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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 .

A聽few hermit crab species 鈥� especially those in a group called the Coenobita 鈥� modify their private property: spending months or even years聽tearing away some of hard material inside the shell to make it a roomier and more comfortable dwelling.聽But remodelled shells are potentially seen as more desirable, which can make them a tempting target for theft by another hermit crab especially when the homeowner is preoccupied.

Laidre wondered whether this risk of theft might explain a curious feature of Coenobita聽hermit crab anatomy 鈥� namely, that some males have penises that can be 60 per cent as long as the rest of their bodies.

Theft risk

Laidre鈥檚 idea was that a male with a larger penis can remain inside his valuable home during sex and simply extend his penis out of the shell towards the female. A male with a shorter penis would have to creep out of his shell for sex, which would mean leaving his private property unoccupied and at risk of being stolen. Laidre dubs this the 鈥榩rivate parts for private property鈥� hypothesis.

He tested the idea by measuring penis length in a number of closely related hermit crab species 鈥� some that don鈥檛 move into snail shells at all, some that do so but don鈥檛 modify the shells, and some that move into snail shells and then remodel them extensively.

In line with his idea, the males in the remodelled shells have penises that are significantly longer than the other hermit crab species. Additionally, among the hermit crabs that remodel their shells, the species that does so most extensively聽has the longest penis of all.

Laidre would now like to follow up these findings by studying in more detail how penis size affects the way crabs behave. 鈥淚t remains unclear if, within species, males with relatively larger penises for their body size are less likely to have their shell stolen or more likely to be successful in mating,鈥� he says.