Mating systems in cactus mice are not well understood. Like other Peromyscus species they are likely to have a promiscuous mating system.
The gestation period for Peromyscus eremicus is 20 to 25 days. Litter size is 1 to 4 offspring and average weight at birth is 2.1 to 2.9 grams (Parker 1990). Females may have up to 4 litters per year. The female has two pairs of teats rather than three, which has been suggested to correlate with the number of offspring (Parker 1990). The weaning period is unknown. Females reach sexual maturity after about two months, and the age of male sexual maturity is unknown. The average age for the female cactus mouse's first estrus is 39.2 days (Nowak 1991). Females have been observed to reproduce continuously and year round, and specifically during January, February, June, and September in central Arizona (Spotorno 1992).
Male Peromyscus eremicus have a simple penis, distinct from the complex penis of some other murids. The os baculum is simple, lacking the three distal prongs typical of complex penes. Young cactus mouse penes were found to have three bluish cartilaginous condensations near the tip of a well-ossified baculum. One condensation was located at the apex and two were located laterally, where the normal distal prongs in a complex penis would occur (Spotorno 1992). This characteristic is not only distinguishing, but it suggests that possession of a 3-pronged baculum may be primitive. The penis is nevertheless broad, and its size has been suggested to increase "locking" during 3.5 percent of copulations (Species Information Library Peromyscus eremicus 1994). Additionally, bone-ossification and sexual maturity are both controlled by testosterone.
Breeding interval: Cactus mice can have multiple litters in a year.
Breeding season: Breeding occurs throughout the year.
Range number of offspring: 1 to 4.
Range gestation period: 20 to 25 days.
Average age at sexual or reproductive maturity (female): 2 months.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
Average birth mass: 2.33 g.
Average number of offspring: 2.6.
Average age at sexual or reproductive maturity (female)
Sex: female: 129 days.
Young cactus mice are born with their ears and eyes closed. The ears open in the first day after birth and the eyes open in 11 to 15 days. Females nurse and care for their young in their nest until they are weaned, probably within a few weeks of birth. Juveniles go through their first molt at 5 weeks old.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)