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Art by Scott Hartman, distributed under a Creative Commons Attribution 2.5 Generic license.

What Do Croc Tools Mean For Dinosaur Innovation?

Alligators, crocodiles, and gharials are exceptional ambush predators. Carrying on a tradition of wait-and-strike that has worked for them since the Mesozoic, living crocodylians can watch from the water’s surface with little more than their eyes and nostrils breaking the surface of the water. But even such effective camouflage isn’t always enough to sneak up on potential meals. Independent of each other, American alligators and mugger crocodiles appear to have learned to use lures to entice avian prey.

Psychologist Vladimir Dinets and coauthors described the sneaky behavior late last year in an Ethology Ecology & Evolution paper. Silly as it may sound, mugger crocodiles at India’s Madras Crocodile Bank and American alligators at the St. Augustine Alligator Farm Zoological Park balanced sticks on their snouts to invite nesting egrets to come within chomping range. The twigs do little to hide the tough-skinned archosaurs beneath, but birds in need of building materials will sometimes risk being ingested to grab a stick from the crocodylians’ snouts.

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A mugger crocodile “stick-displaying.” From Dinets et al., 2013.

This is a rare report of tool use in reptiles, although not everyone is sold on that conclusion. Given that reptiles have traditionally been thought to be slow, stupid, and generally boring, the bar is set high for confirming behaviors as complex as intentionally using lures to snaffle up prey. But let’s say that Dinets and colleagues are right and crocodylians truly are capable of selecting and using tools. According to the study authors, this opens up the possibility that some of the late, great non-avian dinosaurs could have been tool-users, too.

An American alligator and the cattle egret in its stomach form an extant phylogenetic bracket. Birds are dinosaurs and crocodylians are the closest living relatives to the dinosaur group as a whole, and so characteristics shared by both lineages may have been present in the last common ancestor of both groups as well as extinct descendants of that common ancestor. Non-avian dinosaurs, from Triceratops to Tyrannosaurus, fall within this bracket, leading Dinets and coauthors to leave their paper with the coy line “Phylogenetic bracketing by birds and crocodilians suggests that the behavior of non-avian dinosaurs was most likely very complex as well.”

But that’s too simple an argument. The way mugger crocodiles use sticks as traps is quite different from the way birds such as the stunningly smart New Caledonian crows use tools to tackle a variety of tasks. Extant phylogenetic bracketing can work for anatomical features or biochemical characteristics than can be shown to be the same in two distantly-related animals, but behavior is much more malleable and prone to independent evolution of similar habits. Evolutionary bracketing alone can’t draw us to visions of Allosaurus draping ferns over its snout to lure unwary little Diplodocus or feathery little Troodon using sticks to dig insects out of their underground hiding places.

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While they had very small brains, even large dinosaurs – such as this Diplodocus – may not have been as dumb as often thought. Photo by Brian Switek.

Nevertheless, clever crocodylians have opened the possibility of tool-using non-avian dinosaurs in a different way. By dint of their ecotothermic metabolism and relatively smaller brain size, reptiles have often thought to be dumb animals. A surge of interest has shown that this old understanding is wrong. Reptiles are more socially complex and smarter than expected, and the possible discovery of tool-use in crocodylians indicates that even non-avian dinosaurs, oft-ridiculed as small-brained, may have had the mental capabilities to use tools.

We can’t observe non-avian dinosaurs directly. We’re 66 million years too late for that. As far as intelligence goes, all we have left are the internal molds and in-filled casts of their brains. Matched with estimates of body mass, what remains of dinosaur brains have been used to derive an encephalization quotient that in turn acts as a rough proxy for behavioral complexity and intelligence. A 50 foot long sauropod with the brain the size of a walnut probably wasn’t as bright as a roughly human-sized troodontid with a brain as large as an ostrich’s.

Still, such studies have shown that non-avian dinosaurs were not as painfully small-brained as early paleontologists thought. Many non-avian dinosaurs had a relationship between brain and body size similar to living reptiles, including crocodylians, and some of the brainier dinosaurs were more similar to birds. If non-avian dinosaurs were generally on par with living reptiles, and living reptiles are smarter than expected, then it’s not unreasonable to wonder about tool-using dinosaurs.

But how would we know? If non-avian dinosaurs used tools, they would have made use of the materials around them – sticks, leaves, stones. We wouldn’t know a dinosaur tool even if we found one. Perhaps trace fossils could be of assistance – little insects pierced with a particular sort of twig, or scratches in the ground made by sticks rather than claws – but researchers would have to recognize them and find enough of a sample to support the weight of an exceptional hypothesis. We may never know whether any non-avian dinosaurs were innovative animals. I hope the fossil record proves me wrong.

References:

Buchholtz, E. “Dinosaur paleoneurology,” in The Complete Dinosaur, 2nd ed., eds. Brett-Surman, M., Holtz, T., and Farlow, J. Bloomington: Indiana University Press. pp. 191-208

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

Jerison, H. 1969. Brain evolution and dinosaur brains. The American Naturalist. 103, 934: 575-588