The growing pains of orphan chimpanzees

Mothers may be shaping pre-adult growth and offspring muscle mass even without direct provisioning

January 07, 2020

Like humans, young chimpanzees associate with their mothers all the way into adulthood. Unlike in humans, offspring no longer depend on maternal food sharing beyond the weaning age. Therefore, the reasons for and consequences of these years of post-weaning mother-offspring associations are unclear. Using long-term behavioural and hormonal data from wild chimpanzees in the Ta茂 Forest, C么te d鈥橧voire, researchers from the Ta茂 Chimpanzee Project at the Max Planck Institute for Evolutionary Anthropology, have revealed that mothers may be shaping pre-adult growth and offspring muscle mass even without direct provisioning. We compared growth of young chimpanzees with a mother until adulthood compared to those who had experienced maternal loss after weaning.

The prolonged periods of juvenility and nutritional dependence that are characteristic of human development are thought to facilitate brain and somatic growth in children, as well as provide opportunities to learn and accumulate skills required for a productive adult life. In chimpanzees, the benefits of continuing to associate with mothers after becoming nutritionally independent are less well understood.

To address this, the researchers used a dataset comprising 18 years of research effort on three groups of wild chimpanzees at the Ta茂 Forest, C么te d鈥橧voire, to examine the effects of maternal loss and mothers鈥 social status on offspring lean muscle mass development. 鈥淲e assessed the muscle mass of 70 offspring from 41 mothers by means of urinary creatinine concentrations, a by-product of metabolic activity in muscles,鈥 explains Tobias Deschner, a researcher at the MPI for Evolutionary Anthropology. As expected, offspring muscle mass increased with age in both males and females. However, chimpanzee offspring that lost their mother after they were already weaned had less muscle mass than those with living mothers. Furthermore, mothers of high social status measured by dominance rank, produced offspring with higher muscle mass.

This study demonstrates how maternal presence and maternal social status can impact offspring phenotype in wild chimpanzees. Importantly, even once offspring are nutritionally independent, maternal loss has negative consequences on their growth. 鈥淥ur results emphasize the crucial role of mothers and suggest that even in the absence of consistent direct provisioning from mothers to offspring, chimpanzee mothers still indirectly influence food consumption in their offspring鈥, Liran Samuni, one of the two first authors on the study, points out.

鈥淭his may occur in similar ways to humans鈥, suggests Patrick Tkaczynski, the other first author on the study. 鈥淢others may provide support to their offspring during competitive interactions with others, and increase their chance to 鈥榳in鈥 conflicts, or soften reactions of offspring to challenging situations. Also, mothers may provide opportunities for offspring to learn how to find and access hard to extract or rare food items.鈥

鈥淥ur study provides evidence that a mother-offspring association and dependence that lasts beyond the weaning age, although unique in its duration and level of investment in human societies, may be a trait with deep evolutionary origins that we share with one of our closest living relatives鈥, declares Catherine Crockford, head of the ERC project group Ape Attachement and senior author of the study.

[PT]

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