Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 29, Number 11;   November, 2002

Young Investi-'gator' Discovers Ancient Sensory System on Snouts of Crocodilians.

by Katrina MacLeod and Catherine Carr

Reprinted from the newsletter of the Minnesota Herpetoogical Society, Vol.22, No.8, August 2002.

A trip that began as a collecting excursion for young alligators, as part of her graduate work on the auditory system in birds and crocodillians, and curiosity led the University of Maryland student to the discovery of the function and natural history of a novel sensory system, "dome pressure receptors," she says, two years later. These receptors, as Daphne showed in a paper published recently in Nature ("An Ancient Sensory Organ in Crocodilians", Nature, 16 May 2002, V417:241-242), appear to underlie the crocodilians' ability to detect prey as they lie in wait at water's surface, half-submerged. To put it all into context, Daphne says, "these ancient animals have really solved the problem of wearing a suit of armor, for protection, and yet still being able to be sensitive to the environment." Crocodilians, after all, survived through the age of the dinosaurs and into the age of the mammals (all the more to eat, one supposes.)

After that fateful Louisiana trip, and being the person she is, Daphne pursued the question upon her return from the swamplands, and discovered no one knew much about these organs. Some German in the last century described them, of course, but no function was known. So, in her spare time from working on her dissertation, she described the receptors with electron microscopy and light level microscopy. Using a tracer dye, she discovered the fibers running to the receptors innervated via the 5th nerve a hypertrophic trigeminal ganglion ("huge," says Daphne). Interestingly, the trigeminal nerve appears to subserve two other highly specialized modalities: the electrosensory organs in the platypus, and infrared organs in snakes. In the alligator, the trigeminal formed dense branching patterns directly under the 'bumps', which lacked pores or hairs. These nerve bundles, on their way from the skin surface to the brain, course through hundreds of holes in the jawbones, making a beehive-like pattern in the bone.

To determine the function, clearly some physiology was called for. Daphne received a Grass fellowship to work the summer of 2001 at the Woods Hole Marine Biology Laboratory, training grounds for many a prominent physiologist. With her junior alligators in a water tank and recording electrode snug around the trigeminal nerve, she began to test what stimuli would activate the receptors. After testing various stimuli like electrical current, light, "stinky things", she found that disrupting the water by placing her hand in the tank caused the nerve to fire. Ripples! Pressure waves on the water surface are what these receptors like. To test the responses she used a fancy pressure wave stimulator (aluminum beer can attached to a speaker), and found that the nerve fired in phase to the waveform, and fired with increasing probability with increasing water pressure. To determine that this was due to the receptors themselves, she used a piezoelectric "poker" to apply mechanical stimulation to the receptors directly, and the skin surface in between. Only poking the receptors elicited a response.

But did the gators respond behaviorally to such stimuli? Daphne [determined] that little hungry gators [will] turn toward the source of a surface disruption caused by as little as a single drop of water, as if hunting. In complete darkness, and with their ears filled with vaseline, she found that the gators would turn and bite at the exact location of the odorless water drop, with less than 10% error rate over nearly the entire testable tank space. These animals were accurate in both direction and range, and, remarkably, could perform the task even with considerable continuous random surface disruption noise. "They're really good at this," she says. She showed that the dome pressure receptors are necessary for the behavior: gators with beauty mask goo covering their receptors failed to lunge at the drops. Furthermore, only gators sitting at the air-water inter face would perform the task. (Submerged animals and lazy ones sitting on rocks out of the water do not.)

When did alligators and crocodiles evolve such an elegant solution to "combining armour with tactile sensitivity?" By looking for telltale foramina patterning (the beehive in the jaw bones through which the nerve bundles pass) in the skull bones of various species, one can determine who had these receptors and who didn't, including species now extinct. Among living reptiles, all water-dwelling crocodilian species had the pattern, while terrrestrial crocodilian species and lizards did not. Evidence from at fossil specimens of extinct crocodilia show that the modern pattern goes back into the Early Jurassic period, or over 200 million years ago.

News of the crocodile's recently discovered sensitive side seems to have struck a fascination nerve with the media. Daphne's work has made headlines worldwide, with the story covered by London Times, New York Times, Washington Post, Der Spiegel, CNN, BBC, National Public Radio (USA), the Discovery Channel, and National Geographic, each interested in taping interviews of Daphne and her 'crocs'. Daphne, taking it all in stride, uses the opportunity to introduce the general public to the wonderful world of animal behavior and brains: "I said 'neuroethology' every chance I had!"

None of this is terribly surprising for Daphne's thesis advisor, Catherine Carr. "Daphne is driven by a real passion for comparative neurobiology, and is very creative and extremely hard working. She always has more ideas than time, but I have noticed that she finishes what she starts." Not one to be shy, Daphne "talked to every crocodile paleontologist in the world" and "spent weekends at the Smithsonian working on fossil skulls," says Catherine. Prolific, hard-working, and ever curious, she has a number of awards to show for it, including a Graduate Research Award from the University of Maryland, College Park, a Minority Travel Award from the Society for Neuroscience, the Grass Fellowship, and a National Science Foundation fellowship to pursue her postdoctoral studies with Mark Konishi at Caltech.

Are there crocodiles in the future for this spunky researcher? "Oh, yes," she smiles.