Aspects of Reproduction and Development in the Prairie Vole Part 3

GROWTH IN SUBADULTS AND ADULTS

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 s.e.xes. With advanced age, growth in females lags behind that in males to an increasing extent.

Exceptionally large individuals, of either s.e.x, 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 ---------------+------------+------------+------------+------------

SUMMARY

The prairie vole is non-territorial and somewhat social. Several or many individuals of both s.e.xes 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 s.h.i.+fts from one home base to another. s.e.xual relations are probably more or less promiscuous, although a male and female may rest and travel together in a semi-permanent a.s.sociation. 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 v.a.g.i.n.al 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.

LITERATURE CITED

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Jour. Agric. Res., 27: 523-536.

BODENHEIMER, F. S., and F. SULMAN.

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

GREENWALD, G. S.

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.

HATFIELD, D. M.

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HOYTE, H. M. D.

1955. Observations on some small mammals of Arctic Norway. Jour.

Animal Ecology, 24: 412-425.

JAMESON, E. W.

1947. Natural history of the prairie vole. Univ. Kansas Mus.

Nat. Hist. Publ., 1: 125-151.

MARTIN, E. P.

1956. A population study of the prairie vole (_Microtus ochrogaster_) in northeastern Kansas. Univ. Kansas Mus.

Nat. Hist. Publ., 8: 361-416.

SCHMIDT, F. J. W.

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