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.

Fig. 1. The impact of maternal loss on male chimpanzees’ reproductive careers.

Impact of maternal loss on (A) alpha tenure, (B) age at first sire, and (C) likelihood of siring offspring. (D) Impact of alpha tenure on likelihood of siring offspring. Boxes show the median, 25th, and 75th percentiles, and whiskers are interquartile range × 1.5. (D) Model line (dashed) and 95% confidence intervals (gray), and larger points denote larger number of observations.

Fig. 2. Mother, Sumatra, grooming her 9 year old son, Solibra.

Photograph by L.S.