Part 4

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 shut.

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. long.

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.

GROWTH FROM WEANING TO MATURITY

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.

Table 5. Average Growth (in Over-all Length) in Young Voles of Several Sizes. ( = Male; = 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 , 4 ) 103.3 to 127.3 | in 14.9 | 1.61 | 9 (3 , 6 ) 107.5 to 123.4 | in 11.0 | 1.44 | 8 (5 , 3 ) 114.0 to 132.3 | in 17.5 | 1.05 | 6 (5 , 1 ) 118.5 to 136.0 | in 19.7 | .88 | 6 (3 , 3 ) 122.1 to 135.8 | in 16.2 | .85 | 15 (5 , 10 ) 129.3 to 145.5 | in 22.8 | .71 | 4 (all ) 130.6 to 146.1 | in 19.8 | .78 | 12 (all ) 139.8 to 147.5 | in 29.5 | .26 | 10 (all ) 141.2 to 148.8 | in 26.2 | .29 | 23 (all ) ------------------+--------------+-----------+---------------------------

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 winter.

SIZE AND AGE AT SEXUAL MATURITY

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 relationship.

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.

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.

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 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. ( = Male; = Female)