When you leave your house do you ever worry about being torn from the ground by a giant bird or ruthlessly eaten alive by a vicious carnivorous snail? Do you fear your home might literally be ripped off its foundations and stolen by a particularly sly neighbor? No? Well, be thankful you’re not a hermit crab.
When hermit crabs leave their shells to mate or move house, their soft, squishy bodies are suddenly exposed to predators and the precious shell they've left behind becomes fair game for opportunistic crustaceans looking to move up the property ladder.
But according to a new study published in the journal Royal Society Open Science, some hermit crabs have come up with a clever tactic to cope with this – grow a really, really long penis.
Mark Laidre, an assistant professor of biological sciences at Dartmouth College, came to this conclusion after measuring the penises of 328 hermit crabs belonging to nine closely related species. If you’re wondering how that worked, all the crabs were already-dead museum specimens.
Certain hermit crabs, such as those belonging to the Coenobita genus, have especially desirable protective shells as they remodel their shell’s interior. These crabs can have penises that are up to 60 percent as long as their body. That’s the same as a 6-foot man having a 3.6-foot penis.
Laidre found that crabs with the most desirable and most easily stolen shells, like Coenobita, had the longest penises. Those carrying less valuable and less easily stolen shells had shorter penises, and those not carrying a shell at all had the shortest penises. Therefore, Laidre proposes a new hypothesis, the ‘private parts for private property’ hypothesis.
“Paralleling Darwin's hypothesis that long barnacle penises evolved to facilitate sex with distant neighbours, I suggest that some animals might evolve long penises to protect their private property from being stolen during sex,” he writes.
Essentially, crabs with longer penises are able to extend their “sexual tubes” out of their shell and into the shell of their love interest, all from the comfort of their own home. Laidre describes this, along with behavioral adaptations that might complement it, as a kind of home “insurance”.
“The threat posed by conspecific eavesdroppers should place a strong premium on both speedy and secretive sex, particularly in species with more valuable and more easily stolen private property,” he writes.
Being more likely to become victims of theft, Laidre notes that Coenobita crabs have “secretive ‘quickies’” at night among the “secluded waves”.
Laidre believes his theory might hold true for other animals too and that the opposite pattern might be seen in creatures that own valuable yet non-portable properties, ie those that defend territories might actually have smaller penises, but larger weapons like claws, thanks to a trade-off between the two.