This is Scientific Americans 60 Second Science.

I'm Karen Hopkins.

So I never really thought I'd study penis size, but I sort of stumbled on this topic.

Mark Lydra, a biologist at Dartmouth College. Lydra travels to

Costa Rica to study hermit crabs, a species called cenobeda compresses.

These land crabs do some interior remodeling of their adopted shells.

They extensively hollow them out, removing struts called spiral calumella to give themselves some extra elbow

room. The renovation renders the shells more precious to their owners and to other covetous crustaceans as well.

These more valuable shells though are also more easily stolen since without the spiral calumela inside the to grip onto, individuals are pretty liable to have their property

snatch from them, particularly when they are engaged in other activities, like copulation, which requires coming

partway out of the shell. Despite his work in the field, it wasn't until Lydra was wandering through a museum that he

noticed something about his favorite crabs.

The really striking thing was that C-Novita compressus, the one whose social behavior I've been studying for so many years,

had an unusually large penis, in fact bigger than any of the other species.

The observation gave him an idea, which he dubbed the private parts for private property hypothesis.

In essence the hypothesis posits that in large private parts can be an adaptation, extending

a male sexual reach and thus enabling both him and his partner to remain safely tucked away inside their shells while they copulate, thereby protecting the private property of their shells from being stolen during sex.

Darwin proposed a similar idea to explain why barnacles, which are stuck in one place, are

so amply endowed.

To test his private parts for a private property hypothesis, Lydra sized up more than 300 male museum specimens, including

hermit crabs that live on land and at sea. And he found that crabs that carried custom

coverings had the most impressive carnal equipment. At the same time, species that got their shells off the shelf had bigger gear than did crabs that walked around with no shell at all.

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