AS I write, my kitten is having a funny 5 minutes – or rather a funny 2 hours and counting. A toy mouse has been thoroughly tortured and my laptop keyboard co-opted for a tap-dancing session.
It seems obvious to me that Peggy is playing, and that she is enjoying herself. We are used to the idea of certain warm-blooded creatures, especially our pets, larking around. But what about a crocodile toying with a ball or Komodo dragons playing tug of war with their keepers seemingly for the hell of it?
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It could be that these animals really are playing – or that we are projecting our own playful nature onto their behaviour. “There’s lots of anecdotal little stories out there,” says Gordon Burghardt at the University of Tennessee, Knoxville. In the past, “without photographic or film evidence, they could be easily dismissed by scientists”, he says. Today, though, evidence of play in unexpected creatures is building. Biologists have reported examples from the furthest reaches of the animal kingdom: not just primates and house pets, but reptiles, fish, octopuses and even spiders and wasps. Play isn’t universal, says Burghardt, but it is more common than was once thought. So why is it beneficial to spend time mucking about?
To answer this, we need to understand how and why the capacity to play evolved. A key problem is how to define play – especially as we can’t ask animals if they are having fun. Many thinkers have tackled this question, and Burghardt has attempted a . He devised five criteria for whether a behaviour counts as play. The action shouldn’t achieve anything, at least not immediately: my kitten may attack her toy mouse, but she doesn’t get any nutrition from it. It also should be voluntary, perhaps because it is enjoyable, and it needs to be different from the functional equivalent: when animals play-fight, they either don’t bite or only do so gently. What’s more, the behaviour must be repeated, and it only counts as play if animals do it when they are relaxed and sated, not hungry or fearful. “All five criteria needed to be met before I was confident that the behaviour we were seeing was play,” says Burghardt.
This is a good definition, but assessing whether something is play or not still requires skilled judgement, says Isabel Behncke at the University for Development in Santiago, Chile. It is crucial to know how the animal normally acts, otherwise behaviours with real but obscure functions might be misinterpreted as play. Nevertheless, among vertebrate animals, there are now many unambiguous instances of play – and new ones keep being discovered, such as the play-fighting of . Among birds, black swans have been observed repeatedly riding the crests of waves, like human surfers, while .
But it isn’t just warm-blooded mammals and birds that appear to enjoy mucking about. Perhaps the clearest example is the Komodo dragon, the largest lizard in the world. In zoos, they regularly play tug of war with their keepers over plastic rings. , because when the “toy” is covered in blood or tasty oil, the playfulness is replaced by possessiveness. Crocodiles, too, seem to enjoy playing. In 2015, Vladimir Dinets, then at the University of Tennesse, Knoxville, , including crocodiles repeatedly surfing down streams of water, toying with pink flowers and knocking balls around.
Most bizarre of all is a group of thick-toed geckos that were aboard the 2013 Russian BION-M1 space experiment in zero gravity. One of them wriggled out of its identity collar and the geckos proceeded to repeatedly nudge the floating object, whereas mealworms that drifted by were treated mainly with indifference. One individual was particularly playful. “We saw how one of the geckos, with an aimed push of the snout, throws the collar into a hole in the shelter, like a basketball into a basket,” says Victoria Gulimova at the Research Institute of Human Morphology in Moscow.
In 2014, Burghardt and his colleagues even described an instance of , causing it to tip over and then right itself.
Still, vertebrates are only a small part of the animal kingdom. What about all the creatures that don’t have backbones? We often think of inverterbrates as being simpler or more primitive than vertebrates, but this is a gross oversimplification – and evidence for invertebrate play has been accumulating for two decades.
In 1999, Jennifer Mather at the University of Lethbridge in Canada reported that . In the lab, the octopuses fired water jets to make a floating bottle whizz around. In later studies, Mather found that . She has recently used her play studies as part of a for advanced cognition in octopuses and other cephalopods.
Octopuses are one thing – we know they have complex brains, albeit quite unlike ours. What about insects? In 2006, researchers led by Elisabetta Palagi at the University of Pisa in Italy . When immature, the wasps performed exaggerated versions of adult dominance behaviours, which weren’t combined with genuinely aggressive moves. They seemed to be play-fighting. “If we follow the definition [Burghardt’s five criteria],” says Palagi, “we need to speak about play for paper wasps.”
Similarly, in 2011, Burghardt and his colleagues . Juvenile male Anelosimus studiosus spiders performed mock courtship displays to juvenile females, and the pairs even engaged in mock copulations. “The females who have had that playful experience lay more eggs,” says Burghardt, and the males that played are less likely to be killed or injured during courtship when the animals are sexually mature. So although the behaviour did appear to pay off in the long run, it didn’t achieve anything at the time it was performed, so Burghardt argues that it fits his five criteria. Behncke, however, isn’t so sure about this spider behaviour. “I don’t think they play. Of course, I’m happy to be proved wrong,” she says.
According to Palagi, many researchers remain cautious about some claims of invertebrate play, but other examples have been broadly accepted. “Nobody has a problem to think an octopus can play,” she says.
What is clear is that play is scattered like confetti over the animal family tree. But just because one species does this, it doesn’t mean its closest relatives do. Behncke says this is because play doesn’t evolve for one reason, but for many. “Some forms of play train more cognitive aspects,” she says, whereas other types of play are more physical or social. That’s why there is no simple pattern to which animals do it and which don’t.
“Play enriches diversity of experience and, I think, enriches adaptation to complexity,” says Behncke. The prediction is that you will see it in species that live in more complex niches, she says, meaning environments that are highly variable or unpredictable. This can include the social environment, for animals that live in complex groups.
“If Komodo dragons and birds can play, maybe some dinosaurs did too”
For example, in a , Palagi and her colleagues showed that spotted hyenas spend a lot of time play-fighting. At first glance, this seems odd, because in hyena clans the dominant females are “extremely despotic”, as Palagi puts it. However, these females also sometimes form alliances, and she thinks play-fighting may help cement these bonds.
The findings so far suggest that play didn’t evolve just once, early in the history of animals. “If you look at the animal kingdom, very, very few species show the behaviour,” says Palagi. Instead, it seems play arose independently in different lineages. But because so few species have been studied to see if they play, there is currently no way to tell how many times play evolved.
It may well be ancient. If Komodo dragons and birds can both play, it seems possible that some of their dinosaur relatives did, too. Sixty-six million years ago, maybe Tyrannosaurus rex enjoyed tug of war – pulling with its jaws rather than those silly little arms.
