Brief Communication: Orphaned male Chimpanzees die young even after weaning

Abstract

If a social-living animal has a long life span, permitting different generations to co-exist within a social group, as is the case in many primate species, it can be beneficial for a parent to continue to support its weaned offspring to increase the latter's survival and/or reproductive success. Chimpanzees have an even longer period of dependence on their mothers' milk than do humans, and consequently, offspring younger than 4.5-5 years old cannot survive if the mother dies. Most direct maternal investments, such as maternal transportation of infants and sharing of night shelters (beds or nests), end with nutritional weaning. Thus, it had been assumed that a mother's death was no longer critical to the survival of weaned offspring, in contrast to human children, who continue to depend on parental care long after weaning. However, in theory at least, maternal investment in a chimpanzee son after weaning could be beneficial because in chimpanzees' male-philopatric society, mother and son co-exist for a long time after the offspring's weaning. Using long-term demographic data for a wild chimpanzee population in the Mahale Mountains, Tanzania, we show the first empirical evidence that orphaned chimpanzee sons die younger than expected even if they lose their mothers after weaning. This suggests that long-lasting, but indirect, maternal investment in sons continues several years after weaning and is vital to the survival of the sons. The maternal influence on males in the male-philopatric societies of hominids may be greater than previously believed.

Do juveniles help or hinder? Influence of juvenile offspring on maternal behavior and reproductive outcomes in wild chimpanzees (Pan troglodytes)

“…While juvenile chimpanzees are nutritionally independent and do not routinely provide overt help to their mothers, this does not preclude the possibility of a mutually beneficial mother-juvenile relationship. Mounting evidence from the present and earlier studies (Nakamura et al, 2014) suggests that mothers continue to be important to weaned chimpanzees and our results indicate that post-weaning care does not come at an immediate reproductive cost to the mother. Instead, chimpanzee mothers may benefit from increased social interaction with their older offspring.…”

“…We used discrete event history analysis (Singer and Willett, 1993) to test whether having a sibling who was less than 8 years older was related to the odds of an individual’s own survival each year between birth to age 8 years. Examining survival past weaning age (~4–5 years) allows us to investigate potential post-weaning effects of older sibling presence on survival, given known post-weaning effects of maternal loss (Nakamura et al, 2014). Eight and a half years of age is the earliest onset of sexual maturity in females in this study community (Wallis, 1997) and female life expectancy peaks between 6–8 years of age (Bronikowski et al, 2016).…”

“…In chimpanzees, orphaned males face significantly lower odds of survival than non-orphaned individuals, even if they lose their mothers after weaning. This increased mortality in weaned orphans highlights the importance of some type of post-weaning maternal care in this species (Nakamura et al, 2014). Notably, unlike in bonobos and matrilineal cercopthicine primates, maternal interventions in immature chimpanzee agonistic interactions are rare (Markham et al, 2015), although some maternal interventions in interactions between immatures and adults have been reported (Pusey, 1983; Goodall, 1986).…”

Stable nitrogen isotope analysis of dentine serial sections elucidate sex differences in weaning patterns of wild chimpanzees (Pan troglodytes)

“…2). No isotopic study of weaning is currently available for another wild chimpanzee population; however a recent study from Mahale by Nakamura et al (2013) found that orphaned chimpanzee sons die younger than expected even if they lose their mothers after weaning, suggesting that maternal investment in sons at Mahale is often vital to their survival.…”

Maternal rank influences the outcome of aggressive interactions between immature chimpanzees

“…Maternal effects can be realized across this long period of dependency. For example, a recent study demonstrated lower survivorship for offspring that were orphaned after weaning (Nakamura et al, 2014). Similar patterns have been observed in other long-lived mammals, including killer whales, Orcinus orca (Foster et al, 2012) and red deer, Cervus elaphus (Andres et al, 2013).…”