One frequent criticism of evolutionary explanations of natural adaptations is that, while the adaptations themselves may be observable, the proposed reasons for their selection are often speculative, what critics like to call “just so stories.”
This criticism is leveled most strongly at attempts to explain the evolutionary advantage of behaviour. It’s generally easier to identify the adaptational advantage of a body part than it is to guess the purpose of an activity.
A recent guest blog on the website of Scientific American magazine highlights the problem by looking at one of the most intractable of behavioural questions: Why do animals play?
South African animal behaviourist Lynda Sharpe has spent the last fifteen years studying the social interactions of habituated mongooses and meerkats. After all of this work, her answer is that we really don’t know why animals play.
Two answers have long dominated speculation about why animals play. One answer is that animals play to practice, to hone skills that will be important to them as adults. Those young animals which play most, and play best, will survive in greater numbers. The other answer is that social play reduces aggression and/or increases bonding, making mating or pack formation easier.
These are the most popular answers, but in studies with a number of mammals the results simply don’t support the expectations. There is no clear evidence that either practice or socialization increases the survival rate of young animals or the number of later offspring.
Sharpe reports studies of young grasshopper mice, coyote pups, and kittens. None of these studies shows any relationship between youthful play and later success catching prey. Her own studies of meerkats found no correlation between early play and later pack dominance, suggesting that play does not hone the battle skills needed by males who grow up to compete for mates. Sharpe also found that early play had no effect on adult aggression or on social bonding (adults were as likely to clash with former frequent playmates as with those with whom they had seldom played).
But there are studies — with ground squirrels, feral horses, and brown bears — that show that young animals that were active players were more likely to survive or to produce healthy offspring in their first breeding season. With these animals, at least, something seems to be going on.
The key question, Sharpe writes, isn’t what but how. How does play help some animals live to maturity and exhibit reproductive success?
Studies with the common lab rat suggest that play has a homeostatic function. Rats are very playful, apparently, and rats that are allowed to play with others exhibit less tension, showing less fear flight as well as less fear aggression, than rats which are prevented from playing. Sharpe explains:
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… . And we know that play (which normally consists of exciting ‘flight or fight’ behaviors) activates the same neurochemical pathways as stress… . So maybe young animals are using play to prime or fine-tune their own stress response.
There is some evidence that similar responses happen in primates, including us. But there is another consequence which may be even more important to humans:
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.
There is as yet no evidence tying human brain growth to play history, but the animal studies point the way to an area of potentially fruitful research.
While those who don’t like extending evolutionary explanations to complex social behaviours like interactive play will latch onto the uncertainties, the more important point of Sharpe’s summary may be that a neurological preference for being social may not only be hardwired into the brains of some animals, including us, but the resulting behaviours may contribute to the growth of the capacity for the social instinct itself.
That is, there may be some evidence for a mutually reinforcing system, where social play fosters a cognitive environment in which the brain develops more sophisticated ways of being social. The more social we are, the more socially oriented we become, and the larger our brains become to manage our social tendencies.
Those of us who believe that, more than anything else, we are hardwired as social animals won’t be at all surprised if this reinforcing pattern is shown to be the case.