Infant handling by female mountain gorillas: Establishing its frequency, function, and (ir)relevance for life history evolution

Abstract

Infant handling describes cases in which youngsters are temporarily removed from the care of their mothers and "taken care of" (held, carried, etc.) by other conspecifics. Handlers may gain indirect fitness benefits from these actions and can practice mothering skills, thereby improving the odds of survival of their own infants. Great apes are notable for displaying little infant handling. Apart from anecdotal observations, no published data exist on infant handling in wild mountain gorillas. We tested two of the most pertinent explanations ("kin selection" and "learning to mother") in a wild population of mountain gorillas in Rwanda. We predicted that (a) nulliparous females would exhibit infant handling (i.e., carrying) more than parous females and (b) maternal kin would exhibit more infant handling than nonkin. We collated 8 years of data on infant carrying behavior collected in 13 groups monitored by the Dian Fossey Gorilla Fund's Karisoke Research Center. Infant handling is an infrequent behavior (1,783 instances over 25,600 observation hours). A strong positive effect of relatedness and handler parity on the frequency of infant handling emerged. While the nature of handler-infant interactions (affiliative, abusive, etc.) remains unstudied, they could constitute alloparental care and could therefore attenuate maternal energetic burden and ultimately allow increased birth rates. However, the rarity of this behavior makes it an unlikely contributor to mountain gorillas' relatively short interbirth intervals.