Absence of reproductive suppression in young adult female striped mice living in their natal family

• We studied alternative reproductive tactics in female striped mice.
• We tested for reproductive suppression as observed in the males of the same species.
• Of 17 litters, we kept one female in the family while her sister was kept singly.
• Females did not differ in time of puberty, corticosterone or progesterone levels.
• Reproductive suppression cannot explain why in the field young females do not breed.

Alternative reproductive tactics of males have been studied in many species, but few studies have focused on females. In many communally breeding mammals, females can be adult nonbreeding helpers, leave the group and breed solitarily, or be a breeder in their natal group, representing three alternative reproductive tactics. The reasons for delayed breeding are not well understood, but in many sociable species both male and female helpers are reproductively suppressed. Male helpers of communally breeding striped mice, Rhabdomys pumilio, have increased corticosterone levels and delayed sexual maturation compared with their singly housed brothers. In the present study, we tested whether similar effects occur in female striped mouse helpers. In the field, young adult females typically do not breed in their natal group, indicating that they might be reproductively suppressed. Seventeen sister pairs from 17 family groups were studied. One sister of each pair was kept in the family group, while the other was housed singly at 3 weeks of age. Sisters did not differ in either the age at which they reached puberty (at 6 weeks on average) or in their corticosterone and progesterone levels. However, in neutral encounter tests, singly housed sisters showed more amicable behaviours when presented with unfamiliar striped mice of both sexes. Their high sociable motivation might explain why most females remain philopatric under natural conditions. We conclude that philopatric female striped mice in monogamous family groups are not reproductively suppressed, but reproductive competition might occur in natural communal groups with multiple old breeding females, as observed under high population density.