Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Tool use in crocodylians: crocodiles and alligators use sticks as lures to attract waterbirds


Mugger crocodile (Crocodylus palustris) at Madras Crocodile Bank, Tamil Nadu, India, with sticks on its head. What's going on here? Read on. Photo by Vladinir Dinets, from Dinets et al. (2013). Used with permission.

In recent years it has – I really, really hope – become better known that non-bird reptiles (turtles, lizards, snakes, crocodiles, alligators and so on) are not boring dullards, but behaviourally complex creatures that get up to all sorts of interesting things. Play behaviour, complex social interactions, gaze recognition, pair-bonding and monogamy, social hunting, speedy learning abilities and good memories have all been demonstrated across these groups. And another interesting and unexpected bit of complex behaviour has just been published. It’s so interesting that I feel compelled to write about it today. It concerns what seems to be tool use in crocodiles and alligators.

As described by Dinets et al. (2013), Mugger crocodiles Crocodylus palustris in India and American alligators Alligator mississippiensis in the USA have both been observed to lie, partially submerged, beneath egret and heron colonies with sticks balanced across their snouts. Birds approach to collect the sticks for use in nest building and… well, let’s just say that it doesn’t end well for the birds. If the crocodylians really are using the sticks as bait to attract their bird prey, this is tool use, since the sticks are objects that are being employed for a specific function.

American alligator successfully catches Snowy egret (Egretta thula) following stick-displaying behaviour. Photo by Don Specht, from Dinets et al. (2013). Used with permission.

The occurrence of sticks on the crocodylians is not random: stick-displaying behaviour was most frequently observed both in those crocodylians living at rookeries and was exclusively observed during the egret and heron nesting season, being most frequent in late March and April (when the egrets and herons are working hard to find sticks) (Dinets et al. 2013).

The possibility that stick-displaying behaviour results from a random association between rookery-frequenting crocodylians and floating sticks was deemed unlikely by the authors, since floating sticks are extremely rare in the pools concerned, especially at the time of year concerned (partly this is because the local trees – baldcypresses and water tupelos – don’t shed twigs, but also because the nesting birds rapidly remove floating sticks for nest-building). Therefore, deliberate collection and employment of sticks by the crocodylians seems most likely (Dinets et al. 2013): it seems that they are practising baiting behaviour, whereby predators use objects in order to get potential prey to closely approach and hence become easier to catch. Even better, they are seemingly only practicing this baiting behaviour during a specific part of the year.

Green heron using bread as bait to attract fish. From Guido Trombetta's SeaWayBLOG.

Baiting behaviour is already well known for archosaurs. It’s frequently practised by Green herons Butorides virescens: they use feathers, twigs and even berries and bits of bread to attract fish (Norris 1975, Boswall 1983, Walsh et al. 1985, Robinson 1994) [adjacent photo from this article at SeaWayBLOG]. Burrowing owls Athene cunicularia use mammal dung to attract dung beetles (Levey et al. 2004) and gulls of at least two species have been seen using bait to attract finches fish (Henry & Aznar 2006). And it should be noted that this is not the first mention of what seems to be baiting in crocodylians, since Shumaker et al. (2011) anecdotally reported cases in which Saltwater crocodiles C. porosus seemingly used fish fragments to attract birds. What next? Needless to say, we need to be somewhat sceptical about the claimed presence of this behaviour: what's needed now is for someone to catch it on film and hence demonstrate the behaviour in action. Of course, now that people know that it might occur, they'll at least be on the lookout for it.

Crocodiles sometimes do strange things. This Orinoco croc (C. intermedius) is eating leaves. Photo by John Brueggen, from Brueggen (2002), taken at St. Augustine Alligator Farm Zoological Park.

As Dinets et al. (2013) note, the apparent presence of this behaviour in two extant crocodilian species raises the possibility that it’s more widespread within the group, and even – given its presence in both crocodylians and birds – that tool use involving bait was practised by extinct archosaurs. A number of surprising and unusual bits of behaviour have been documented in extant crocodylians in recent years (several of which have been covered at Tet Zoo), including fruit eating, leaf eating, adoption of babies, the possible feeding of babies, climbing, co-operative hunting, pair-bonding and monogamy, plus it’s long been known that they have a complex, sophisticated repertoire of vocal and postural communicative signals. These all show that crocodiles, alligators and gharials are complex, adaptable beasts that do many things that we might not consider likely had they not been documented. What's next? Stay tuned...

Many thanks to Vladimir Dinets for his help. For previous Tet Zoo articles on behaviour in living crocodylians, see…

For for other Tet Zoo articles on crocodiles, see...

Refs - -

Boswall, J. 1983. Tool-using and related behavior in birds: more notes. Avicultural Magazine 89, 94-108.

Brueggen, J. 2002. Crocodilians: fact vs. fiction. In International Union of the Conservation of Nature, Crocodile Specialist Group 1, pp. 204-210.

Dinets, V., Brueggen, J. C. & Brueggen, J. D. 2013. Crocodilians use tools for hunting. Ethology Ecology & Evolution in press

Henry, P.-Y. & Aznar, J.-C. 2006. Tool-use in Charadii: active bait-fishing by a Herring gull. Waterbirds 29, 233-234.

Levey, D. J., Duncan, R. S. & Levins, C. F. 2004. Use of dung as a tool by burrowing owls. Nature 431, 39.

Norris, D. 1975. Green Heron (Butorides virescens) uses feather lure for fishing. American Birds 29, 652-654.

Robinson, S. K. 1994. Use of bait and lures by Green-backed Herons in Amazonian Peru. Wilson Bulletin 106, 569-571.

Shumaker, R. W., Walkup, K. R. & Beck, B. B. 2011. Animal Tool Behavior: the Use and Manufacture of Tools by Animals. The Johns Hopkins University Press, Baltimore.

Walsh, J. F., Grunewald, J. & Grunewald, B. 1985. Green-backed Herons (Butorides striatus) possibly using a lure and using apparent bait. Journal of Ornithology 126, 439-442.

The views expressed are those of the author and are not necessarily those of Scientific American.

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