Part 13

Figures 18 and 19 show that some litters grow appreciably faster than others, but the end results are about the same. Since the young were measured at irregular intervals, statistical procedures for calculating confidence limits of the curves were not applicable.

Solitary young reared by one female of each species, attained maximum size more rapidly than animals having litter mates (Fig. 19). Nevertheless, solitary individuals and individuals from litters all reach essentially the same size 50 days after birth.

The gestation time of P. truei is several days longer than that of P. maniculatus, and the young of truei are fewer and heavier than those of maniculatus. As would be expected, truei remains in the nest longer and nurses longer than maniculatus.

Young of each species grow rapidly for the first month, and attain, in that time, the largest percentage of their adult size; they grow rapidly up to sometime between the thirtieth and fiftieth days. Thereafter the rate of growth diminishes and the animals begin to gain weight rather than continuing to extend the lengths of the body and appendages.

Figure 19 reveals that the appendages of young maniculatus attain most of their length about a week earlier than those of truei. Young truei acquire mobility and coordination somewhat later than young maniculatus, but both species are seemingly equal in these respects by about the end of the second week.

Length of gestation period, number and size of embryos, amount of time spent in the nest, and time required for bodily growth are all of major importance in determining the relative success of truei and maniculatus. These parameters will be considered further in the discussion.

PARENTAL BEHAVIOR

In the laboratory, pregnant females were supplied with either kapok, cotton, or a piece of burlap with which to make a nest. The kapok or cotton was used directly by the mice in constructing a hollow, compact, moundlike nest. When burlap was used for nest building, the female first completely frayed the cloth by chewing it into a fluffy mass of fibers.

When the top of a nest was opened to inspect young, the female would attempt to pull the nesting material back into shape by means of forefeet and teeth. The mother's defensive posture was to cover the young with her body, often lying over them and facing upward, toward the investigator. In this semi-recumbent position, the female would attack the investigator's fingers with her forefeet and teeth. Often the female would stand bipedally and use the forefeet and teeth to mount the attack. If at this time a young chanced to wander away from the mother, she would quickly pick it up and place it in the nest at her feet.

When disturbed, females of both species, but especially P. maniculatus, often dove headlong under their nest or into the wood shavings on the floor of the cage. This type of retreat was most often used when young were nursing. Time is required even by the mother to disengage nursing young, and this mode of escape is the most expedient. The mother disengaged nursing young by licking around their faces and pushing with her paws.

Nursing females of both species tolerated the male parent in the nest. A male and female often sat side by side in the nest and by means of their bodies participated in covering the young. Males were not observed to attempt any defense of the nest, or of the young. Females were tolerant of older young in the nest when another litter was born and was being nursed. In one nest, a female of P. truei gave birth to a litter of three when her older litter was 29 days old. The three older young continued to nurse until they were 37 days old, at which time they were removed from the cage. The female appeared tolerant of this nursing by members of the older litter, but appeared to give preference to the wants of the younger offspring.

One female of P. truei lost or killed all but one young of her litter; at about the same time, a P. maniculatus and all but one of her young inexplicably died. Since the remaining young maniculatus, a male, was just weaned and was considered expendable, I placed him in the cage with the female truei and her 33-day-old, male offspring. The reaction to the newcomer was unexpected. The female immediately covered the P. maniculatus and her own young and prepared to defend them against me. Later, when the P. maniculatus was disturbed, he had only to emit a squeak and the female truei would run to cover and protect him. When the young male of P. truei was 69 days old the female kept him out of the nest, but still kept the male maniculatus in the nest with her. Although the female was somewhat antagonistic to her own young, she did not injure him, but only kept him out of the nest. The male truei was left in the cage with his mother and the P. maniculatus from September 23 to December 10. None of the mice had any apparent cuts on the ears or tail to indicate fighting. As much as seven months after the P. maniculatus was introduced into the cage, the female truei continued to cover him with her body whenever there was a disturbance. The male maniculatus not only tolerated this attention, but ran under the female truei when frightened. "Adoption" of young of another species has been reported for a number of animals, but, without further evidence, it is not possible to postulate that such adoptions occur between species of Peromyscus in nature.

Young males are tolerated by their mothers after weaning. One young male maniculatus was left in the cage with his mother from the time of his birth in autumn until late February of the following year. A litter was born on February 24. A young male P. truei was also left in the cage with his mother until he had acquired most of his postjuvenal pelage; the female and male usually sat together in the cage.

Females of both species sometimes eat their young when the young die shortly after birth. One female of each species killed three of her four young, and ate their brains and viscera. In one of these cases, the female, of P. maniculatus, also died; the female of P. truei was the same one that adopted the surviving P. maniculatus. The female truei continued to nurse her one remaining young for at least several days after killing three of his litter mates. A reason for this cannibalism might have been that I had fed these mice for several weeks on a mixture of grains low in protein content. Inadequacy of this diet for nursing females may have caused them to become cannibalistic. The feed of all captives was changed to Purina Laboratory Chow after the young were killed.

Transportation of Young

Females of both species transported their young either by dragging them collectively while the young were attached to mammae, or by carrying them one at a time in the mouth. Since mice of the subgenus Peromyscus have three pairs of nipples, they probably transport only six young collectively. Svihla (1932:13) has stated that both pectoral and inguinal teats are used in transporting young, in contrast to Seton's reputed assertion that only inguinal nipples were used. But Svihla neglected to cite Seton's complete statement. Seton (1920:137) recorded a litter of three as using only the inguinal mammae, but on the following page recorded the use of both inguinal and pectoral mammae by another litter of four. My findings agree with those of Svihla. Nursing females of both species were removed periodically from cages by lifting them by the tail. The young would hang onto the mammae and the female would clutch the young to her with all four feet. Young two weeks old or older crawled behind the mother while nursing.

The method of transporting young in the mouth has been mentioned by Seton (1920:136) and described by Lang (1925) and Hall (1928:256). These authors report that the mother picks the young up in her paws, and places it ventral-side up in her mouth, with her incisors around it. The young are not picked up by the skin on the nape of the neck, as are the juveniles of dogs and cats. I have found that females of both species of Peromyscus carry their young ventral-side up in their mouth while the young are small, and sometimes when the young are older. Generally, when females of P. truei moved young weighing more than 10 grams, the female grasped the young from the dorsal side, across the thorax just posterior to the shoulders, and held them with the incisors more or less around the animal. Perhaps this method was used with older young because of the observed tendency of the larger young to resist being turned over and grasped from the ventral side, and because their increased weight would have made it difficult, if not impossible, for the mother to pick them up with her paws. The young rarely resisted the efforts of the mother to move them by this method; when grasped across the thorax by the mother, the young would remain limp until released. Some females of P. truei would drag almost fully grown young back into the nest in this manner. I have not observed older young of a comparable age to be moved by females of P. maniculatus. The females of P. maniculatus appear to be somewhat less concerned than those of P. truei for the welfare of their young once they are mobile and close to being weaned.

The following listing describes changes in postnatal development of young, of each species, from birth to nine weeks of age.

P. maniculatus P. truei ----------------------------------+------------------------------------- FIRST WEEK: At birth, young are | At birth, young are helpless, red helpless, red overall, small | overall, smaller than P. truei, with wrinkled skin. Pinna of ear | skin wrinkled. Ear, eyes, and folded over and closed; eyes | digits as in P. truei. closed; digits not separated | from rest of foot. | | Redness diminished by fourth day. | Redness decreases and disappears by | fourth day. | Hair apparent by fifth day; | Hair apparent by fourth day; body dorsal one-half or two-thirds of | bicolored by end of week. body more darkly pigmented than | venter by fourth day. | | Young squeak loudly and suck; | Young squeak loudly; sucking more sometimes crawl, but drag hind | pronounced than in P. truei; may legs. | crawl, but drag hind legs. ----------------------------------+------------------------------------- | SECOND WEEK: Appreciable increase | As in P. truei. in size; head about 60 percent | larger than at birth, by 14th | day, and still large in | proportion to body. | | Toes on hind foot separated more | As in P. truei, but somewhat more from foot. | advanced. | Body well haired by end of week; | Body well haired by end of week; dorsum dark gray, venter whitish; | dorsum dark gray with brownish tail bicolored in most, but not | tint; venter whitish; tail haired. | bicolored in most, but not haired. | Pinna of ear unfolded and open by | As in P. truei, but development end of week. | somewhat more advanced. | Through day 10, use hind legs to | Crawl well by end of week; push, but by end of week use legs | difficult to hold, squirm but do to crawl; difficult to hold, | not bite; agile. squirm but do not bite. | | Walk behind mother while nursing; | agile. | ----------------------------------+------------------------------------- | THIRD WEEK: Eyes open on 16th to | Eyes open on 16th to 20th day, 21st day. | partly open earlier. | Gray pelage of dorsum brownish. | Pelage of dorsum brownish; molt Apparently there is a molt line | line across shoulders progressing progressing posteriorly from | posteriorly; browner anterior to nose; the molt line has moved to | line, grayer posterior to it. shoulder region by end of week; | pelage anterior to line browner, | grayer posterior to it. | | Tail haired and weakly bicolored | Tail haired and bicolored in all in some individuals by end of | individuals. week. | | Young walk and jump well; squirm | Young walk and jump well; fight and but rarely bite. | bite when handled. ----------------------------------+------------------------------------- | FOURTH WEEK: Begin to eat solid | Some young eat grain by 24th day; foods at 23-29 days, but also | others continue to nurse. nurse. | | Molt line about 3/4 inch | Juvenal pelage complete; no sign of posterior to head. Juvenal pelage | postjuvenal molt. completed by end of week. Some | young have brownish hair on front | legs. | | Young roll over on backs and use | As in P. truei; also, all jump feet to ward off litter mates | well, and fight fiercely when that are dropped into nest, or | handled. into container, with them. | ----------------------------------+------------------------------------- | FIFTH WEEK: Young weaned on 30th | All young weaned before or by end to 40th day; some nurse beyond | of week; none observed to nurse 30th day if female is lactating. | beyond 30th day, even if female is | lactating. | Juvenal pelage complete and no | Juvenal pelage complete; postjuvenal molt apparent on | postjuvenal pelage not apparent on dorsum. | most, but probably present on all, | and concealed under juvenal pelage. ----------------------------------+------------------------------------- | SIXTH WEEK: Postjuvenal pelage | Postjuvenal molt apparent in most apparent in most individuals | young; almost complete in some, under juvenal pelage, especially | except above tail and on flanks. along lateral line. | ----------------------------------+------------------------------------- | SEVENTH WEEK: Postjuvenal pelage | Postjuvenal pelage apparent in all apparent in most young; in some | young; less distinct molt line than the molt line has progressed well | in P. truei. up on the sides, but not to | mid-dorsum. | ----------------------------------+------------------------------------- | EIGHTH WEEK: All individuals | Growth completed in some growing; total lengths of 156-170 | individuals; those in larger millimeters; weight 17-22 grams. | litters have total lengths of | 128-144 millimeters; weight | 14-17 grams. ----------------------------------+------------------------------------- | NINTH WEEK: Testes partly scrotal | Scrotum in season usually large, in one male on 59th day. | vaginae open, evidence of coitus | common. (McCabe and Blanchard, | 1950:39). | New brown pelage encroaching on | Postjuvenal molt completed in some saddle and on hind legs; | individuals by end of week. New postjuvenal molt completed in | pelage tends to be concealed under some individuals by eleventh | juvenal pelage longer than in P. week. | truei. ----------------------------------+-------------------------------------

CHANGES OWING TO INCREASE IN AGE

Increase in length of limb bones, changes in proportion of bones in the skull, eruption and degree of wear of teeth, and changes in pelage can be used to ascertain relative age. Different investigators might choose different limits for the three categories young, subadult, and adult. Museum specimens were assigned to one of five age groups listed below mostly on the basis of tooth wear, essentially as described by Hoffmeister (1951:1).

Juvenile: M3 just breaking through bony covering of jaw or showing no wear whatsoever.

Young: M3 worn smooth except for labial cusps, and M1 and M2 showing little or no wear.

Subadult: M3 worn smooth; labial cusp may persist, but is well worn; M1 and M2 having lingual cusps worn, but not smooth; labial cusps showing little wear.

Adult: Lingual cusps worn smooth and labial cusps showing considerable wear; labial cusp of M3 may persist.

Old: Cusps worn smooth; not more than one re-entrant angle per tooth discernible, frequently none.

For live animals examined in the field, criteria based on pelage and breeding condition were used, as follows:

Juvenile: Only gray, juvenal pelage present.