Fig. 18: Scatter diagram of postnatal growth of captive mice, showing increase in length of bodies from birth to 70 days of age. The records for P. truei represent 11 individuals of five litters; those for P. maniculatus represent 17 individuals of four litters.
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.
Fig. 19: Graphs showing postnatal growth of solitary captive individuals of P. truei and P. maniculatus, representing the only young in each of two litters.
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.