Tough But Sensitive: Croc Skin Is Dotted With Sensory Micro-Organs.

Despite appearances to the contrary, this guy is actually highly sensitive. Credit: Michel Milinkovitch

You could be forgiven for thinking that crocodiles aren’t sensitive. With that thick keratinous skin it’s difficult to imagine they could feel much, but as it turns out, they can feel a great deal more than we realised. Research published today in EvoDevo reveals that crocodilians have sense organs in their skin that are sensitive to mechanical, thermal and even chemical stimuli.

These integumentary sense organs (ISOs), or dome pressure receptors as they’re also known, have a well documented hypersensitivity to changes in pressure. When a potential prey item breaks the surface of a water body, these pressure receptors can detect the resulting waves, allowing the croc to identify the source of the disturbance. In this way, crocodilians can detect prey even if visibility is impaired.

Today’s study shows that the function of these receptors extends beyond pressure detection. By studying these micro-organs in Nile crocodiles and spectacled caiman, researchers at The University of Geneva‘s Laboratory of Artificial and Natural Evolution (LANE) found that they were also sensitive to temperature and  pH. Human somatosensory systems allow us to detect sensations such as heat, coldness, wetness and pressure, but the mechanism is quite different.

Close up of integumentary sense organs (The dark dot within each scale). Credit: Michel Milinkovitch

Close up of integumentary sense organs (The dark dot within each scale). Credit: Michel Milinkovitch

Our skin allows us to perceive these sensations through the stimulation of a suite of specific receptors across the entire skin surface. In crocodilians, only the ISOs are stimulated, and these tiny organs are responsible for delivering the entire range of sensation. This sensory system (unique among vertebrates)  means crocodilians can have the best of both worlds, as  ISOs allow the animal to have a tough leathery skin without sacrificing sensitivity.

Previous research conducted at LANE revealed that ISOs play a key role in shaping a crocodiles facial scales. Unlike scales elsewhere on the body, crocodilian facial scales do not emerge from genetically governed developmental units. Instead, the rapid growth of the animal’s skull during embryonic development causes the overlying skin to crack under pressure, forming the distinctive scalation pattern we are so familiar with. ISOs emerge at an earlier stage in the animal’s development than the cracking, and as the “fault-lines” appear, they tend to concentrate in those areas of the skin where the density of ISOs is lowest.

Here we see a high density cluster of ISOs. And teeth...Lots of teeth. Credit: Michel Milinkovitch

Here we see a high density cluster of ISOs. And teeth…Lots of teeth. Credit: Michel Milinkovitch

Interestingly, in crocodiles and gharials these ISOs are distributed evenly all over the body, but in alligators and caiman the receptors are restricted to the cranial scales.  The reason for this discrepancy is unknown, though the authors of the study suggest that the presence of post-cranial ISOs may offer crocodiles and gharials a competitive advantage in environments where pH, and temperature fluctuate. This advantage may be one of the reasons for their wider geographical distribution relative to alligators and caiman.

Related articles: Croc Scales: Not All They’re Cracked Up To Be…

Source: Nicolas Di-Poi and Michel C Milinkovitch. “Crocodylians evolved scattered multi-sensory micro-organs” EvoDevo 2013, 4:19 doi:10.1186/2041-9139-4-19

Photos: All photographs provided by Michel C. Milinkovitch.

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