregnancies occur when females are
approximately one month old, but most are several weeks older before
they become pregnant. Rate of growth declines steadily from a length
increment of approximately 2 mm. per day in voles less than two
weeks old to an increment of approximately one-fourth mm. per day in
subadults. Growth rate is highly variable among individuals at all
stages, and especially in those that have attained adult size. Even
adults tend to gain in length, slowly, as well as in weight, and the
largest individuals are all many months old.



  1924. Breeding, feeding and other life habits of meadow mice.
        Jour. Agric. Res., 27: 523-536.


  1946. The estrous cycle of _Microtus guentheri_ D. and A. and
        its ecological implications. Ecol., 27: 255-256.


  1956. The reproductive cycle of the field mouse, _Microtus
        californicus_. Jour. Mamm., 37: 213-222, 2 figs., 1 pl.

_Hamilton, W. J., Jr._

  1941. The reproduction of the field mouse, _Microtus
        pennsylvanicus_. Cornell Univ. Agric. Exp. Sta. Mem.,
        237: 1-23.


  1935. A natural history of _Microtus californicus_. Jour. Mamm.,
        16: 261-271.


  1955. Observations on some small mammals of Arctic Norway. Jour.
        Animal Ecology, 24: 412-425.


  1947. Natural history of the prairie vole. Univ. Kansas Mus.
        Nat. Hist. Publ., 1: 125-151.


  1956. A population study of the prairie vole (_Microtus
        ochrogaster_) in northeastern Kansas. Univ. Kansas Mus.
        Nat. Hist. Publ., 8: 361-416.


  1931. Mammals of western Clark County, Wisconsin. Jour. Mamm.,
        12: 99-117.



Institutional libraries interested in publications exchange may
obtain this series by addressing the Exchange Librarian, University
of Kansas Library, Lawrence, Kansas. Copies for individuals, persons
working in a particular field of study, may be obtained by
addressing instead the Museum of Natural History, University of
Kansas, Lawrence, Kansas. There is no provision for sale of this
series by the University Library which meets institutional requests,
or by the Museum of Natural History which meets the requests of
individuals. However, when individuals request copies from the
Museum, 25 cents should be included, for each separate number that
is 100 pages or more in length, for the purpose of defraying the
costs of wrapping and mailing.

    * An asterisk designates those numbers of which the Museum's
    supply (not the Library's supply) is exhausted. Numbers
    published to date, in this series, are as follows:

  Vol.  1. Nos. 1-26 and index. Pp. 1-638, 1946-1950.

 *Vol.  2. (Complete) Mammals of Washington. By Walter W. Dalquest.
           Pp. 1-444, 140 figures in text. April 9, 1948.

  Vol.  3.  *1. The avifauna of Micronesia, its origin, evolution,
                and distribution. By Rollin H. Baker. Pp. 1-359,
                16 figures in text. June 12, 1951.

            *2. A quantitative study of the nocturnal migration of
                birds. By George H. Lowery, Jr. Pp. 361-472, 47
                figures in text. June 29, 1951.

             3. Phylogeny of the waxwings and allied birds. By M. Dale
                Arvey. Pp. 473-530, 49 figures in text, 13 tables.
                October 10, 1951.

             4. Birds from the state of Veracruz, Mexico. By George H.
                Lowery, Jr. and Walter W. Dalquest. Pp. 531-649,
                7 figures in text, 2 tables. October 10, 1951.

            Index. Pp. 651-681.

 *Vol.  4.  (Complete) American weasels. By E. Raymond Hall. Pp. 1-466,
             41 plates, 31 figures in text. December 27, 1951.

  Vol.  5.   1. Preliminary survey of a Paleocene faunule from the
                Angels Peak area, New Mexico. By Robert W. Wilson.
                Pp. 1-11, 1 figure in text. February 24, 1951.

             2. Two new moles (Genus Scalopus) from Mexico and Texas.
                By Rollin H. Baker. Pp. 17-24. February 28, 1951.

             3. Two new pocket gophers from Wyoming and Colorado. By
                E. Raymond Hall and H. Gordon Montague. Pp. 25-32.
                February 28, 1951.

             4. Mammals obtained by Dr. Curt von Wedel from the barrier
                beach of Tamaulipas, Mexico. By E. Raymond Hall. Pp.
                33-47, 1 figure in text. October 1, 1951.

             5. Comments on the taxonomy and geographic distribution of
                some North American rabbits. By E. Raymond Hall and Keith
                R. Kelson. Pp. 49-58, October 1, 1951.

             6. Two new subspecies of Thomomys bottae from New Mexico
                and Colorado. By Keith R. Kelson. Pp. 59-71, 1 figure in
                text. October 1, 1951.

             7. A new subspecies of Microtus montanus from Montana and
                comments on Microtus canicaudus Miller. By E. Raymond
                Hall and Keith R. Kelson. Pp. 73-79. October 1, 1951.

             8. A new pocket gopher (Genus Thomomys) from eastern
                Colorado. By E. Raymond Hall. Pp. 81-85. October 1, 1951.

             9. Mammals taken along the Alaskan Highway. By Rollin H.
                Baker. Pp. 87-117, 1 figure in text. November 28, 1951.

           *10. A synopsis of the North American Lagomorpha. By E.
                Raymond Hall. Pp. 119-202, 68 figures in text. December
                15, 1951.

            11. A new pocket mouse (Genus Perognathus) from Kansas. By E.
                Lendell Cockrum. Pp. 203-206. December 15, 1951.

            12. Mammals from Tamaulipas, Mexico. By Rollin H. Baker. Pp.
                207-218. December 15, 1951.

            13. A new pocket gopher (Genus Thomomys) from Wyoming and
                Colorado. By E. Raymond Hall. Pp. 219-222. December 15,

            14. A new name for the Mexican red bat. By E. Raymond Hall.
                Pp. 223-226. December 15, 1951.

            15. Taxonomic notes on Mexican bats of the Genus Rhogeëssa.
                By E. Raymond Hall. Pp. 227-232. April 10, 1952.

            16. Comments on the taxonomy and geographic distribution of
                some North American woodrats (Genus Neotoma). By Keith R.
                Kelson. Pp. 233-242. April 10, 1952.

            17. The subspecies of the Mexican red-bellied squirrel,
                Sciurus aureogaster. By Keith R. Kelson. Pp. 243-250,
                1 figure in text. April 10, 1952.

            18. Geographic range of Peromyscus melanophrys, with
                description of new subspecies. By Rollin H. Baker.
                Pp. 251-258, 1 figure in text. May 10, 1952.

            19. A new chipmunk (Genus Eutamias) from the Black Hills.
                By John A. White. Pp. 259-262. April 10, 1952.

            20. A new piñon mouse (Peromyscus truei) from Durango, Mexico.
                By Robert B. Finley, Jr. Pp. 263-267. May 23, 1952.

            21. An annotated checklist of Nebraskan bats. By Olin L. Webb
                and J. Knox Jones, Jr. Pp. 269-279. May 31, 1952.

            22. Geographic variation in red-backed mice (Genus
                Clethrionomys) of the southern Rocky Mountain region. By
                E. Lendell Cockrum and Kenneth L. Fitch. Pp. 281-292,
                1 figure in text. November 15, 1952.

            23. Comments on the taxonomy and geographic distribution of
                North American microtines. By E. Raymond Hall and E.
                Lendell Cockrum. Pp. 293-312. November 17, 1952.

            24. The subspecific status of two Central American sloths. By
                E. Raymond Hall and Keith R. Kelson. Pp. 313-337. November
                21, 1952.

            25. Comments on the taxonomy and geographic distribution of
                some North American marsupials, insectivores, and
                carnivores. By E. Raymond Hall and Keith R. Kelson. Pp.
                319-341. December 5, 1952.

            26. Comments on the taxonomy and geographic distribution of
                some North American rodents. By E. Raymond Hall and Keith
                R. Kelson. Pp. 343-371. December 15, 1952.

            27. A synopsis of the North American microtine rodents. By E.
                Raymond Hall and E. Lendell Cockrum. Pp. 373-498, 149
                figures in text. January 15, 1953.

            28. The pocket gophers (Genus Thomomys) of Coahuila, Mexico.
                By Rollin H. Baker. Pp. 499-514, 1 figure in text.
                June 1, 1953.

            29. Geographic distribution of the pocket mouse, Perognathus
                fasciatus. By J. Knox Jones, Jr. Pp. 515-526, 7 figures
                in text. August 1, 1953.

            30. A new subspecies of wood rat (Neotoma mexicana) from
                Colorado. By Robert B. Finley, Jr. Pp. 527-534, 2 figures
                in text. August 15, 1953.

            31. Four new pocket gophers of the genus Cratogeomys from
                Jalisco, Mexico. By Robert J. Russell. Pp. 535-542.
                October 15, 1953.

            32. Genera and subgenera of chipmunks. By John A. White.
                Pp. 543-561, 12 figures in text. December 1, 1953.

            33. Taxonomy of the chipmunks, Eutamias quadrivittatus and
                Eutamias umbrinus. By John A. White. Pp. 563-582,
                6 figures in text. December 1, 1953.

            34. Geographic distribution and taxonomy of the chipmunks of
                Wyoming. By John A. White. Pp. 584-610, 3 figures in
                text. December 1, 1953.

            35. The baculum of the chipmunks of western North America.
                By John A. White. Pp. 611-631, 19 figures in text.
                December 1, 1953.

            36. Pleistocene Soricidae from San Josecito Cave, Nuevo Leon,
                Mexico. By James S. Findley. Pp. 633-639. December 1, 1953.

            37. Seventeen species of bats recorded from Barro Colorado
                Island, Panama Canal Zone. By E. Raymond Hall and William
                B. Jackson. Pp. 641-646. December 1, 1953.

            Index. Pp. 647-676.

 *Vol.  6.  (Complete) Mammals of Utah, _taxonomy and distribution_. By
            Stephen D. Durrant. Pp. 1-549, 91 figures in text, 30 tables.
            August 10, 1952.

  Vol.  7.  *1. Mammals of Kansas. By E. Lendell Cockrum. Pp. 1-303,
                73 figures in text, 37 tables. August 25, 1952.

             2. Ecology of the opossum on a natural area in northeastern
                Kansas. By Henry S. Fitch and Lewis L. Sandidge. Pp.
                305-338, 5 figures in text. August 24, 1953.

             3. The silky pocket mice (Perognathus flavus) of Mexico. By
                Rollin H. Baker. Pp. 339-347, 1 figure in text. February
                15, 1954.

             4. North American jumping mice (Genus Zapus). By Philip H.
                Krutzsch. Pp. 349-472, 47 figures in text, 4 tables.
                April 21, 1954.

             5. Mammals from Southeastern Alaska. By Rollin H. Baker and
                James S. Findley. Pp. 473-477. April 21, 1954.

             6. Distribution of Some Nebraskan Mammals. By J. Knox Jones,
                Jr. Pp. 479-487. April 21, 1954.

             7. Subspeciation in the montane meadow mouse, Microtus
                montanus, in Wyoming and Colorado. By Sydney Anderson.
                Pp. 489-506, 2 figures in text. July 23, 1954.

             8. A new subspecies of bat (Myotis velifer) from southeastern
                California and Arizona. By Terry A. Vaughn. Pp. 507-512.
                July 23, 1954.

             9. Mammals of the San Gabriel mountains of California. By
                Terry A. Vaughn. Pp. 513-582, 1 figure in text, 12 tables.
                November 15, 1954.

            10. A new bat (Genus Pipistrellus) from northeastern Mexico.
                By Rollin H. Baker. Pp. 583-586. November 15, 1954.

            11. A new subspecies of pocket mouse from Kansas. By E.
                Raymond Hall. Pp. 587-590. November 15, 1954.

            12. Geographic variation in the pocket gopher, Cratogeomys
                castanops, in Coahuila, Mexico. By Robert J. Russell and
                Rollin H. Baker. Pp. 591-608. March 15, 1955.

            13. A new cottontail (Sylvilagus floridanus) from northeastern
                Mexico. By Rollin H. Baker. Pp. 609-612. April 8, 1955.

            14. Taxonomy and distribution of some American shrews.
                By James S. Findley. Pp. 613-618. June 10, 1955.

            15. The pigmy woodrat, Neotoma goldmani, its distribution and
                systematic position. By Dennis G. Rainey and Rollin H.
                Baker. Pp. 619-624, 2 figs. in text. June 10, 1955.

            Index. Pp. 625-651.

  Vol.  8.   1. Life history and ecology of the five-lined skink, Eumeces
                fasciatus. By Henry S. Fitch. Pp. 1-156, 26 figs. in text.
                September 1, 1954.

             2. Myology and serology of the Avian Family Fringillidae, a
                taxonomic study. By William B. Stallcup. Pp. 157-211, 23
                figures in text, 4 tables. November 15, 1954.

             3. An ecological study of the collared lizard (Crotaphytus
                collaris). By Henry S. Fitch. Pp. 213-274, 10 figures in
                text. February 10, 1956.

             4. A field study of the Kansas ant-eating frog, Gastrophryne
                olivacea. By Henry S. Fitch. Pp. 275-306, 9 figures in
                text. February 10, 1956.

            5. Check-list of the birds of Kansas. By Harrison B. Tordoff.
               Pp. 307-359, 1 figure in text. March 10, 1956.

            6. A population study of the prairie vole (Microtus
               ochrogaster) in northeastern Kansas. By Edwin P. Martin.
               Pp. 361-416, 19 figures in text. April 2, 1956.

            7. Temperature responses in free-living amphibians and reptiles
               of northeastern Kansas. By Henry S. Fitch. Pp. 417-476, 10
               figures in text, 6 tables. June 1, 1956.

            8. Food of the crow, Corvus brachyrhynchos Brehm, in
               south-central Kansas. By Dwight Platt. Pp. 477-498,
               4 tables. June 8, 1956.

            9. Ecological observations on the woodrat, Neotoma floridana.
               By Henry S. Fitch and Dennis G. Rainey. Pp. 499-533,
               3 figures in text. June 12, 1956.

           10. Eastern woodrat, Neotoma floridana: Life history and
               ecology. By Dennis G. Rainey. Pp. 535-646, 12 plates,
               13 figures in text. August 15, 1956.

           Index. Pp. 647-675.

  Vol.  9.   1. Speciation of the wandering shrew. By James S. Findley.
                Pp. 1-68, 18 figures in text. December 10, 1955.

             2. Additional records and extensions of ranges of mammals from
                Utah. By Stephen D. Durrant, M. Raymond Lee, and Richard M.
                Hansen. Pp. 69-80. December 10, 1955.

             3. A new long-eared myotis (Myotis evotis) from northeastern
                Mexico. By Rollin H. Baker and Howard J. Stains. Pp. 81-84.
                December 10, 1955.

             4. Subspeciation in the meadow mouse, Microtus pennsylvanicus,
                in Wyoming. By Sydney Anderson. Pp. 85-104, 2 figures in
                text. May 10, 1956.

             5. The condylarth genus Ellipsodon. By Robert W. Wilson.
                Pp. 105-116, 6 figures in text. May 19, 1956.

             6. Additional remains of the multituberculate genus
                Eucosmodon. By Robert W. Wilson. Pp. 117-123, 10 figures
                in text. May 19, 1956.

             7. Mammals of Coahuila, Mexico. By Rollin H. Baker. Pp.
                125-335, 75 figures in text. June 15, 1956.

             8. Comments on the taxonomic status of Apodemus peninsulae,
                with description of a new subspecies from North China.
                By J. Knox Jones, Jr. Pp. 337-346, 1 figure in text,
                1 table. August 15, 1956.

             9. Extensions of known ranges of Mexican bats. By Sydney
                Anderson, Pp. 347-351. August 15, 1956.

            10. A new bat (Genus Leptonycteris) from Coahuila. By Howard
                J. Stains. Pp. 353-356. January 21, 1957.

            11. A new species of pocket gopher (Genus Pappogeomys) from
                Jalisco, Mexico. By Robert J. Russell. Pp. 357-361.
                January 21, 1957.

            More numbers will appear in volume 9.

  Vol. 10.   1. Studies of birds killed in nocturnal migration. By
                Harrison B. Tordoff and Robert M. Mengel. Pp. 1-44, 6
                figures in text, 2 tables. September 12, 1956.

             2. Comparative breeding behavior of Ammospiza caudacuta and
                A. maritima. By Glen E. Woolfenden. Pp. 45-75, 6 plates,
                1 figure. December 20, 1956.

             3. The forest habitat of the University of Kansas Natural
                History Reservation. By Henry S. Fitch and Ronald R.
                McGregor. Pp. 77-127, 2 plates, 7 figures in text,
                4 tables. December 31, 1956.

             4. Aspects of reproduction and development in the prairie vole
                (Microtus ochrogaster). By Henry S. Fitch. Pp. 129-161,
                8 figures in text, 6 tables. December 19, 1957.

            More numbers will appear in volume 10.

Transcriber's Notes

With the exception of the list of typographical corrections below and
minor corrections not noted here, the text presented here is the same
as in the original printed version. Tables 1 and 2 were rotated so that
the width would be less than the limit of 75 characters wide. In the
tables, the title and some column headers were printed in small caps.
Those are usually displayed as ALL CAPS; but for ease of reading, they
were left as mixed case. The list of UKMNH publications was compiled
at the end of the article's text.

Typographical Corrections

 Page(s)       Correction
 ============  =======================================
 129, 130, ii  This publication: 4 tables => 6 tables
 138           cyle => cycle

Text Emphasis

 _Text_ - Italic

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