National Geographic

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.

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

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.

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.

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

There are 7 Comments. Add Yours.

  1. Richard Dashnau
    January 9, 2014

    This rather limited study is certainly getting a lot of attention. I volunteer at a 5000 acre park with a free (non-managed) population of many kinds of animals–including alligators (approx 300 breeding size plus youngers ones living freely in 7 lakes). In that time I have observed and filmed alligators doing many amazing things. In fact, I’ve also observed and filmed alligators with all kinds of “junk” on their snouts and backs; usually a product of having surfaced under said “junk” (sticks of various sizes, plants, algae). But I haven’t seen(and have yet to hear of) any report of an alligator baiting any prey with twigs. I (and the other park staff) interact with thousands of people each year (250,000 visited the park last year). Many visitors and staff have stories to share. No stories feature alligators baiting with sticks. Alligators are amazingly complex creatures; and have many fascinating hunting behaviors. It will be interesting if any further study is done on this subject, but it seems premature at this point to say that they “use tools” and then try to postulate such behavior in distant relatives.

  2. Ethan Cowgill
    January 11, 2014

    The thought of non avian dinosaurs using tools is an entertaining one to say the least. If they did use tools, then it just might mean that therapods were somehow cooler than they already are.

  3. Dan Peterson
    January 13, 2014

    To Richard, the difference may be that in the places where this activitity was observed were in close proximity to wading bird rookeries. Unlike these parks where the behavior was reliably reported, perhaps people cannot normally get close enough to the rookeries (if there even are any in your park), to observe the activity. The key point of the paper was that the crocodilians only lured near nesting colonies where there was the greatest likelihood birds would be searching for sticks.

    I agree with the authors that crocodilians are more complex and intelligent than most people realize. For ten years I shared an apartment with a Nile Crocodile that pretty much had the run of the house. He knew that his food was kept in the refrigerator, and if he was hungry he would scratch the floor of his pool with a hind foot when he heard the refrigerator door open, to remind me to get something out for him.

    Then there is the legendary Gustave, who has allegedly captured and consumed literally hundreds of supposedly far more intelligent humans and is still out there.

  4. chris y
    January 14, 2014

    Advanced Tyrannosaurids and Abelisaurids had clearly been using prosthetic forelimbs for long enough for their own to become vestigial. All is explained!

  5. HERMAN DIAZ
    January 19, 2014

    The discovery of tool-using crocs may be the most interesting news I’ve heard in a while (& yes, that includes the discovery of new Deinocheirus material). Among other things, it opens up the possibility of tool-use in non-maniraptoran dinos. I already figured that at least some non-avian maniraptorans practiced some kind of tool use (hence my “Anting Alvarezsaurid”: http://dinotoyblog.com/forum/index.php/topic,1720.0.html ). Maybe someone w/actual artistic talent (I.e. Not me) should draw a spinosaur practicing baiting behavior. Now that I think of it, maybe someone should also draw a deinonychosaur using rocks to break open large eggs.

  6. Simon Evans
    January 22, 2014

    re: ‘The twigs do little to hide the tough-skinned archosaurs beneath, ‘ I don’ not agree, I think they make the Alligator look like some floating tree trunk, not least because the colour and texture of the creature matches that of the twigs and such flotsam closely. On the other hand as the first post suggests, this may have been inadvertent, and not intentional on behalf of the beast, in that it just happened to surface where these broken pieces of bocage happened to be floating.

  7. george hancock
    January 24, 2014

    In reply to richard.

    This behaviour would appear to be specific to certain colonies and locations. It likely that olny select groups of alligators have learnt this or only certain individuals of the species. This behaviour also seems to take place only outside of large nesting sites for ardeidae(herons) or other water birds.

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