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 post-reproductive lifespans. Prolonged dependency must therefore have evolved under conditions in which reproductive success is gained with parental investment and diminished with early parental loss. Catherine Crockford and colleagues tested this idea using data from wild chimpanzees, which have similarly extended immature years as humans and prolonged mother-offspring associations. They found that males who lost their mothers after weaning but before maturity began reproducing later and had lower average reproductive success.
Major theories in human evolution argue that parents continuing to provide food to their offspring until they have grown up has enabled our species to have the largest brains of any species on the planet relative to our body size. Brains are expensive tissue and grow slowly leading to long childhoods. Ongoing parental care through long childhoods allow children time to learn the skills they need to survive in adulthood. Such long childhoods are rare across animals, equaled only by other great apes, like chimpanzees.
Chimpanzees may have long childhoods, but mothers rarely directly provide them with food after ages four to five years when they are weaned. Mostly mothers let their offspring forage for themselves. So then what do chimpanzee mothers provide their sons that gives them a competitive edge over orphaned sons? We do not yet know the answer but scientists do have some ideas.
One idea is that mothers know where to find the best food and how to use tools to extract hidden and very nutritious foods, like insects, honey and nuts. Offspring gradually learn these skills through their infant and juvenile years. Researchers can speculate that one reason offspring continue to travel and feed close to their mothers every day until they are teenagers, is that watching their mothers helps them to learn. Acquiring skills which enable them to eat more nutritious foods may be why great apes can afford much bigger brains relative to their body size than other primates.
Another idea is that mothers pass on social skills. Again a bit like humans, chimpanzees live in a complex social world of alliances and competition. It might be that they learn through watching their mothers when to build alliances and when to fight.
References: Catherine Crockford, Liran Samuni, Linda Vigilant and Roman M. Wittig, “Postweaning maternal care increases male chimpanzee reproductive success”, Science Advances, 2020, Vol. 6, no. 38, eaaz5746 DOI: 10.1126/sciadv.aaz5746 link: https://advances.sciencemag.org/content/6/38/eaaz5746