Does Paternity Certainty Elicit Protection and Support of Offspring by Male Gelada Monkeys (Theropithecus gelada)?

Abstract

In most mammals, mothers raise offspring independently, whereas in humans, fathers often contribute substantially to parental care. How and why more intensive paternal care emerged in humans remains an unanswered question. Paternity certainty has long been considered necessary for males to invest substantially in parenting effort. However, genetic studies of mammals and birds have repeatedly revealed mismatches between the mating system inferred from social behavior and the mating system revealed through genetic relationships. Thus, extra-pair or extra-group copulations can have a substantial impact on paternity certainty. Further, while males are most likely to gain fitness benefits from investing in their own offspring, recent studies have revealed that paternity certainty may be neither necessary nor sufficient to account for the continuum of paternal care observed across primates, from minimal to more extensive forms. Recent studies have highlighted that humans live in multi-level societies. While we cannot know for certain when this trait evolved in our ancestral past, fossil evidence suggests that australopiths had high body size sexual dimorphism, consistent with polygyny, and ecological conditions specific to food and sleeping site distribution may have resulted in australopiths dispersing in ways similar to other multi-level primates, such as gelada monkeys (Theropithecus gelada). Geladas thus represent an interesting model species to explore conditions under which more intense paternal care could emerge. To examine the degree to which gelada males invest in parenting effort, I completed genetic analyses to determine paternities of 98 offspring who are part of a larger wild population studied in the Guassa Conservation Area, Menz Highlands, Ethiopia. With known paternities, I then analyzed behavioral data and long-term demographic data to examine the degree to which immatures benefit from the presence of a father, particularly during unstable periods following take-over events when infanticide risk increases for unweaned offspring. Finally, I explored the ways mating or parenting effort by gelada males manifest through different patterns of male-immature interactions. I found that leaders sired most offspring (80%), while there was only a single case of a follower siring an offspring. Thus, leaders invest heavily in mating effort, but deposed leaders who transition to the position of follower acquire very few mating opportunities. My results of behavioral analyses indicate that followers shift their efforts towards parenting effort once deposed. The increased mortality risk associated with social instability following take-over events, in addition to the loss of either parent, substantially reduced offspring survival and measures of well-being (e.g., feeding and play duration, and agonisms received from other individuals). However, males can reduce these risks by providing extended periods of unit stability as a leader or, as a follower aiding in unit defense to reduce the risk of take-over. If australopiths were polygynous within multi-level societies like geladas, these findings suggest that males would have focused more on mating than parenting effort, at least until males were deposed, with more extensive forms of paternal care observed in humans emerging later.