Part 12

Both species constructed nests that resembled inverted bowls. Solitary mice naturally built smaller nests than those built by females with young.

The entrance to the closed nests varied; often the female would bolt through the side of the nest where there was no opening. Sometimes the mice would exit and enter through the top of the nest. In some cases it appeared that the entire nest was closed; probably the occupant had closed the entrance. Such a closed nest would have the advantage of greatly moderating the microenvironment within the nest, and would allow the animal within to remain comfortable with a minimum expenditure of energy. The larger nests found in the trapping grid resembled those built by captives. Nests built of grasses were always larger than those built of juniper bark. Juniper bark is as easily worked into nests as are grasses, in my judgment. Therefore, difficulty of construction of nests from this material probably does not account for the smaller size of the nests composed of bark. I think the difference in insulating characteristics between the two materials probably accounts for the difference in size of the nests.

REPRODUCTION

In Mesa Verde, Peromyscus reproduces from April through September. Reproduction is greatly reduced in the autumn, and most females complete reproduction before October.

Ten of the 20 females of P. maniculatus, taken in May, contained embryos; five others were lactating. Lactating and pregnant females were collected on May 5, 1962, indicating that reproduction in some females began in early April. In September, 15 of 34 females were pregnant or lactating, whereas in October only two out of 15 females of P. maniculatus were reproducing. Only one female of P. maniculatus was found to contain embryos in October. This large adult was taken on October 3, 1963, and had six embryos, each five millimeters long. She probably would have produced a litter later in October, and would have been nursing into November. A report of October breeding in north-central Colorado described nine of 23 females of P. maniculatus as being in a reproductive state; seven were lactating and one was pregnant between October 26 and 31, 1952 (Beidleman, 1954:118).

In the Museum of Natural History, the University of Kansas, there are 35 females of P. maniculatus more than 144 millimeters in total length taken from Mesa Verde in November, 1957 (Anderson, 1961:53). None of these contained embryos, and no pregnant females have been taken from the park in November.

P. truei and P. maniculatus reproduce at about the same time. A female of P. truei prepared as a specimen on May 10, 1964, contained four embryos, each 20 millimeters long, indicating a breeding time in mid-April. Svihla (1932:19) reported the gestation period for non-lactating P. truei to be 25 to 27 days and for lactating individuals, 40 days. Lactation tends to increase the gestation period of other Peromyscus by about five days (Asdell, 1964:266). The gestation period of nine non-lactating females of P. m. rufinus was reported by Svihla to be 23 to 24 days. Lactation increased the length of the period of gestation in this subspecies to between 23 and 32 days (mean for seven females 26.57 +- 0.73, Svihla, 1932:19).

Females of P. truei were observed in various stages of reproduction from June through September. Ten of the 20 females of P. truei taken in September were reproducing; four contained embryos and the other six were lactating. In October, only one of 17 females caught in snap traps was lactating. Lactating females were caught in live-traps as late as October 23, although most females had ceased reproduction by then. No pregnant or lactating females were observed in November.

In P. maniculatus, puberty has been placed at 32 to 35 days for females weighing 13 grams, and in males at from 40 to 45 days, at weights of 15 to 16 grams (Jameson, 1953:45). In P. truei, the weight of the testes is reported to rise in March and diminish through September, with accessory organs following the same cycle (Asdell, 1964:267). Young of P. truei nurse for about one month, although some litters may not be weaned until 40 days of age. Young of P. maniculatus are weaned between 22 and 37 days of age (Svihla, 1932:30).

Twenty-six pregnant females of P. maniculatus, taken in the breeding seasons of 1961-1964, contained from one to eight embryos each; the mean was 4.65 +- 1.67. Other investigators have found similar mean values in this species (Asdell, 1964:266).

Thirteen females of P. truei taken in the breeding seasons of 1961-1964, contained from three to six embryos each; the mean was 4.0 +- .912. Svihla (1932:25) reported litter sizes, at birth, of two to five and a mean of 2.84, in 19 litters. Other investigators have reported litter sizes of one to five with a mean of 3.4, and one to six with a mean of 3.6 (Asdell, 1964:268). Apparently P. truei does not have more than six young per litter.

In captivity, females of both species began reproduction in early February. These captives had been kept for several months at a temperature of 21 degrees Centigrade, and on a daily photoperiod of 15 hours. Some captive males had enlarged, scrotal testes in January; the extended photoperiod and warm temperature probably influenced the breeding condition. In both species testes of wild males caught in autumn after late September and on through the winter were abdominal, except for one male of P. maniculatus which had enlarged, scrotal testes on October 15.

Dates at which different animals arrived at breeding condition varied, in part owing to subadults (young of the year) appearing in the catch from early summer to late autumn. Some adult females appeared to be pregnant or lactating throughout much of the summer and early autumn, whereas other females, that were caught a number of times, apparently reproduced only once in the summer.

Some females may fail to breed even though they are mature enough to do so. One female of P. truei captured eight times (August 30 to September 20) was a juvenile when first caught, and was classed as young (in postjuvenal molt) on September 10. She did not reproduce in her first breeding season, unless she did so after September 20, which is unlikely. Another female of P. truei was an adult when first caught, and was caught 12 times (August 21 to October 25). At no time were her mammae enlarged and she was not lactating or pregnant. It is improbable that she reproduced earlier in the season, for teats of mice that have reproduced earlier usually are enlarged to such a degree that previous parturition is clearly indicated. It was surprising to catch a female, of any age, 12 times in two months without sign of reproductive activity.

Only one female of P. maniculatus did not show reproductive activity. She was a juvenile on July 19 when first caught; a subadult on August 28 when caught the third time, and an adult on October 23 when caught the fifth time.

Burt reported a rest period of a month or more in the summer, in Michigan, during which many females of P. leucopus did not reproduce. They began to breed again in late summer at about the time when young of the year began reproducing (Burt, 1940:17, 19). Abundant mast was correlated with reproductivity in autumn, according to Jameson (1953:54), who thought that "food is a basic determinant of the autumn reproduction" of P. leucopus.

Little has been written about the length of time males remain in breeding condition. Difficulties in determining breeding condition are many. Fertility customarily is determined by sectioning testes and noting the presence or absence, and relative abundance, of sperm. This procedure necessarily sacrifices the individual and indicates the breeding condition at only one moment and for only the individuals sacrificed. My observations of males caught a number of times in live traps shed some light on the breeding condition of males, but the investigator is likely to err in extrapolating physiological data from morphology when he notes whether the testes are abdominal or scrotal and whether they are enlarged or small. It was assumed that testes that have not descended, and that lie within the abdominal cavity, are not capable of producing viable sperm. This is the condition in most juveniles, and in all males during winter. As the breeding condition is attained, testes descend into the scrotum. Soon the testes and their accessory organs enlarge and are readily apparent.

Howard (1950:320) reported that numerous males of P. leucopus sired litters when their testes appeared to be abdominal, and therefore questioned whether the criterion of descended testes is valid as an indicator of breeding condition. My captive males of P. maniculatus and P. truei did not sire litters when their testes were abdominal, even though such males were left with adult females for as long as four to five months (August through December). Captive pairs of both species yielded no evidence of reproductive activity until January when, as mentioned earlier, some of the males had scrotal testes. Young were born first in early February, although their parents had been confined together since the preceding August. Jameson reported the testes of fecund males of P. maniculatus as almost always 8.0 millimeters or larger (Jameson, 1953:50). Testes that are at least partly scrotal must be considered as being capable of producing motile sperm, even though this may not be the case for all individuals.

Toward the beginning and end of the breeding season the testes and accessory organs of wild mice were small and probably produced few if any sperm. At these times some males apparently were so frightened by being handled that the testes were retracted into the inguinal canals. It would have been easy to consider such males as having abdominal testes when in fact they did not. In such cases the scrotum usually was noticeably enlarged; it was found also that in many cases the testes returned to the scrotal position if the mouse was held gently for a few minutes. Careful handling of animals was found to prevent, or at least retard, retraction of the testes. Retraction of the testes from the scrotum was not a problem at the height of the breeding season when the testes were engorged.

I had originally assumed that all adult males would be fertile throughout the breeding season, and that any males with abdominal testes would be subadults or young of the year. This assumption was an oversimplification; all adult males did not reach breeding condition at the same time of year. My data do not support a firm conclusion, for it is difficult to follow non-captive individuals throughout a breeding season, owing to sporadic appearance of animals in traps. Nevertheless, observations of mice that were trapped a number of times indicated the following:

1) Some adult males that had abdominal testes in the middle of July reached breeding condition as late as late August and even late September.

2) Some juvenal males had scrotal testes at the time their postjuvenal molt was just beginning to be apparent on their sides. Most juvenal males did not have scrotal testes, and many juveniles that appeared repeatedly in traps from mid-July through late October did not attain breeding condition. A mouse that was a juvenile in mid-July must have been born in mid-June.

3) Apparently animals born early in the breeding season may reproduce later in that season, whereas those born later in the breeding season tend not to breed until the following year.

Possibly cooler evening temperatures in July and August, due to the relatively larger amounts of precipitation in those months, inhibit reproductive development of late-born young. Most plants have ceased vegetative growth and have produced seeds by this time; but the interrelationships between growing seasons, climatic conditions, and reproductive physiology are unknown.

Only one adult of each species had scrotal testes after late September; the P. truei had scrotal testes on October 24, 1963, and the P. maniculatus had scrotal testes on October 15 of that year.

GROWTH

Growth of captive P. maniculatus and P. truei is discussed in several reports. One of the most complete is that of McCabe and Blanchard (1950) on P. m. gambelii and P. t. gilberti in California. A detailed discussion of the dentition in P. truei and wear of the teeth in different age groups is given by Hoffmeister (1951). Molt in these species has been considered by a number of authors (Collins, 1918; McCabe and Blanchard, 1950; Hoffmeister, 1951; Anderson, 1961). The report by McCabe and Blanchard is valuable because molt is compared between the two species from the first to the twenty-first week of postnatal development.

The thoroughness of the above-mentioned studies is readily apparent to those who have worked with mice of the genus Peromyscus. Nevertheless, the ecology of local populations of P. maniculatus and P. truei as reported for the San Francisco Bay area (McCabe and Blanchard, 1950) has little relationship to the ecology of mice of other subspecies of these species, in southwestern Colorado. Indeed, the preferred habitats, and to some extent the behavior, differ strikingly in Colorado and California.