Sunday, December 18, 2011

Bird poop solves the paradox of the snail

I used to love a certain board game called “Snail’s Pace Race”.  For some reason, the inch-by-inch progress of the colorful wooden snails along a flat piece of cardboard was exciting to my five-year-old brain.  Perhaps it was my aversion to competitive situations- no one loses in a game in which snails race.

As even a five-year-old knows, snails are not exactly skillful travelers.  Yet paradoxically, they can be found all over the world.  How exactly does a snail, of all creatures, get to remote places like islands in the middle of the Pacific Ocean? Even Darwin was puzzled by the incongruity of a widespread animal with such little talent for dispersion.

In a way, plants are also like snails.  They don’t move very well, but some live across areas separated by thousands of miles (see previous post).  In order to accomplish this, plants produce seeds that are much smaller than their adult form and which can be more easily transported by wind or on the fur of animals.  Snails are larger than the typical seed, but perhaps they could still be accidentally blown to new places by strong winds or hitch a ride on the legs on birds.  These events don’t seem very likely, but all it takes is a few successful travelers for the snails to start living in a new place.

A group of Japanese researchers were interested in how frequently these chance movements of snails across large distances might occur, but since it is impossible to track every snail across even a relatively small area, they used a more indirect approach.  All across the island of Hahajima they collected snails and analyzed their DNA.  When snails mate with each other, their offspring have DNA from both parents.  If snails are able to move across the island easily and do not just mate with the individuals they are closest to, then different DNA sequences will get passed all around the island and there won’t be huge differences between the type of sequences that can be found on the northern end versus the southern end.  However, if groups of snails living far apart had very different DNA sequences then this would imply that snails are only rarely transported across the island.

Unexpectedly, the genetic analysis showed that DNA was getting swapped between snails across the island and that distant places did not have different DNA sequences- something must be moving the snails around!  Even more interesting was that places with lots of Japanese white-eyes (a bird that eats snails) had greater variation in DNA sequences among snails living there, which meant that more sex was happening there between snails from different places.  The evidence pointed to the birds as the main snail-movers, but when the researchers looked at the birds’ legs, they never saw little hitch-hikers.

It wasn’t until they looked more closely at the birds’ excrement that the mystery began to unravel.  The snail they were studying (Tornatellides boenigi) is a very small snail, and when the white-eyes eat them, many of the shells come out in the poop still in-tact.  What if some of the shells could come out with both shell and living snail still in-tact? To test this, the researchers went to the Yokohama Zoo and started feeding snails to white-eyes and brown-eared bulbuls (another common bird on Haha-jima).  Incredibly, 15% of the snails came back crawling out of the poop- one even had babies afterward.  Like the Millenium Falcon in the belly of the giant space worm, the snails somehow survived the corrosive acidity of the stomach, though in this case they came out the other end.

Like bats that eat figs and subsequently rain their seeds all across the forest, or humans that have transported corn from its home in Mexico to fields all over the world, this new study shows that predators don’t always have a purely negative influence on the things they eat.

You can find this article at:

ResearchBlogging.orgWada, S., Kawakami, K., & Chiba, S. (2012). Snails can survive passage through a bird’s digestive system Journal of Biogeography, 39 (1), 69-73 DOI: 10.1111/j.1365-2699.2011.02559.x

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