Part 3

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.

SIZE AT BIRTH

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 --------------+------------+------------+-----------

EARLY GROWTH

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 enemies.

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.

CARE OF YOUNG

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 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.

EARLY DEVELOPMENT OF YOUNG

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 sounds.

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