Thursday, May 18, 2006

Snapping turtles, part III: bite, lunge, lure and snap

Pterosaurs and dinosaurs have taken up all of my research time within the last few days. The phylogeny and taxonomy of azhdarchoids, the obscure sauropods of the Kimmeridge Clay Formation, and the theropods of the Brazilian Santana Formation, among others. I will post about all of these things when the time is right. As I noted here a while back, one of the recurrent themes in my blog posts will be turtles. I still have to write up the stuff on Cuthbert and the other departmental turtles, on J-Lo the araripemydid, and on Branston the pickled Pelomedusa, and I also have posts planned on the turtles that time forgot (viz, Mesozoic-grade taxa that made it well into the Cenozoic) and on the many newly discovered geoemydids that have recently been named from China.

Forgive me if it seems like dinosaurs are the things I post about the least but, well, while I find them fascinating, I honestly don’t find them more fascinating than all the other types of tetrapods. Larks, fringe-toed lizards, tree frogs, okapis and sewer rats really are AS INTERESTING as dinosaurs, and I relish telling this to those psychologists who wonder why some of us humans are fascinated by long-extinct Mesozoic reptiles.

In two previous turtle posts I discussed chelydrids, the snapping turtles. But a third post was planned, and here it is. Those two previous posts covered, respectively, general stuff about taxonomy, evolution and body size (They bite, they grow to huge sizes, they locate human corpses: the snapping turtles, part I), and physiology and distribution (Snapping turtles, part II: hyperexcitability, supercooling and recolonisation of Europe in the Anthropocene).

What perhaps makes chelydrids most interesting is what they eat, or at least what they bite. ‘Large individuals are known to have caused injury to people unwary enough to step into or swim in the water near them, and they are quite capable of removing a toe or finger if given the opportunity’ (Alderton 1988, p. 112). Indeed, they are anecdotally credited with being able to ‘bite through a broom handle’. For his excellent TV series ‘O’Shea’s Dangerous Reptiles’, Mark O’Shea decided to test this dubious assertion. With the help of a colleague he caught the largest and nastiest alligator snappers he could, pissed them off by poking them, and then pissed them off some more by shoving a broom handle into their mouths. All the turtles bit happily, and bit hard. And as impressive as it was, sad to say not one turtle was powerful enough to cleave neatly through 25 thick mm of solid wood, which to be honest isn’t much of a surprise.

Rather more rigorous tests were applied by Herrel et al. (2002) who tested the bite force of numerous diverse turtles, including both snappers and alligator snappers. They found snappers to have bite forces of between 208 and 226 Newtons, and alligator snappers of between 158 and 176 N.

This powerful bite can have its practical uses. Zoo vet David Taylor, one of the best sources for zoological anecdotes (think tapir-eating hippos and a zebra with a tooth embedded in one of its testicles), tells the story in one of his books of how an alligator snapper in Indiana was used by its Native American owner to retrieve lost human corpses. The turtle (which was kept on a long leash) was taken to the lake where the person was missing, and released. Eventually its owner would know from the pull on the leash that the turtle was tugging around on a carcass, somewhere on the bottom. It would then be pulled slowly to the surface, and it would successfully bring the corpse with it. I couldn’t find the relevant Taylor book that relates this tale, but I did find a version of the story in Alderton (1988).

Ordinarily snapping turtles don’t feed on broom handles or human corpses of course. They are sit-and-wait predators that lunge with great speed at passing objects, but they also consume static objects like molluscs and some plants. Chelydra likes to skulk in aquatic vegetation, though it also floats near the water surface. Aquatic insects, crustaceans, worms and fish are eaten, and it also preys on a diversity of tetrapods. These include frogs and toads, salamanders, snakes, smaller turtles, birds and small mammals. As mentioned, they also eat various plants, including Canadian pondweed and parts of water lilies, and they are also reported to eat snails, clams and freshwater sponges. This interests me, as if they eat plants and clams and sponges then they obviously don’t regard moving objects only as potential prey. Do they use olfaction to determine whether or not these non-moving objects are edible? I don’t know, and I can’t find an answer in the literature. They are reported to consume more plant material during the warmer months, when plant growth is more abundant.

Macroclemys, the alligator snapper, is altogether different from Chelydra in terms of what it does and what it can do. As is reasonably well known, it possesses a lure-like organ on the floor of its mouth. This isn’t the tongue as sometimes stated but a specialised mobile organ attached to the tongue at mid-length. Grey at rest, it is red when active, and the turtles deploy it by sitting still with the mouth wide open, and by wriggling it. Some herpetologists have proposed that the lure is only used during the daytime when the turtles sit passively on the substrate (as opposed to the nightime, when the turtles actively forage for prey and don’t, in theory, deploy the lure), and while there are some observations recorded from captive individuals, it doesn’t seem that much is known about the use of the lure.

Like Chelydra, Macroclemys grabs crustaceans, worms, fish, frogs and snakes, and it also eats static prey like plants and clams. It is more dedicated to turtle-killing than Chelydra is, and well able to catch and kill turtles that are about as big as it is. Allen & Neill (1950) reported alligator snappers to eat Painted turtles Chrysemys picta, Chicken turtles Deirochelys reticularia, mud turtles Kinosternon, as well as other alligator snappers. In fact their predation on smaller turtles may be significant enough to keep populations of other species low in areas where alligator snappers are abundant. As suggested by that story about the corpse-finder alligator snapper, members of this species are also adept at carrion feeding, though how important this is in their natural diet is uncertain.

Unless you already know the answer (clever you), you might be wondering how slow, clumsy turtles are able to catch speedy, agile aquatic prey like fish. The feeding styles of aquatic turtles are pretty fascinating, and have been studied recently by Patrick Lemell and colleagues (see Lemell et al. 2000, 2002). Using expandable throats, rapidly opening jaws and streamlined skulls, aquatic turtles can dart their head forward, open their jaws, and engulf water and prey within – literally – a fraction of a second.

Along these lines, Lauder & Prendergast (1992) used high-speed video recording to study feeding behaviour in Chelydra. They showed that Chelydra lunges and engulfs fish prey within 78 ms, with ‘peak head extension velocities of 152.5 cm per second’. It doesn’t lunge as quickly when feeding on worms, darting its head forward at a sedate 54 cm per second in these cases, and engulfing worms within a leisurely 98 ms. In contrast to matamatas and other aquatic turtles that generate massive negative pressures and thereby employ suction to engulf prey, Chelydra is predominantly a ram-feeder that doesn’t generate negative pressure when it lunges.

I’d like to talk more about aquatic feeding behaviour in turtles, but it’ll have to wait for another post. It’s a story that involves plethodontid salamanders, placodonts and the evolution of filter-feeding.

The photo above, showing the lure on the floor of the mouth, is from here on Richard Butler’s excellent site on Oklahoman reptiles. And this isn’t the same Richard Butler who works on ornithischian dinosaurs. For the latest news on Tetrapod Zoology do go here.

Refs - -

Alderton, D. 1988. Turtles & Tortoises of the World. Blandford (London).

Allen, E. R. & Neill, W. T. 1950. The alligator snapping turtle, Macrochelys temminckii, in Florida. Special Publication of Ross Allen’s Reptile Institute 4, 1-15.

Herrel, A., O’Reilly, J. C. & Richmond, A. M. 2002. Evolution of bite performance in turtles. Journal of Evolutionary Biology 15, 1083-1094.

Lauder, G. V. & Prendergast, T. 1992. Kinematics of aquatic prey capture in the snapping turtle Chelydra serpentina. Journal of Experimental Biology 164, 55-78.

Lemell, P., Beisser, C. J. & Weisgram, J. 2000. Morphology and function of the feeding apparatus of Pelusios castaneus (Chelonia; Pleurodira). Journal of Morphology 244, 127-135.

- ., Lemell, C., Snelderwaard, P., Gumpenberger, M., Wochesländer, R. & Weisgram, J. 2002. Feeding patterns of Chelus fimbriatus (Pleurodira: Chelidae). The Journal of Evolutionary Biology 205, 1495-1506.


Blogger Dr. Vector said...

Regarding the tongue-lure business...

For a few years I had a pet snapper (Chelydra serptentina), and it would sometimes lure fish by wagging its non-lure-equipped tongue. I wonder if this behavior is apomorphic for chelydrids, or if it might be more broadly distributed in turtles. Presumably the wiggling tongue behavior evolved in chelydrids before the specialized lure organ appeared in Macroclemys.

Loved your comments about finding all tetrapods interesting, not just dinosaurs. I find that it is impossible for me to learn very much about a group without becoming very interested in its members. I started working on dinosaurs because I was more interested in them than in anything else (except maybe turtles). Having finally gotten to the point where I'm interested in everything, I find that dinosaurs are the only group I know enough about to work on. Who knows, that may change also.

Keep up the grotesquely fascinating work.


9:35 PM  
Anonymous Richard Butler said...

Sounds like you have a pretty busy and fascinating life. Good luck with your doctorate!

Richard Butler

12:10 AM  

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