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You see when a baby animal experiences stress, its brain changes so that it’s subsequently less sensitive to stress hormones. This means that, as an adult, the critter recovers more rapidly after a hair-raising experience (21). And we know that play (which normally consists of exciting ‘flight or fight’ behaviors) activates the same neurochemical pathways as stress (22). So maybe young animals are using play to prime or fine-tune their own stress response.

The other very important thing we’ve learnt from the humble rat is that when they’re reared with lots of companions and interesting objects, they develop larger brains than rats that grow up in austere surroundings. These enriched rats not only have heavier cerebral cortexes, with more neural connections, they learn more quickly too.

Researchers teased apart the factors that promoted this brain growth and found that sensory stimulation and arousal (even together) couldn’t increase cortical growth unless they were coupled with interactive behavior (i.e. play or training). And it was play that had the biggest impact; in fact, the more a young rat played, the more rapidly its brain grew (23).
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