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Friday, June 22, 2012

Snakes that chew their food

I have to admit right up front that the title of this article is not really accurate. No snakes chew their food the way we do. Almost all snakes must swallow their food whole, which limits their (often considerable) gape to items they can jaw-walk their kinetic skulls over. Taken as a whole, there are few animals on Earth that snakes do not eat -- whales and dolphins, elephants, animals endemic to the polar regions, some very toxic millipedes. There are snakes that swallow leopards whole, snakes that eat porcupines without removing the quills, snakes that tolerate stabs from catfish spines, snakes that eat other snakes longer than they are. Here's a video of a Tantilla eating a giant centipede. As a group, they can eat nearly anything. They all swallow their prey whole. Almost.
Except for this one
Its genus name is Fordonia, which is probably meaningless, seeing as it was biologist J.E. Gray of the British Museum of Natural History,"well known for inventing many apparently meaningless scientific names", who came up with it (he also named the North American Farancia). Commonly known as the Crab-eating Water Snake or White-bellied Mangrove Snake (after the specific epithet), Fordonia leucobalia is native to the mangrove swamps and tidal mud flats of southeast Asia and northern Australia. It lives in mud lobster and fiddler crab burrows, and moves by jumping across the soft mud, into which it would sink if it tried to slither.

Part of a small but interesting group of live-bearing snakes known as homalopsids, Fordonia is southeast Asia's answer to the North American natricine Nerodia, for many the archetypical semi-aquatic snake. What sets Fordonia apart from other homalopsid snakes, which feed mostly on fishes, is that it eats crabs, an observation first made by Cantor in 1847. (This may be highly cathartic for the snakes, whose primary predators as juveniles include large crabs.)

Those are hard-shelled decapod crustaceans, for you biologists out there 
Like many other arthropods, crabs have an anti-predator adaptation called leg autotomy, similar to tail autotomy in lizards, salamanders, and some snakes. This means that their legs can break off when grabbed and will later regrow - better to lose a limb and escape than to be eaten whole. But Fordonia has evolved behaviors that exploit the crabs' ability to autotomize their legs - it pins the crab's body to the mud and pulls of its legs, eating them one at a time! Sometimes they also consume the crab's body, but often they just leave it behind. This makes Fordonia the only snake that breaks its prey apart prior to eating it, although we must admit that it is somewhat helped along by the crab's autotomy. This discovery was sufficiently exciting to be published in the prestigious journal Nature.

The five crab legs at the top, eaten by this snake, came from a crab about the size of the one on the bottom. The white circle represents the maximum-sized prey item the snake could have eaten whole. Figure from Jayne et al. 2002
The adaptations of Fordonia to cancrivory don't end there. As anyone who has eaten crab legs knows, a crab's exoskeleton is very tough - we humans must use tools to break into it. In order not to be internally lacerated by their prey, Fordonia have evolved extra tough, muscular stomach lining. Other crustacean-eating snakes, such as the North American crayfish snakes (genus Regina), as well as the arthropod-eating False Hook-nosed Snake (Pseudoficimia frontalis, a sonorine snake from western Mexican dry forests), also have thickened muscles surrounding their stomachs, to prevent internal damage from they prey's sharp exoskeletons.

Digestion in snakes is an intense process: their digestive enzymes are very strong, capable of breaking down  even bone. Still, a little mastication can help the digestive process along considerably. For most snakes this isn't an option, because their needle-like teeth and highly mobile skull bones are ill-suited to both cutting and generating bite forces. However, snake biologist Alan Savitzky reported that recently ingested crab legs extracted from Fordonia stomachs were crushed. How is this possible? In fact, Fordonia possess remarkably robust and compact teeth for a snake, almost like molars! Although this is an extreme morphological modification, Savitzky remarked that it is almost surprising that the teeth and skulls of Fordonia aren't more abnormal, considering their unusual diet. Finally, Fordonia has evolved a large salt gland to help maintain osmotic balance on a high-salt diet (crabs are isosmotic to their environment, meaning that they have the same salt content as sea water).

Left: Tooth of Cerberus rynchops; Right: Teeth of Fordonia leucobalia
While Fordonia does all this with hard-shelled crabs, another homalopsid species found in the same mangroves, the Cat-eyed Watersnake (Gerarda prevostiana), has been found to consume freshly-molted (and therefore soft-shelled) crabs in much the same way. This kind of specialization is also found among the four species of North American Crayfish Snakes (Regina) - two of which (R. rigida, R. alleni) have hinged teeth to help them consume hard-shelled crayfish, and two of which (R. grahamii, R. septemvittata) seek out freshly-molted crayfish by smelling their molting secretions. Incredibly, although Gerarda lacks the morphological adaptations for cancrivory of Fordonia, it was observed tearing apart the soft carapaces of crabs after eating their legs, which probably allows Gerarda to consume crabs that would otherwise be too large for them to swallow whole. The feeding mechanisms used by Fordonia and Gerarda differ in the modes of attack and prey restraint, the usual orientation for swallowing crabs, and how pieces were torn from prey, suggesting that they might have evolved their crab-eating habits independently and convergently, rather than inheriting them from a shared common ancestor (although they are evolutionary sisters, one another's closest relatives). Two other closely related genera of homalopsine, Myron and Cantoria, also consume some crustaceans, but are less well-known. How many snakes are out there with strange dietary adaptations that remain to be discovered? We may never know.

This snake only eats soft-shelled crabs - what a snob


Thanks to A. Captain and Brendan Schembri for photographs.


Alfaro ME, Karns DR, Voris HK, Brock CD, Stuart BL (2008) Phylogeny, evolutionary history, and biogeography of Oriental-Australian rear-fanged water snakes (Colubroidea: Homalopsidae) inferred from mitochondrial and nuclear DNA sequences. Molecular phylogenetics and evolution 46:576-593

Jayne BC, Voris HK, Ng PKL (2002) Snake circumvents constraints on prey size. Nature 418:143

Savitzky AH (1983) Coadapted character complexes among snakes: fossoriality, piscivory, and durophagy. American Zoologist 23:397-409

Shine R, Schwaner T (1985) Prey constriction by venomous snakes: a review, and new data on Australian species. Copeia 1985:1067-1071

Voris HK, Jeffries WB (1995) Predation on marine snakes: a case for decapods supported by new observations from Thailand. Journal of Tropical Ecology 11:569-576

Voris HK, Murphy JC (2002) The prey and predators of Homalopsine snakes. Journal of Natural History 36:1621-1632

Creative Commons License

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.


Ivan said...

I find it awesome that both Fordonia and Gerarda are native to mangroves in Singapore. In fact, the sequence from the BBC's Life In Cold Blood depicting the feeding habits of Fordonia was filmed in Singapore.


Andrew Durso said...

Very high on my list of places to visit and hunt snakes. Thanks for the link!