An asymmetric diet of snails

One of the new entries on the Map of Life highlights some of the extraordinary convergences relating to snake and lizard feeding. Perhaps the most charismatic example relates to some beautiful neo-tropical snakes who have become truly adept at extracting snails from their shells and scoffing them at will. Here is a taster (as it were) and do head over to our “Feeding in snakes and lizards” entry for even more to chew on…

Snails may not be everyone’s first choice on the menu but several distinct colubrid snakes have evolved expert techniques for gorging on these nutritious gastropods. Southeast Asian pareatine snakes (e.g. Pareas iswasaki) and neo-tropical dipsadine snakes (e.g. Dipsas) both have asymmetrical mandibles (lower jaws) with many more teeth on the right than the left side.  

Catesby's snail-eating snake, photo by G. Gallice

Catesby’s snail-eating snake, photo by G. Gallice

The right and left mandibles are inserted into right-coiling snail shells and then by repeated and alternate retractions, they delicately extract the snail’s soft body. In an incredible instance of convergence, not only the pareatine and dipsadine snakes, but also certain insectivorous beetle larvae also have asymmetrical mandibles for snail predation. Because the right-coiling snails are preferred this  means that the much rarer atypical left-coiling variants can gain a selective advantage where these predators abound.

Still within the reptiles we find another highly adapted snail-eater and with it another exemplary case of convergence. The Australian pink-tongued skink (Cyclodomorphus gerrardii) is a rainforest-dwelling lizard with a pair of large ‘hammer’ teeth for cracking snail shells. Within the same geographic region fossils of a Miocene marsupial (Malleodectes) have been found whose dentition is all but identical to Cyclodomorphus. It seems that both animals preyed on rainforest snails in the same way, and in the increasing competition resulting from climate change the lizards, in the end, came up trumps.


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