Friday, April 2, 2010

Hermit Crab Shell-Choice Behavior: How to find a dream home!

Of course, everyone wants to live in the nicest possible house, along with periodic upgrades. A new study by biologists at the New England Aquarium and Tufts University reveals that hermit crabs might locate new housing by using previously unknown social networking skills.

The scientists combined field studies, lab experiments and computer models to uncover some surprising new tricks that could lead to better house-hunting strategies for humans and hermit crabs alike. Their research was published yesterday in the British scientific journal, "Behavioral Ecology," and reveals that, contrary to their name, hermit crabs often find the best new shells when they gather together. Play the video to see this happening.

Hermit crabs have an unusual lifestyle because they require empty snail shells for shelter. And, they also need to regularly search out new shells as they grow bigger throughout their lives. "Hermit crabs are really picky about their real estate choices, "Maybe it's because, in addition to having to locate food and escape predators, they're constantly being thrown back into the housing market," says Randi Rotjan, research scientist at the New England Aquarium and leader of the research team who co-authored the study with Sara Lewis, a professor at Tufts University. Rotjan will be lecturing on this new discovery April 5 at the New England Aquarium (Click here to register for the free lecture).

Sometimes there aren't enough suitable shells to go around and some crabs have to go around naked. The exposed soft abdomen of homeless crabs makes them more vulnerable to predators. "I've seen hermit crabs dragging around in bottle caps and even ballpoint pen tops," Lewis says. "It's pretty pathetic."

So how do hermit crabs win this life-or-death shell game?

It seems one strategy that helps each hermit crab find the very best shell for them is by joining a lively group activity known as a synchronous vacancy chain. When a new shell becomes available, crabs gather around it and queue up in a line from largest to smallest [photo above]. Once the largest crab moves into the vacant shell, each crab in the queue swiftly switches into the newly vacated shell right in front of them. So in the end, a single vacant shell kicks off an entire chain of shell vacancies that ultimately leads to many crabs getting new, and generally improved, housing.

By seeding vacant shells into field populations and watching night after night to see what happened, the scientists discovered some previously unknown hermit crab behaviors. When a hermit crab discovers an empty but oversized shell, it waits nearby rather than abandoning the unsuitable shell. Once a small group gathers, crabs begin piggybacking by holding onto the shell of a larger crab and riding along. Such waiting and piggybacking behaviors seem to increase the chances that a synchronous vacancy chain will happen. "They spend hours queuing up, and then the chain just fires off in seconds, like a chain of dominoes," says Rotjan.

Computer models populated with virtual hermit crabs and shells confirmed that synchronous vacancy chains depend not only on crab density, but also on how long crabs are programmed to wait near an unsuitable shell.

According to Rotjan, synchronous vacancy chains can occur with any animal that relies on discrete, limited and reusable resources, such as anemone-dwelling fish, or hole-nesting woodpeckers. Studying vacancy chains in hermit crabs might even lend some important insights into human behavior, since people regularly participate in vacancy chains, including synchronous ones. For example, every fall, many neighborhood streets in Boston, Mass., are clogged with U-Haul trucks and moving vans that signal the city's many students and residents participating in synchronous vacancy chains: everyone moves on the same day (September 1); the most popular annual lease start date. Just like hermit crabs, humans carefully assess their potential housing options before switching; and any savvy apartment-hunter knows that there is more inventory to choose from with a Sept. 1 start date, but moving fast is critical to ensure that you get the home you want.

For both humans and hermit crabs, the main advantage to synchronous switching is a bountiful inventory, but plentiful choices come with a cost: higher competition for the perfect house. Apartments with less popular starting lease dates resemble an asynchronous vacancy chain with less inventory available throughout the year, but also less competition for each house.

Social networking via internet sites like Craigslist surely wouldn't work for hermit crabs (they resort to waiting near an empty shell or recruiting other crabs to the scene), but human social networking sites have made it simpler for people to assess homes and organize moving dates. In essence, human participation in synchronous vacancy chains shows that humans have learned what the crabs have always known: social networking might make suitable new homes easier to come by. If you're organized.

[Visit the research project page for more information. Read the article abstract in Behavioral Ecology.]

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