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This is Scientific Americans' 60-second science. I'm Karen Hopkin.

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

They extensively hollow them out, removing struts called spiral calliomella 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 colliamella inside the shell to grip onto, individuals are pretty

liable to have their properties snatch from them, particularly when they are engaged in other

activities, like copulation, which requires coming part way 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 Cina Vita 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's 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

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