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. [Illustration: 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_. 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.