Of all the words to describe a crocodile, ‘sensitive’ may not be the one that immediately comes to mind. These animals are known for its almost impregnable armor, but now researchers from the University of Geneva have discovered that there may be more to it than just thick skin.
A new study shows that the entire body of Nile crocodiles (Crocodylus niloticus) is covered in hundreds of scales, each with a tiny spot full of nerve endings. During embryo development, these spots start forming in the head but eventually reach the whole body. Each spot reacts to touch, as well as changes in temperature and pH. “We used molecular techniques and, to our great surprise, found clear expression of not only mechano-receptors but also of chemo- and thermo-receptor channels”, said Prof Michel Milinkovitch, biologist and lead author in the study.
The spots allow these reptiles to detect prey even in total darkness, by feeling water ripples caused by any unlucky fish swimming nearby. What’s more, with temperature and pH sensors in the skin, crocodiles quickly pick up on weather changes and move to find the best spot for a snooze or a snack. It is likely the presence of hundreds of these sensory organs have contributed to the wide geographical distribution of crocodiles, in contrast to more specific habitats of, for example, alligators and caimans, which only have these spots in the head.
This sensory system, reported last week in EvoDevo, is unique among vertebrates. While we, and other mammals, are virtually covered in nerve endings, this is at the expense of a more vulnerable skin. A diffused sensory system would be highly inefficient in thick-skinned crocodiles. However, “the re-organisation of the skin sensory system into an array of multi-sensory micro-organs is probably one of the best solutions to that conundrum”, said Prof Milinkovitch.
With a ‘spotty’ distribution of sensors, crocodiles can maintain an impressive armored protection, but still benefit from sensitive skin. These spots provide them not only with exquisite sensitivity to touch (of the same magnitude as our own fingertips) but also with the ability to quickly detect cold, heat and chemical changes in the environment despite their impressive thick skin.
The team is keen to continue studying non-model species, as they believe these animals are fantastic tools to decipher mechanisms of evolution and development which may not be possible to evaluate with the more classic animal models, such as fruit fly and mouse.
Di-Poï N, & Milinkovitch MC (2013). Crocodylians evolved scattered multi-sensory micro-organs. EvoDevo, 4 (1) PMID: 23819918
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