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Friday, January 10, 2014

Crocodile redemption

We all know that humans are not the only animals that use tools. Obviously, other primates do, as well as many kinds of birds and other mammals. However, we don’t usually think of crocodiles as having the brain power for tool use. Don’t they spend their days imitating logs? Turns out we may have seriously misjudged these giant reptiles. Vladimir Dinets of the University of Tennessee and his colleagues have documented tool-use in crocodilians.

Dinets observed crocodiles and alligators balancing twigs and sticks on their snouts like so:

Mugger crocodile (Crocodylus palustris) at Madras Crocodile Bank, Tamil Nadu, India, with sticks on its head.
Photo by Vladinir Dinets, from Dinets et al. (2013).

The animals were careful to maintain the sticks in position even while moving. Why?

Some wading birds, notably egrets, like to nest in trees above crocodile  or alligator ponds. Presumably, the crocodiles discourage other predators from entering the area. While the birds do lose the occasional hapless chick that topples out of its nest, the benefits seem to outweigh the losses.

During nesting season, the egrets are in great need for building materials like sticks and twigs. Although those things literally grow on trees, they aren’t shed in the numbers that nesting birds require. This means that, for a while each year, loose sticks are a hot commodity. 

Naturally, any bird desperate to complete its nest would find sticks that are just floating in a pool too tempting to pass up. With the following result:

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

It appears that the crocodiles are using the sticks as lures. 

Before you skeptically (and rightly so) note that the crocodiles could just happen to surface under some loose sticks, I’d like to add two bits of evidence. First, during nesting season there are effectively no loose sticks lying around. At the very least, the crocodiles would have had to actively seek them out. Second, the crocodiles only collect sticks on their noses during nesting season. And of course, egrets have no interest in sticks when it isn’t nesting season.

This really looks like tool-use to me.

You can read more about this at Tetrapod Zoology.

V. Dinets, J.C. Brueggen, & J.D. Brueggen (2013). Crocodilians use tools for hunting Ethology, Ecology & Evolution DOI: 10.1080/03949370.2013.858276.

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