Tuesday, July 12, 2011

Snails Can Survive Being Digested by Birds

Japanese White-eye / Photo by Trisha Shears
A story has been making the rounds about some small snails being able to survive despite being eaten by a bird and subjected to its digestive processes.
Studies of the diets of birds on the island of Hahajima identified the Japanese white-eye's preference for the tiny land snail Tornatellides boeningi.

In the lab scientists fed the birds with the snails to find out whether any survived the digestive process.

"We were surprised that a high rate, about 15 percent, of snails were still alive after passing through the gut of [the] birds," explained researcher Shinichiro Wada.
Snails being able to survive digestion is fascinating in itself. It conjures mental images of near-death experiences or even zombies. However, in this case, it has additional significance. Scientists on the island Hahajima in Japan had noticed odd distribution patterns of Tornatellides boeningi. They found genetic evidence of interbreeding among seemingly isolated populations of the snail and other evidence of dispersal over longer distances than they would expect. If these snails can survive digestion, it is possible, indeed likely, that Japanese White-eyes carry live snails to new locations in their guts.
One snail in particular helped researchers identify how numerous snails could travel over distances via bird droppings.

"One of the snails fed to the bird gave birth to juveniles just after passing through the gut," Mr Wada told the BBC.
The idea that some snail distribution patterns can be explained by passive transportation is not a new one. Malacologist and snail blogger Aydin Örstan has discussed dispersal by cars on a few occasions (including here, here, and in a recent paper). The idea of transportation by birds is not entirely new either, as some speculation had involved snails hitching rides on birds' feet. This seems to be the first indication, though, that snails can survive the digestion process and land somewhere other than where they were eaten. It will be interesting to see if this turns out to be true for other snail-bird pairings as well.

Being eaten by a bird will not work for all snails as a means of transportation. The tiny Tornatellides boeningi are able to survive a white-eye's digestive tract because their small size makes it easier for a bird to swallow them whole. After being swallowed, they spend a relatively short time in the digestive tract. (Most songbirds defecate their meals shortly after eating them. This short turnaround makes it possible for intact seeds to survive digestion and be dispersed through defecation.) While some snails are swallowed whole, others are not so lucky. Gulls are known for dropping mollusks to break their shells if they are unable to pry into the shell by other means. Snail Kites use their hooked bills to extract a snail's soft tissues being eating them and discarding the shell. Snails subjected to such harsh treatments are unlikely to survive.