In order to study play in animals, however, we first need a clear definition of exactly what it is we're talking about. A common model used today is the one defined by Gordon Burghardt in a 2005 book on the subject, which defines play as a physical activity meeting four key criteria.
Firstly, the behaviour has to be something that the animal does of its own volition; it has to be something that the animal wants to do and isn't being forced into. Secondly, and in a similar vein, it has to be performed by healthy, unstressed individuals, rather than something it only does when (say) trapped in a bare and empty cage for days on end. Thirdly, it has to be something that the species in question performs frequently, not some one-off reaction to an unusual situation - although there's no requirement that all members of a species should play in exactly the same way.
Finally, there has to be no obvious point to it. Play does not achieve anything concrete, either because it doesn't resemble other activities, or because, if it does, it doesn't produce the same results. For instance, a cat might play with a toy as if it were a mouse, but it's clear that it doesn't expect to get an actual edible mouse out of this once it's finished.
So why bother? One could argue that it's just a side-effect of necessary instincts - that a cat plays with a mouse toy because that's just what it does with things that are mouse-shaped. Under this argument, evolution hasn't gotten rid of play activity because, even if it isn't beneficial, there's no real downside to it, either. But it's debatable how true that really is. Playing takes up energy that could be used in looking for food or, for that matter, just relaxing so as to divert scarce resources to bodily growth. It's also potentially dangerous, in that a predator might sneak up on you while you're distracted by playing with a leaf or whatever.
So the benefits must outweigh those costs or evolution would scrap the activity and let animals focus on something more useful. There have been several studies to try and identify what those benefits might be, and the findings are, perhaps, largely what you might expect. Play is more common in young animals than adults and it's likely that it promotes motor or social skills that may be useful later in life. For example, if you play at fighting as a juvenile, you'll probably be better at the real thing when it matters, both physically and psychologically. One study, for instance, found that wild horses were more likely to survive to adulthood and be physically fitter if they had more opportunities to play when they were foals.
It has also been proposed that play helps develop an animal's temperament - what we'd more likely call 'personality' in humans. The argument here is that it refines a young animal's responses so that it is better able to react appropriately to unexpected situations as an adult. In this sense, we can define at least three ways in which personality might vary among animals. Loosely speaking, we can describe these as bravery, curiosity, and sociability. Depending on the exact circumstance, different levels of each of these three traits may be beneficial. For instance, bravery may be helpful sometimes, but caution and shyness more so at others.
Belding's ground squirrel (Urocitellus beldingi) is common in and around the Great Basin of the western US. They live in inland grassy regions of California and Oregon, in southern Idaho, northern Nevada, and a small corner of northwestern Utah. They spend about nine months of each year hibernating, mating soon after they wake and giving birth 25 days later. The young emerge from the mother's burrow at around four weeks old and spend the first few hours of each morning in rough-and-tumble play with one another before exploring the outside world. Most of this play activity takes place during their first week above ground, tailing off in the second week as they begin to travel further afield, and essentially stops after that.
A study in 2017 showed that the baby squirrels which played the most during these two weeks were bolder and more curious in unfamiliar but apparently safe environments, spending more time exploring and thus finding it easier to escape from a test site and return home. This is, however, not the same dimension on the temperament scale as bravery/shyness, which is a measure of how they react in the face of a potential threat. So a new study decided to expand on the previous work by measuring how easily frightened the young squirrels were.
Obviously, we don't want to actually put baby squirrels in danger (or no more danger than that posed by the real world of nature). So, instead, the researchers measured the squirrels' bravery by simply walking up to them while they were playing. The question being: how close did they have to get before the squirrels would run away?
The result was broadly consistent with the previous one; the young squirrels that played the most in the mornings were the most likely to run away when approached by a potentially hostile human. This apparently wasn't the case on the first couple of days after they emerged from the burrows, which the researchers suggest means that it's not an innate aspect of their personality - that is, that the most nervous squirrels are the ones who play the most with their fellows. Similarly, it wasn't that they were any better at spotting intruders, since they started perking up and paying attention at about the same distance regardless, they just let the human get closer before making a run for it.
Either the act of engaging in social play rewires their young brains to make them more cautious around humans (and presumably other animals) or there's some third factor that makes them both play more and become cautious. For example, their life experiences while exploring in the afternoon might affect how they react both to situations and to each other the following morning. The study also showed that squirrels born to first-time mothers were more cautious than those with older mothers, perhaps due to the quality of the parental care they had received before leaving the burrow for the first time.
There is some evidence that play in young mammals reshapes the neural pathways in their brain, much as other experiences might, and this can enable them to make better judgements when they respond to new situations as they age. In this case, it seems that social play both makes the squirrels more curious and willing to explore the outside world and tempers that with an understandable caution about the potential risks they might face as a result.
Regardless of whether it's the ultimate cause of this behaviour or there's something else that happens first, playing in these squirrels is helping them learn to develop a very sensible attitude to the world around them.
[Photo by V.J. Anderson, from Wikimedia Commons.]
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