Abstract
Extra-group paternity (EGP) has been described in various mammalian species; however, little is known about which factors contribute to the variation in EGP, as the majority of studies were restricted in time and the number of groups considered. Using longitudinal demographic and genetic data, we aim to investigate which factors predict rates of EGP in the free-ranging rhesus macaque population of Cayo Santiago, Puerto Rico (USA). Of the 1649 infants considered which were born into six social groups over 9 years, we identified an average of 16% of infants resulting from EGPs. We tested the influence of group size, breeding group sex ratio, female reproductive synchrony, and group instability on the occurrence of EGPs. Our results suggest a tendency for EGPs to increase as the proportion of females increased in larger groups, but no such effect in smaller groups. Furthermore, as group instability and female reproductive synchrony decreased, the number of EGPs tended to increase. Our results support the hypothesis that group structure affects the occurrence of EGPs, which might be mediated by male mating opportunities, male monopolization potential, and/or female choice.Significance statementIn several species, both sexes seek alternative reproductive strategies to enhance their reproductive success. For instance, females may pursue EGPs to potentially increase genetic compatibility with males, or males may seek EGPs to improve their mating opportunities. Our longitudinal analysis, including demographic and genetic data over 9 years of six social groups of rhesus macaques, revealed high variation in the occurrence of EGPs across groups and years, and this variation tended to depend on group characteristics such as breeding group size, sex ratio, female synchrony, and group instability. The data suggest that group structure affects the number of EGPs in this group-living primate. Our results show that EGPs can affect the distribution of paternity within social groups and should be taken into account when assessing reproductive success.
Highlights
The proportion of offspring fathered by males outside of the breeding group or pair plays a critical role in increasing genetic variability of populations while shaping the evolution of 67 Page 2 of 11Behav Ecol Sociobiol (2017) 71: 67 complex social systems (Isvaran and Clutton-Brock 2007)
The results suggested an interaction between sex ratio and group size
In the present analysis, results point towards a complex interplay between the effects of group size and sex ratio on the rate of Extra-group paternity (EGP)
Summary
The proportion of offspring fathered by males outside of the breeding group (i.e., extra-group paternity, EGP) or pair (i.e., extra-pair paternity, EPP) plays a critical role in increasing genetic variability of populations while shaping the evolution of 67 Page 2 of 11Behav Ecol Sociobiol (2017) 71: 67 complex social systems (Isvaran and Clutton-Brock 2007). EGP seems more likely when males are unable to fully monopolize receptive females (van Noordwijk and van Schaik 2004), suggesting that females, under such scenarios, are able to seek surreptitious matings that could result in EGPs. the rates of EGP within social groups should depend on the reproductive strategies that the individuals of a given group pursue. A relationship between the proportion of females in a social group and the number of EGPs was found in a study on Verreaux’s sifaka, where the number of offspring sired by resident males in a social group decreased when the sex ratio was biased toward females (Lawler et al 2003). Across 26 mammal species, EGP was only weakly related to the sex ratio of breeders (Isvaran and Clutton-Brock 2007). The relationship between the breeding group sex ratio, group size, and extra-group paternity is far from being resolved
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