Deer mice are polygynous, meaning that male deer mice each mate with more than one female.
Mating System: polygynous ; polygynandrous (promiscuous)
Females deer mice can have many litters in a year. In the wild, reproduction may not occur during winter or other unfavorable seasons. Females are able to become pregnant again shortly after giving birth. The pregnancy of a female deer mouse that is not nursing young lasts from 22.4 to 25.5 days and the pregnancy of a female deer mouse that is nursing young lasts 24.1 to 30.6 days. Deer mice may have litters containing from one to eleven young with typical litters containing four, five, or six babies. Litter size increases each time a female deer mouse gives birth until the fifth or sixth litter and decreases afterwards.
Deer mice are very helpless at birth but develop quickly. At birth, each baby has a mass of about 1.5 g. The young are born hairless with wrinkled, pink skin, closed eyes, and folded over ears. Juvenile hair begins to develop on the second day after birth. On the third day, the ears unfold with the ear canal opening on the tenth day. Eyes open on the fifteenth day, and the young are weaned between day 25 and 35.
Deer mice can reproduce when they are 35 days old, but they usually breed for the first time at 49 days.
Breeding interval: Deer mice breed every three to four weeks during the warmer months and less frequently during the winter.
Breeding season: Deer mice breed year round, but most breeding occurs during the warmer months.
Range number of offspring: 1 to 11.
Average number of offspring: 4 to 6.
Range gestation period: 22.4 to 30.6 days.
Range weaning age: 25 to 35 days.
Average time to independence: 35 days.
Range age at sexual or reproductive maturity (female): 35 (low) days.
Average age at sexual or reproductive maturity (female): 49 days.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous ; post-partum estrous
Average birth mass: 2 g.
Average number of offspring: 5.
Like all female eutheria, deer mice provide nourishment to their young before birth through the placenta. After the young are born, mother deer mice produce milk for their offspring. While nursing, the mother carries her young clinging to her nipples or one at a time in her mouth. Once weaned, the young usually leave the nest and become independent of their mother, although sometimes the mother will tolerate their presence for longer periods. Often when the mother has a second litter, she forces the first litter out of the nest.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)
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