Factors Affecting Population Densities
The production of young, and success in rearing them, is essential to continuity of any population. P. maniculatus is favored in this respect, because the females produce more young and wean them sooner than do females of P. truei. In addition, lactating females of P. maniculatus require significantly less water than do females of P. truei. Since young mice of both species require no more water per gram of body weight than do adults, the young can disperse into any area that is habitable by their species. P. maniculatus probably is affected less by prolonged drought than is P. truei. Since lactating females require the most water of any animal in the population, they are the weakest link in the system. Females of Peromyscus are known to reabsorb embryos when conditions are unfavorable for continued pregnancy. If prolonged drought occurred in the reproductive season, and desiccated the vegetation upon which the mice depend for moisture, the populations should diminish the following year. Lactating females of P. truei would be affected more seriously by a shortage of water than would lactating females of P. maniculatus.
Of two species, the one producing the more young probably would be subjected to more parasitism and predation than the species producing fewer young. A favorable season for botflies, Cuterebra sp., revealed that P. maniculatus has a higher incidence of parasitism by these flies than has P. truei; possibly the adult flies concentrate in the open, grassy areas where P. maniculatus is more abundant, rather than in the woodlands where P. truei lives. Perhaps the lower parasitism of P. truei by warbles is related to the physiology of this species of mouse. Near Boulder, Colorado, the incidence of infection by warbles is lower in P. difficilis, a species closely related to P. truei, than in P. maniculatus (V. Keen, personal communication).
Although predation by carnivores would be expected to be higher on P. maniculatus, because this species does not climb, my data show that more individuals of P. truei were taken by coyotes. I lack confidence in these findings, suspecting that another sample might indicate the reverse. Birds of prey probably catch more individuals of P. maniculatus, because this species lives in more open habitats. My data do not warrant firm conclusions regarding predation.
The length of time females must care for their young influences the rate at which individuals can be added to the population. Females of P. truei nurse their young longer and keep them in the nest longer than do females of P. maniculatus. Although this may enhance the chances of survival of young of P. truei, it also reduces the number of litters that each female can have in each breeding season. Females of P. maniculatus can produce more young per litter, and each female probably can produce more litters per year than females of P. truei.
Captives of P. truei were tolerant of other individuals of the same species, even when kept in close confinement. However, when there was slight shortage of food or water they killed their litter mates, or females killed their young. Only a short period of time was necessary for one mouse to dispatch all others in the litter. The attacked mice were bitten through the head before being eaten; the brains and viscera were the first parts consumed. The population might be decimated rapidly if drought forced this species to cannibalism. When the supply of food or water was restored, the captive mice resumed their tolerant nature.
In captivity, P. maniculatus is amazingly tolerant of close confinement with members of the same species; individuals did not tend to kill their litter mates, or their young, even during shortage of food and water. This tolerance, especially under stressful conditions, probably enables P. maniculatus to persist in relatively unfavorable areas.
Each of the two species of Peromyscus illustrates one or more adaptations to its environment. P. truei is adapted to climbing by possession of long toes, a long tail, and large hind feet. The tail is used as a counterbalance when climbing (Horner, 1954). When frightened, individuals of P. truei often ran across the ground in a semi-saltatorial fashion, bounding over clumps of grass that were as much as 18 inches high. Such individuals usually ran to the nearest tree and climbed to branches 10 to 20 feet above the ground.
Large eyes are characteristic of the truei group of mice, and may be an adaptation to a semi-arboreal mode of life. A similar adaptation is shared by some other arboreal mammals, and of arboreal snakes. The large eyes of P. truei in comparison to those of P. maniculatus, probably increase the field of vision, and permit the animal to look downward as well as in other directions.