Crocodiles are more sensitive than your grandmother...Who knew?

If you ever observe crocodilians closely, you will notice a peppering of black dots over their ossified skin.  If you speculate about their function, you will not be the first, nor be much more clueless than your predecessors.  Herpetologists have proposed numerous functions for these structures, and as recently as 2011 new hypotheses have been set forth.  These have proposed that the tiny organs sense electrical fields, as fish do, or produce oils for keeping crocodile skin waterproof.  They have been assigned the ability of magnetoreception for navigation and the capacity to judge salt concentrations.  All these ideas have had little backing and little research with which to discriminate between them.  Until now.  Leitch and Catania (2012) clearly demonstrate at the physiological and behavioral level that these little spots relay pressures, especially in water.  Dubbed the distinctively vague moniker ‘integumentary (skin) sensory organs,’ or ISOs, this crocodile pepper no longer lurks in the shadows. 

 Integumentary sensory organs can be seen as small, raised black spots peppering the scales of crocodilians.  Notice the concentration around the jaws and teeth. Alligator mississippiensis.

 Individual ISOs can be seen in this more detailed image.

The ability to sense pressure is generally termed mechanoreception.  This includes the sense of touch, hearing (think of pressure waves in the form of sound), and sense of underwater pressure waves from movement.  The latter is common in fishes, possessing pressure sensitive pits in the scales around their head and along their lateral line.  The advantage is the ability to locate a source of activity in quiet, underwater darkness, allowing hunting by feel alone.  It seems, then, no coincidence that crocodilian ISOs are concentrated most heavily around the jaws and teeth, and that the nerves these sensors plug into go straight to the Trigeminal nerve – a nerve used in the process of biting.  In fact, Leitch and Catania (2012) found that Alligator mississippiensis have over three thousand ISOs on their head, and none on the rest of their body.  Crocodylus niloticus, typical of crocodiles, has a similar concentration of ISOs over its cranium but also has them on nearly every scale over the rest of the body, with a total of around 9000.  

Leitch and Catania (2012) tested just how sensitive these organs are, and were surprised at the results.  Using tiny, filamentous wires precisely gauged for their stiffness and therefore the pressure they could exert, Leitch and Catania began poking crocs.  To do so qualitatively, the authors had to anesthetize their alligators and nile crocodiles and connect an electrode to the nerve pathway trailing from the ISOs.  First, ISOs were found to be more responsive when wet.  With this established, the scientists began testing the limits of ISOs’ ability to detect pressure.  Eventually, they found themselves using a filament wire so hair-thin that a human fingertip could not feel a poke from it.  Even still, the crocodilians could sense it, and the researchers ran out of smaller wires to use, never finding the lower limit of ISO sensitivity. 

To test other hypotheses for ISO function, the authors of this study also exposed ISOs to underwater electric fields and salt solutions, with no response.  It seems likely that all other proposed functions can be eliminated in favor of mechanoreception.  But conscious crocodilians also give further demonstration of the practical utility of this ability.  American alligators and Nile crocodiles were individually placed in a darkened tank of water, saturated with white noise.  In this way, alternative senses were eliminated.  Then food was dropped in the water and the crocodilians were observed (using infrared cameras) honing directly to the source of the splash.  Interestingly, a resting crocodilian typically has just its eyes, ears, and nose above water.  ISOs are concentrated, among other areas, right below the eye, just below the surface of the water when at a typical resting position.  This allows for surface ripples to be efficiently sensed, just as this experiment demonstrated.  Further, both crocodiles and alligators were observed hunting live fish with precision in this darkened, noise-polluted environment.  Of greatest interest was that crocodiles, unlike alligators, posses ISOs over their whole body.  Thus they were able to accurately react to fish activity below the surface even while their head was elevated above the surface, while alligators could not.

 

Mother crocodilians, such as this Nile crocodile, use their jaws to transport their offspring.

What makes these organs so sensitive?  They are absolutely loaded with exposed nerve endings, and the epithelial layer over them is 60% thinner than elsewhere on the crocodilian’s body.  Without integumentary sensory organs, these heavily armored reptiles would be extremely insensitive.  That could be an issue when handling young, as mothers do when delicately picking their hatchlings up in their jaws and carrying them to the water.  Leitch and Catania propose that the heavy concentration of ISOs around the teeth relay jaw pressure and allow crocodilians to gauge their gentleness with their young.  Further function of ISOs around teeth and even in the mouth could include aid in handling captured prey while attempting to orient it correctly for swallowing.  

 

Maneuvering prey can be tricky when you have stiff jaws, a non-functional tongue, and no hands. Crocodylus porosus.

This revealing research, however, brings up questions requiring further illumination: why do the families Crocodylidae and Gavialidae have ISOs over their whole body, while family members of Alligatoridae posses them on the head only?  How important is this sense in hunting in the wild?  Is it important in courtship and mating?  Is it disproportionately crucial for piscivorous crocodilians?  When sight is possible underwater, which sense – mechanoreception or sight – is more important?  

 

Crocodilians - such as these American alligators - have extremely tender and tactile-oriented courtship rituals.  Even in this photo the ISOs are visible.

These questions and more will have to be answered in the future.  For now, we can be amazed at this belated sensory discovery that deepens our awe for crocodilians. And we can take note: not everything has been discovered!

Leitch, D. B. and Catania, K. C. (2012). Structure, innervation and response properties of integumentary sensory organs in crocodilians. J. Exp. Biol. 215, 4217-4230. http://jeb.biologists.org/content/215/23/4217.abstract