Reproductive performance of a laboratory colony of the deermouse, Peromyscus maniculatus gambelii.

During a 23-mo period approximately 600 litters and 2,500 offspring were produced in a laboratory colony established with wild-trapped deermice (Peromyscus maniculatus gambelii). The incidence of pregnancy and size of litters increased with domestication. Average litter size was 4.3 for all litters and 4.1 and 4.7 for litters born to wild-trapped and first generation females, respectively. Litter size increased up to the seventh sequential pregnancy for both wild-caught and first-generation females and then tapered off. In all litters more males were born than females. The departure from a 1:1 ratio was significant for the combined data and for litters of first-generation females, but not for litters of wild-caught females.