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

Sunday, December 4, 2011

Different worms for different gulls

Old wives may say that the early bird gets the worm, but there are many worms that birds may not want to get.  These are the helminths (a.k.a parasitic worms), and whether they are early to the table or late, birds end up with a lot of them.  Helminths come in many shapes and sizes; nematodes are round and generally look like a worm, digeneans have two suckers and are solid all the way through, and cestodes (tapeworms) are basically a miniature toothy q-tip head trailing a toilet paper tail.  Birds and other vertebrate animals have such a diverse collection of these gut parasites that scientists have started to think about the insides of an animal as a mini-ecosystem and are asking whether ecological theory developed for larger animals also works in the same way for parasites.

A study published online this month in Ecography asked whether ring-billed gulls have more similar helminthes in their guts if they live closer to each other or if they are closer together in age.  The idea that as places become farther apart they have more different plants and animals is called ‘distance decay’ and biogeographers have gotten pretty excited about measuring it in the last ten years, even though for geographers it has been the ‘first law of geography’ since 1970.

For gulls living along 300 km of the St. Lawrence River in Quebec, Canada, the differences between the parasites living in different gulls was more related to their age than how far apart they lived from each other.  Older gulls had a wider variety of parasites than younger gulls because they forage for food in many places and ingest different types of parasites, whereas chicks and juveniles don’t venture as far.  However, when the researchers compared the parasites living inside Quebec gulls to those living 3000 km away in Alberta, their differences in parasites were much more related to how far apart gulls lived- age barely had any effect.

This distance decay in parasite communities has also been looked at in the parasites of fish, mammals, and molluscs.  The gull researchers wanted to know whether different characteristics of the host animal, such as its size and mobility, or whether it is homeothermic (warm-blooded) affects how quickly the difference between the parasites of two hosts increases as the distance between them increases.  One might think that animals that migrate across wide areas would have similar parasites even across large distances because each animal is likely to pick up most of the same parasites, whereas mussels (that are affixed to rocks on the ocean shore) could have very different types of parasites living inside them even across short distances.

Intriguingly, this doesn’t actually seem to be the case.  In a preliminary analysis of 25 studies of different host animals, the researchers found that only two factors affected the rate of distance decay in parasites; latitude and how widely across space animals were sampled in the study- host mobility didn’t matter at all.  Studies that analyzed parasites in animals across a large extent (1,000 -10,000 km) tended to find slower rates of distance decay.  Parasites in animals living farther north at higher latitudes had faster rates of distance decay.  That is, there is greater variation in the parasites that animals have over shorter distances. To me, it’s not exactly clear why this may be, but the researchers suggest that it could be because animals living up north have to deal with a wider range of climates.  As more scientists study the ecology of parasites it will be interesting to see whether host characteristics affect other aspects of parasite communities.

You can find this article at:

ResearchBlogging.orgLocke, S., Levy, M., Marcogliese, D., Ackerman, S., & McLaughlin, J. (2011). The decay of parasite community similarity in ring-billed gulls Larus delawarensis and other hosts Ecography DOI: 10.1111/j.1600-0587.2011.07244.x