Extragroup paternity in gelada monkeys, Theropithecus gelada, at Guassa, Ethiopia and a comparison with other primates

Following the introduction of DNA fingerprinting in the 1980s, studies have repeatedly revealed mismatches between the mating system inferred from social behaviour and the mating system revealed through genetic relationships. In this study, we examined the occurrence of extragroup paternities (EGPs) in gelada monkeys at Guassa, Ethiopia and explored how EGPs may be acquired in this species, which is conventionally considered to have a single breeding male (‘leader’) per social group (‘unit’). We found that while leaders sired most offspring, a substantial proportion (17 of 88 offspring, 19%) were sired by extra-unit males. Offspring were more likely to be sired by extra-unit males when they were: (1) conceived in the 6 months following the start of peak rainfall (July–December) and (2) the first successful conception for a mother. Furthermore, eight of the 17 extra-unit paternities were directly or indirectly the result of a single infertile leader, indicating that infertility plays an important role in the occurrence of EGPs. Compiling published data revealed that the 19% EGP rate observed among Guassa geladas was also the median for polygynous nonhuman primates (range 0–57%, N = 5 populations), in contrast to geladas in the Simien Mountains, where EGPs are absent or rare. Thus, intraspecific variation in EGP rates in geladas encompasses much of the range described across polygynous primates more broadly. The Guassa gelada EGP rate was similar to the median rate reported for mammals. Primates generally had a lower median rate of EGPs, more similar to the median rate reported for birds. Understanding variation in EGP rates requires understanding the outcomes of competing interests of males and females within the same species, as well as how these features contribute to the evolution of social systems and influence variation in EGP rates at higher taxonomic levels.