Penis Size Matters for Hermit Crabs Trying to Keep Their Shells During Sex

"Really, you can view their whole social life as a bit of a waiting game and a bit Machiavellian."

When you’re a human, coming out of your shell is a journey toward self-actualization. When you’re a crab, coming out of your shell is a nightmare. Bad things happen when you come out of your shell, like other crabs stealing it away and leaving you out to die. But according to new research in Royal Society Open Science, that’s less likely to happen if you’re a crab with a big penis.

Mark Laidre, Ph.D., is an assistant professor of biology at Dartmouth University, an experimental biologist, and author of the new paper on well-endowed crabs. He was studying land-dwelling hermit crabs at the Smithsonian’s National Museum of Natural History when he realized that the animals with the most “valuable shells” also had the largest penises.

“I went to the museum and I saw this kind of ridiculous variation across these really closely related species and it got me wondering,” Laidre tells Inverse. “And it just clicked that, wow, these things could be an adaptation that allowed them to not lose their shell during what’s one of the most dangerous and risky moments for them.” By “dangerous and risky moments,” he’s referring to sex. By “things,” he’s referring to the penises.

If a male hermit crab wants to mate, he needs to leave his shell.

Unsplash / Thomas Lipke

“Dangerous and Risky Moments”

When hermit crabs have sex, male crabs have to come out of their shells at least part way. Their goal is to transfer a packet of sperm called a spermatophore into a female’s shell and onto the outside of her body. While hermit crab sex doesn’t involve internal fertilization (where eggs are fertilized by sperm inside the body), the male crab still needs to get his business into the female crab’s shell. And to do this, he must leave his own.

"Really, you can view their whole social life as a bit of a waiting game and a bit Machiavellian.

But the hermit crab’s neighbors can sometimes be a mood killer. That’s because third-party intruders often target sexual pairs as they are mating and attempt to steal the male’s shell. In some cases, even the male’s sexual partner tries to take his shell. Everyone wants the shell, especially if its the sort that’s seen as the most valuable: spacious and light-weight, with a smooth, carved-out interior. Females like these shells because they can carry more eggs in them, and males like these shells because they can grow bigger — a key predictor of reproductive success.

Here is how an intruding crab would successfully evict another one: It would bolt out of its shell, leap into the new one, and — if it’s better — run away with a new house. All the while, however, it would have to maintain a grip on its original shell, in case it turns out the whole effort wasn’t really worth it and it needs to get back into its own. Sometimes this happens when the evictee is trying to have sex, but sometimes this also happens when the evictee is just minding its own business.

“Really, you can view their whole social life as a bit of a waiting game and a bit Machiavellian,” Laidre says. “If somebody bites the dust or someone else is weakening, the other crabs very quickly update on that, get to that spot, and sometimes, in a team effort, will try to pull that individual out of their shell.”

But when you’re a crab, a good way to avoid this is to have a big penis.

A "naked" hermit crab outside of his shell. The red arrow designates the length the length of his "'penis" or "sexual tubes." 

Mark Laidre

“Things” (aka Penises)

When you are a crab with a large penis trying to have sex, you don’t have to stray so far from your shell in the first place. Laidre determined this in his study by measuring the genitalia of land crabs in the genus Coenobita. This group includes the terrestrial hermit crab, which wear “remodeled” shells. He also examined closely related species of crabs, some of which modify their shells and some which do not. Measuring the penis-to-body-size ratio of 328 museum specimens led to the realization that crabs with more valuable, removable shells also had longer penises. Laidre writes:

“The results suggest larger penises evolved as morphological adaptations to facilitate safe sex, in which individuals retain their valuable property by extending a long penis outside the shell to copulate.”

This hypothesis builds on what Charles Darwin observed as he discovered that barnacles have the largest penis size relative to body size in the animal kingdom. Long penises, Darwin posited, evolved as specialized adaptations, allowing a creature to “overcome ecological challenges” because of their increased “sexual reach.”

And over time, strong selection pressure led to this scenario, which Laidre describes with his “private parts for private property” hypothesis. Larger shells with worn-down insides are more valuable and easily stolen because of their one-size-fits-all nature, so these shells are filled with crabs with a “significantly larger penis size” than species carrying less valuable property.

The term “property,” Laidre concedes, can be “a little anthropomorphic,” but animals do have things, like nests, burrows, and holes filled with acorns. Sometimes, they have shells that are so precious that losing them will result in death within 24 hours. You’ve got to take advantage of what you can to stay alive, especially when that thing helps you stay close to home.

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