Thursday, June 22, 2006

Dinosaurs come out to play

As a kid I always got the impression from textbooks that the only tetrapods (and thus only animals) that engage in play behaviour are (1) mammals and (2) a few really smart birds, like corvids and some parrots. Raptors are also known to engage in play behaviour, with it being relatively well documented that adults will drop feathers in front of their flying juveniles. The juveniles catch the feathers as if they’re pretend prey.

But it would seem that play behaviour is not allowed to occur in lissamphibians, non-avian reptiles, or the majority of birds. They just don’t do it, or at least no one has ever recorded them doing it. So why do mammals and oh-so-clever corvids and parrots, and predatory raptors, play, and why do other tetrapods not? Maybe so-called ‘higher tetrapods’ engage in play behaviour because full-blown endothermy allows this sort of superfluous, energy-wasting behaviour; maybe it’s a result of enhanced encephalisation; or maybe it’s only possible if extensive parental care allows juveniles enough behavioural ‘security’ to indulge in carefree behaviour.

Well, here’s the news. All of the above is crap. You might be surprised to hear that play behaviour is far from unique to mammals and a minority of birds, but has also been documented in turtles, lizards, crocodilians and even lissamphibians and fish (Bekoff 2000, Burghardt 2005). But because the reports discussing or mentioning play behaviour in these animals have been mostly anecdotal, and hence only mentioned as brief asides in larger behavioural studies or in brief one-page notes published in obscure journals, they have largely gone overlooked until recently.

Hold on: play behaviour in reptiles, amphibians and fish? Before looking at this further we need to sort out exactly what ‘play’ really is. How can it be defined? Of course this is something that ethologists have been arguing about for decades, and lengthy papers and virtually entire books (see Smith 1984 and Bekoff & Byers 1998) have been devoted to this topic alone. A rough working definition of play might be: a repeated behaviour, lacking an obvious function, initiated voluntarily when the animal is unstressed.

Most play behaviour – namely that observed in mammals and the more intelligent birds – is easily recognized by us because it resembles the sort of activities that we ourselves already recognize as playful. But this creates the obvious problem that play behaviour in other animals might be difficult to recognize because it is rather different from the sort of behaviours we ‘expect’ to represent play. Juvenile mammals tend to employ obvious honest signals when they’re playing: we’re all familiar with the ‘play face’ and bow-like action that canids (wild and domestic) use to initiate play, for example, and the play behaviour that they indulge in – chasing, play-biting, tussling and role-reversing – recalls human play behaviour.

However, if we employ the rough definition used above, behaviours reported widely among tetrapods can be seen in a new light. It turns out that several non-mammalian, non-avian vertebrates engage in repeated, apparently functionless behaviour that is initiated voluntarily in unstressed individuals. Sometimes this behaviour is directed toward inanimate objects (so-called manipulative play or object play).

Most of the key research in this area has been produced by Gordon M. Burghardt (his website is here), and if you’re interested in his research it’s worth checking out his new book (Burghardt 2005). There’s stuff here about apparent play behaviour in fish and – shock horror – even, outside of vertebrates, in cephalopods. I’m particularly interested in the play behaviour that’s now been documented in captive trionychid and emydid turtles (Burghardt 1998, Burghardt et al. 1996, Kramer & Burghardt 1998).

Thinking about this reminded me of an activity indulged in by one of the Red-eared sliders Trachemys scripta we used to have in my UOP office. One of the terrapins used to regularly remove the plastic hose from the filter box in its tank, and then nudge the filter box around the tank. This was irritating as we (we = myself and Sarah Fielding) had to keep repositioning the box and reconnecting the hose. I honestly didn’t think at the time that this behaviour ‘meant’ anything, but I’m wondering now if it was a form of play. Certainly those animals were bored with nothing to do in their little tank, so maybe they were in need of behavioural enrichment, and hence searching for objects to manipulate.

By introducing objects like wooden blocks and chains into enclosures, Burghardt and colleagues noted exactly this occurring in turtles, crocodilians and lizards. An Orinoco crocodile Crocodylus intermedius rated particularly high in terms of its response to the objects, and appeared to exhibit both curiosity and playfulness toward them. There’s also a published account of an American alligator Alligator mississippiensis exhibiting playful behaviour directed at dripping water (Lazell & Spitzer 1977), and there are also accounts of crocodilians possibly playing with carcasses, and apparently surfing in waves (go here for more on these accounts). I’ve seen a short sequence of film of two sibling Nile crocodiles Crocodylus niloticus tussling with one another in what looked like play behaviour.

The best data however comes from monitor lizards, and in fact from one individual monitor lizard in particular. Kraken is a well-studied female Komodo dragon Varanus komodoensis kept at the Smithsonian National Zoological Park in Washington, D. C. Developing a close bond with her keepers, it began to be noticed that she directed an unusual amount of curiosity toward shoe laces and to objects concealed in people’s pockets (such as handkerchiefs and notebooks). Kraken would tug at or sever shoe laces (with her teeth), and would gently pull objects out of people’s pockets. The keepers then began to introduce boxes, blankets, shoes and Frisbees into Kraken’s enclosure, and many of Kraken’s reactions would be interpreted as playful if witnessed in a mammal. Kraken has also been recorded to play tug-of-war with her keepers.

In a detailed, thorough study of Kraken’s interactions with objects and her keepers, Burghardt et al. (2002) concluded that play-like behaviour in Komodo dragons definitely meets the formal criteria for play: ‘Kraken could discriminate between prey and non-prey and showed varying responses with different objects (i.e., ring and shoe). Large lizards, such as the Komodo dragon, might be revealed as investigative creatures, and further expressions of play-type behaviors should be confirmed and explored. These findings would imply that non-avian reptiles in general and large long-lived species in particular are capable of higher cognition and are much more complex than previously thought’ (p. 116). It’s interesting to note that probable play behaviour was reported in Komodo dragons as early as 1928, incidentally. Other people have now documented play behaviour in captive monitors: for an article devoted to this go here.

So – if you’ll excuse me here for bringing in some vertebrate palaeontology - did non-avian dinosaurs play? Several authors have speculated about this, but only in fictional essays: Stout & Service (1981) depicted baby tyrannosaurs chasing, wrestling and play-biting one another, and Bakker (1995) imagined dromaeosaurids and troodontids sliding down snowy slopes in a Cretaceous winter (which explains the Luis Rey painting you can see here). Of course we don’t know whether dinosaurs played, and we never will, but given how widespread play behaviour is in living reptiles, phylogenetic bracketing indicates that at least some extinct dinosaurs almost certainly would have engaged in this. So, artists, feel free to depict baby dromaeosaurs running around with feather or stick toys in their mouths.

And, finally, here is the proof showing the tyrannosaurs really did play with micro-machines (and for details on the photo used above, go here).

PS - for the latest news on Tetrapod Zoology do go here.

Refs - -

Bakker, R. T. 1995. Raptor Red. Bantam Press, London.

Bekoff, M. 2000. The essential joys of play. BBC Wildlife 18 (8), 46-53.

Burghardt, G. M. 1984. On the origins of play. In Smith, P. K. (ed). Play in Animals and Humans. Basil Blackwell, Oxford, pp. 5-41.

- . 1998. The evolutionary origins of play revisited: lessons from turtles. In Bekoff, M. & Byers, J. A. (eds). Animal Play: Evolutionary, Comparative, and Ecological Perspectives. Cambridge University Press, Cambridge, pp. 1-26.

- . 2005. The Genesis of Animal Play: Testing the Limits. MIT Press, Cambridge, MA.

- ., Chiszar, D., Murphy, J. B., Romano, J., Walsh, T. & Manrod, J. 2002. Behavioral complexity, behavioral development, and play. In Murphy, J. B., Ciofi, C., de La Panouse, C. & Walsh, T. (eds) Komodo Dragons: Biology and Conservation. Smithosonian Institution Press (Washington, DC), pp. 78-117.

- ., Ward, B. & Rosscoe, R. 1996. Problem of reptile play: environmental enrichment and play behavior in a captive Nile soft-shelled turtle, Trionyx tringuis. Zoo Biology 15, 223-238.

Kramer, M. & Burghardt, G. M. 1998. Precocious courtship and play in emydid turtles, Ethology 104, 38-56.

Lazell, J. D. & Spitzer, N. C. 1977. Apparent play behavior in an American alligator. Copeia 1977, 188-189.

Smith, P. K. 1984. Play in Animals and Humans. Basil Blackwell, Oxford.

Stout, W. & Service, W. 1981. The Dinosaurs. Bantam Books, New York.


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