Intersexual conflict influences female reproductive success in a female-dispersing primate

Abstract

In group-living mammals, individual efforts to maximize reproductive success result in conflicts and compromises between the sexes. Females utilize counterstrategies to minimize the costs of sexual coercion by males, but few studies have examined the effect of such behaviors on female reproductive success. Secondary dispersal by females is rare among group-living mammals, but in western gorillas, it is believed to be a mate choice strategy to minimize infanticide risk and infant mortality. Previous research suggested that females choose males that are good protectors. However, how much female reproductive success varies depending on male competitive ability and whether female secondary dispersal leads to reproductive costs or benefits has not been examined. We used data on 100 females and 229 infants in 36 breeding groups from a 20-year long-term study of wild western lowland gorillas to investigate whether male tenure duration and female transfer rate had an effect on interbirth interval, female birth rates, and offspring mortality. We found that offspring mortality was higher near the end of males' tenures, even after excluding potential infanticide when those males died, suggesting that females suffer a reproductive cost by being with males nearing the end of their tenures. Females experience a delay in breeding when they dispersed, having a notable effect on birth rates of surviving offspring per female if females transfer multiple times in their lives. This study exemplifies that female counterstrategies to mitigate the effects of male-male competition and sexual coercion may not be sufficient to overcome the negative consequences of male behavior. Significance Individual reproductive success is maximized through conflicts and compromises between the sexes in social mammals. In species with high dimorphism, females can experience sexual coercion by males and develop counterstrategies to reduce these costs. We studied this link in western lowland gorillas, which exhibit female secondary dispersal, a strategy that is believed to reflect female choice for the protective abilities of males. We found that females are faced with the dilemma of staying with a silverback at the end of his tenure and risk higher infant mortality versus dispersing and suffering reproductive delays and lower Communicated by M. A. van Noordwijk Electronic supplementary material The online version of this article (

Infant survival in western lowland gorillas after voluntary dispersal by pregnant females

“…Dispersal dates were estimated as the midpoint between the last visit to the bai of the group of origin and the first visit of the group of destination (Manguette et al 2019 ). We accounted for the maximum possible error by assuming that a female transferred and conceived on the day she was last observed in the previous group and that she gave birth on the first day she was observed with her offspring in the new group (Table S2).…”

“…The infanticide rate after death of the silverback (leading to group disintegration) is quite high (12%) in western lowland gorillas at Mbeli (Breuer et al 2010 ; Robbins et al 2013 ). With their strategy of voluntary secondary dispersal, female gorillas may be able to choose a better male before the death of the current silverback and protect their offspring from infanticide (Palombit 2015 ; Manguette et al 2019 , 2020 ).…”

Female dispersal patterns influenced by male tenure duration and group size in western lowland gorillas

“…In western gorillas, infant mortality was significantly higher during the last 5 years of the male tenure than earlier in the tenure, suggesting that those males became less capable of providing protection against predators or infanticidal males (Manguette et al 2019). At Mbeli, the infanticide rate after group disintegration is around 12% (Breuer et al 2010;Robbins et al 2013).…”

“…include inbreeding avoidance, reduction of feeding competition, reduction of predation and infanticide risk, reduction of mate competition, mate choice, disease avoidance, and increase genetic diversity (Amos et al 2001;Lukas and Clutton-Brock 2011) (Table 1). The potential costs of female dispersal include aggression towards immigrants (Isbell and Van Vuren 1996), loss of habitat familiarity (Bonte et al 2012), loss of opportunity to help and be helped by kin (Young et al 2005), or reproductive costs such as longer intervals between births (Debeffe et al 2015;Walker et al 2018;Manguette et al 2019). To determine if and when to transfer, a female may gather information on other social groups and evaluate the costs and benefits of such a decision (Isbell and Van Vuren 1996;Clobert et al 2009).…”

“…Studies of feeding competition have shown little or no influence of group size upon female fertility (Robbins et al 2007;Manguette et al 2019). Yet, if females leave groups as soon as feeding competition begins to intensify, dispersal may be a more sensitive measure of within-group competition than studies of female fertility (Watts 1990;Robbins et al 2007).…”