Hermit crabs make the perfect home out of a shell Bazzano Photography / Alamy

Some hermit crabs have unusually large penises, and now we might know why. A long penis allows a male crab to have sex without leaving the shell he calls home, reducing the risk it will be stolen while he is busy.

Many species of hermit crabs protect themselves from predators by moving into empty mollusc shells, which they then carry around with them. Mark Laidre at Dartmouth College, New Hampshire, calls the mollusc shells a form of hermit crab private property.

A few hermit crab species …