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 male.

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 teat.

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 sounds.

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 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.

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.

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.

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:

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.

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.

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