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Who's your daddy? The causes and consequences of male-immature relationships in wild mountain gorillas (Gorilla beringei beringei)


Among mammals, humans are unusual for their bi-parental care. Expensive gametes, gestation, and lactation necessitate the Phylum-typical extensive female investment, but in all cultures across the globe men take part in caring for children. The form, duration, and extent of their care varies, but it is nonetheless a human universal. Across the primate order, about 40% of species provide male care of immatures in some form. The majority of studies of male parenting in our closest extant relatives have taken place in a few small-bodied, monogamous or polyandrous species of New World monkeys, the Callithricids. While highly informative, the social structure and ecology of these species bears little resemblance to that of modern-day humans or the hominins who shared our recent evolutionary past. By studying male-immature relationships in primates that are more closely related to us and that live in social systems closer to "species-typical" (i.e. highly flexible) for Homo sapiens, we can get a clearer picture of the evolution of male care and investment in offspring.

The mountain gorilla (Gorilla beringei beringei) is an excellent model for examining the form and function of such relationships in a close human relative. The genus Gorilla split from the genus Homo ~10 million years ago. Although not our closest living relative, Gorilla beringei is one of the few primates species besides humans in which both males and females can either reproduce in their natal group or disperse. For years they were described as strictly polygnous with one male and multiple females in each group, but after 47 years of research we now know that they are also capable of remarkabe flexibility in their social system. About 40% of the the gorillas monitored by the Dian Fossey Gorilla Fund's Karisoke Research Center in the Virunga Massif in central Africa occur in multi-male, multi-female groups.

In mountain gorilla groups, male-immature relationships are conspicuous for their closeness. Males are highly tolerant of young animals, and immatures often cluster around adult males as soon as they are old enough to be independently mobile. Besides their mother, an adult male (usually but not always the dominant animal) is typically an immature's closest adult social partner. Adult males are key to infant survival, protecting them from infanticidal outsiders, and probably historically, predators as well.

In this dissertation, I examine 1) whether males and immatures are capable of discriminating paternity in multi-male, multi-female groups; 2) if, and how, mother gorillas facilitate relationship development between their infants and a protective adult male(s); and 3) whether social preferences early in an immature's life for certain adult males predict their social preferences at two later developmental stages, including young adulthood. Together, these chapters give us a more nuanced understanding of male parenting behavior and its evolutionary implications in one of our closest living relatives, Gorilla beringei.

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