Wild bonobo and chimpanzee females exhibit broadly similar patterns of behavioral maturation but some evidence for divergence

Abstract

ObjectivesPrimates exhibit variation in rates of growth and development. Variation in female growth and development across ape species appears to be explained by the Ecological Risk Aversion Hypothesis (ERAH). Indeed, existing data on variation in somatic growth and reproductive maturation between humans' closest living ape relatives, bonobos and chimpanzees, appear to be consistent with this hypothesis. However, existing data on behavioral maturation between the two species appear to contradict this hypothesis. We present novel behavioral data on infant and juvenile females from wild populations of both species in order to further evaluate predictions of the ERAH as it relates to the speed of behavioral maturation.Materials and methodsWe analyzed 3 years of behavioral data on 17 female bonobos (<8 years of age) from LuiKotale, Democratic Republic of the Congo and 40 years of behavioral data on 30 age‐matched female chimpanzees from Gombe, Tanzania. We compared the timing of (a) the attainment of independence from mothers and (b) the development of social skills using the following proxies: proximity between females and their mothers and the time that females spent engaged in eating, suckling, social play, social grooming, and riding on their mothers.ResultsWe did not find species differences in the proportion of time that females spent in contact with their mothers or engaged in eating, suckling, social play, or social grooming. Female bonobos spent more time riding on their mothers than did female chimpanzees. Female bonobos spent more time at distances greater than 5 m from their mothers during the ages of 3–8 years, but females did not differ during the ages of 0–3 years.DiscussionBehavioral maturation is largely similar between females of the two species based on the ages and proxies considered herein. We propose alternative explanations for the differences that we found in proximity and riding that do not invoke differences in underlying rates of maturation.

Systematic mapping of developmental milestones in wild chimpanzees

“…A study in the Taї chimpanzee population recently showed that offspring of low‐ranking mothers and those whose mothers die post weaning experience lower growth than those of high‐ranking mothers or mothers who stay alive (Samuni et al, 2020). Whether maternal effects also impact on motor and sociocommunicative milestones remains to be investigated in chimpanzees (Lee et al., 2019), but is known to be influential in humans (Fraley, Roisman, Booth‐LaForce, Owen, & Holland, 2013). The fact that both species have more inter‐individual variation in the timing of later milestones highlights a shared element of developmental plasticity important for a long‐lived species.…”

Transition to siblinghood causes a substantial and long-lasting increase in urinary cortisol levels in wild bonobos

“…Immature bonobos depend heavily on their mothers and maintain close spatial and physical contact during the first 2 years of life ( De Lathouwers, 2004 ; Kuroda, 1989 ; Lee et al, 2020 ). After the age of 5 years, spatial distance to the mother increases ( Kuroda, 1989 ; Toda et al, 2021 ), but in the case of sons, associations between mothers and offspring persist even when sons reach adulthood ( Hohmann et al, 1999 ; Surbeck et al, 2019 ).…”

Cell-Mediated Immune Ontogeny Is Affected by Sex but Not Environmental Context in a Long-Lived Primate Species

“…Another possible explanation is that the change of neopterin in adolescent females is stimulated by exposure to novel pathogen environments. At the age of 5-6 years, females start to emigrate from their natal group by visiting neighboring groups until they become an established resident in one of them (Lee et al, 2020;Sakamaki et al, 2015). In the process, they are probably in contact with a large number of unfamiliar individuals and this may expose them to new pathogens (Ryu et al, 2020).…”

“…Within the African great apes, bonobos (Pan paniscus) and chimpanzees (P. troglodytes) are closely related to each other, and to humans. Given the importance of developmental changes for the reconstruction of phylogenetic trends, there is increasing interest in the postnatal development of bonobos and chimpanzees (Hare and Yamamoto, 2015;Lee et al, 2020;Behringer et al, 2020). Yet, information on immune ontogeny in these species is rare.…”