Loss of mother reduced reproductive success in male chimpanzees

A chimpanzee mother cleans the fur of her nine-year-old son. / Catherine Crockford et al. / Science Advances, 2020

Male chimpanzees that are motherless between weaning and early adulthood become fathers later and have fewer children. This is the conclusion reached by primatologists who studied the life of two dozen chimpanzees from three communities in the rainforests of Côte d'Ivoire. As noted in an article for the journal Science Advances, the study's findings demonstrate how important maternal care is to long-lived species. Perhaps this is one of the reasons why great apes, including humans, have such a long childhood.

Compared to most animals, humans have a very long childhood: more than ten years pass between weaning and puberty, and all this time it depends on the parents. It is believed that a long childhood stimulated the development of the brain of human ancestors, allowing them to more effectively master social skills. However, it remains unclear exactly how this feature took hold.

A team of researchers led by Catherine Crockford of the Max Planck Institute for Evolutionary Anthropology decided to investigate this issue. Scientists have hypothesized that a long childhood can directly enhance reproductive success. To test this idea, they focused on chimpanzees ( Pan troglodytes ), whose parents care for their offspring for almost as long as they do in humans.

The authors analyzed data on three chimpanzee communities from the Tai National Park in Côte d'Ivoire: on average, each of them was observed for 16.7 years. During this time, the researchers were able to track the fate of 23 males: 11 of them lived with their mothers before the start of independent life (that is, up to 12 years), and 11 were orphaned in the period between weaning and puberty.

In the studied chimpanzee population, the fathers of most of the pups are alpha males, so in the first stage, the authors checked whether the loss of the mother affects the male's ability to take the leading position in the group. It turned out that orphan chimpanzees are less likely to become alphas, although statistically this pattern is not entirely reliable, possibly due to the small sample (p = 0.068).

More convincing results were obtained by comparing the age at which males first become fathers. Chimpanzees who lost their mothers had their own children, on average, three years later than their counterparts from "full families" (p = 0.0008). This was one of the reasons for their low reproductive success: on average, orphans had half the number of cubs that survived to two years of age than other males.

Thus, the loss of a mother between weaning and the onset of independent life makes male chimpanzees less competitive, indicating how important maternal care is to their development. Although the female chimpanzee rarely feeds her cubs after lactation, she teaches them to find food. In addition, in her presence, relatives are less likely to take away food from the child. Also, interaction with the mother reduces the stress level of the cubs, which has a beneficial effect on their health.

In future studies, the authors hope to clarify which aspects of maternal care have a positive effect on male reproductive success. Nevertheless, the data obtained can already be considered a serious argument in favor of the hypothesis about the evolutionary benefits of a long childhood.

Chimpanzees share many similarities with humans. For example, this is the only animal species in which, as in humans, functional asymmetry of the cerebral cortex is observed. As it turned out in a recent study, in chimpanzees, one of the structures analogous to the one that provides human speech perception in the left hemisphere is not the same as anatomically similar to it in the right.

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