A childhood dilemma: Growth or play

Active play promotes motoric development but at the same time it uses a lot of energy, which is required for an unimpeded growth process. Evolutionary biologists examining play behavior in animals are faced with a Darwinian paradox: Most definitions of play behavior include that the behavior does not serve any immediate purpose and is not assignable to an obvious function. Any behavior that generates costs but no benefits should disappear through natural selection. The prevalence of play behavior in the animal kingdom was therefore explained by the notion that it produces indirect or long-term benefits but occurs only when the animals have sufficient energy available: Playing promotes the motoric, cognitive and social development and only takes place when the animals are healthy, well fed and safe. "Our findings on Assamese macaques contradict this notion," says Andreas Berghänel, first author of the published study.

Young Assamese macaques, who spend a lot of time wrestling and romping in the jungles of Thailand, grow more slowly than their less playful conspecifics. "Thus, unconstrained development does not appear to be more important than play, young monkeys overexert themselves so much by playing that they cannot keep up with the growth process," says Julia Ostner, head of the study. The more playful monkeys thereby risk maturing later and having fewer offspring. However, there is also a clear benefit: The more time an infant spent playing intensely before acquiring a new motoric skill, the earlier in life it masters this motoric task. A faster motoric development is very beneficial if one is involved in fights or must flee from enemies. "Thus, my recommendation to parents: send your kids out to play and feed them a good dinner afterwards to make them grow tall and smart," says Julia Ostner.

Since 2014 Julia Ostner is Professor and Head of Department at the Johann-Friedrich-Blumenbach Institute for Zoology and Anthropology at the University of Göttingen. In addition, she heads the Research Group Social Evolution in Primates at the German Primate Center -- Leibniz Institute for Primate Research since 2015. Julia Ostner studies the behavior of Assamese macaques at a research station in Phu Khieo Wildlife Sanctuary in Thailand. Since 2014, the research station is financed by the German Primate Center.