Reithrodontomys megalotis is a polygynous species, in which the dominant male mates with females during their estrus period.
Mating System: polygynous
Few individuals live more than a year. As would be predicted from this short lifespan, young reach sexual maturity early, at about 1 month of age, and full maturity is reached at about 4 to 5 months. This species breeds from early spring to late autumn, foregoing reproduction only in the most severe winter weather.
Females have a high reproductive potential, having early sexual maturity and short gestation period of 23 to 25 days. The average litter size varies geographically, but is around 4, and as many as 9 pups can be born at one time.
Newborns are born naked, pink and blind. Neonates weigh 1 to 1.5 g, are 7 to 8 mm in length, and are totally helpless. They have a slight coating of fur by the time they start to crawl, around 5 days of age. Their incisiors erupt around this time. The eyes and ears are open by around 11 days of age. The young are weaned by 24 days. Young are reported to leave their natal nest around 3 weeks of age.
Reithrodontomys megalotis is known to undergo a post partum estrus cycle, allowing rapid production of litters. As females reach the age of approximately 45 weeks, there is a reduction in litter size, signalling senility.
Breeding interval: Breeding interval varies geographically, with animals in mild climates breeding approximately once per month, year round.
Breeding season: Wild western harvest mice breed from early spring to late autumn, foregoing reproduction only in the worst of winter weather..
Range number of offspring: 1 to 9.
Average number of offspring: 4.
Range gestation period: 23 to 25 days.
Average weaning age: 24 days.
Average time to independence: 3 weeks.
Average age at sexual or reproductive maturity (female): 4 to 5 months.
Average age at sexual or reproductive maturity (male): 4 to 5 months.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization ; viviparous ; post-partum estrous
Average birth mass: 1.33 g.
Average number of offspring: 3.3.
Average age at sexual or reproductive maturity (male)
Sex: male: 80 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 107 days.
Females care for their young in a nest made of grass, nursing them for up to 24 days. The young are born blind and helpless, but grow quickly. The young can leave their natal nest as early as three weeks of age. Males apparently play no role in parental care.
Parental Investment: altricial ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)