Postweaning maternal care increases male chimpanzee reproductive success

Abstract

Humans are unusual among animals for continuing to provision and care for their offspring until adulthood. This “prolonged dependency” is considered key for the evolution of other notable human traits, such as large brains, complex societies, and extended postreproductive lifespans. Prolonged dependency must therefore have evolved under conditions in which reproductive success is gained with parental investment and diminished with early parental loss. We tested this idea using data from wild chimpanzees, which have similarly extended immature years as humans and prolonged mother-offspring associations. Males who lost their mothers after weaning but before maturity began reproducing later and had lower average reproductive success. Thus, persistent mother-immature son associations seem vital for enhancing male reproductive success, although mothers barely provision sons after weaning. We posit that these associations lead to social gains, crucial for successful reproduction in complex social societies, and offer insights into the evolution of prolonged dependency.

Group-level cooperation in chimpanzees is shaped by strong social ties

“…This is not the case for male offspring. Mother–son cohabitation during reproductive ages of sons has the potential to improve the reproductive success of sons, for instance in bonobos 54 and in this population of chimpanzees 55 . Here, we showed that females with sons approaching reproductive age in the community were more likely to participate in intergroup encounters in comparison to females with no such sons in the community.…”

Two wild female bonobos adopted infants from a different social group at Wamba

“…Physical, cognitive, emotional, and social development can be impaired as well in both bonobos and chimpanzees 40 , 45 – 48 . Even after weaning, the loss of a mother negatively affects males’ reproductive success 49 , 50 . Only one case of adoption has been reported in wild bonobos: a 4 year old male infant was adopted by his older brother after their mother’s death and survived for > 2 years 51 .…”

Early maternal loss leads to short- but not long-term effects on diurnal cortisol slopes in wild chimpanzees

“…While the effect of maternal loss on wild animal survival and reproduction has been recently established ( Foster et al, 2012 ; Andres et al, 2013 ; Tung et al, 2016 ; Walker et al, 2018 ; Surbeck et al, 2019 ; Crockford et al, 2020 ; Zipple et al, 2021 ), the mechanisms underlying these fitness costs remain understudied. Our study provides one of the rare empirical tests of the BEM (see also Rosenbaum et al, 2020 ) and ACM in wild long-lived mammals by assessing the short- and long-term physiological impacts of early maternal loss.…”

“…In mammals, mothers are essential for the early development of their infants since they provide postnatal care ( Maestripieri and Mateo, 2009 ). Maternal loss in mammals reduces growth ( Samuni et al, 2020 ), survival ( Watts et al, 2009 ; Andres et al, 2013 ; Tung et al, 2016 ; Stanton et al, 2020 ), and long-term reproductive success ( Andres et al, 2013 ; Strauss et al, 2020 ; Crockford et al, 2020 ; Zipple et al, 2021 , reviewed in Clutton-Brock, 2016 ).…”

“…Specifically, our dataset allowed us to assess both the short- and long-term effects of maternal loss on the HPA axis activity in wild chimpanzees. We thereby investigated one of the potential physiological mechanisms explaining the fitness costs associated with maternal loss reported in wild chimpanzees such as reduced growth, survival, and reproductive success ( Nakamura et al, 2014 ; Samuni et al, 2020 ; Stanton et al, 2020 ; Crockford et al, 2020 ). Furthermore, studying physiological effects using diurnal cortisol slopes is an underused paradigm in wild animal subjects despite its prevalence in the human health literature.…”