Size does matter, at least when it comes to some hermit crabs, who appear to have evolved longer penises so they can stay in their shells to protect their homes during sex.

Crabs that live in shells face the risk that interlopers will try to steal their homes while they are otherwise distracted by mating. That risk is even more pronounced for certain types of hermit crabs, who “remodel” their shells by removing some internal structures. The renovations make the shells more spacious and desirable, but also more difficult to cling onto, particularly in the throes of passion.

“In theory, longer penises could enable individuals to reach out to sexual partners while simultaneously maintaining a safe grip on their property with the rest of their body, thus safeguarding property against thieves while having sex,” Mark Laidre, an assistant professor of biological sciences at Dartmouth University, wrote in the study in Royal Society Open Science, published on Wednesday.

To test the theory, he set about measuring members — specifically, seeing how the penises of 328 specimens of various types of hermit crab stacked up.

Mr. Laidre found that the hermit crab with the largest penis relative to body size was the Coenobita species, the only one occupying the remodelled and more vulnerable shell homes. Crabs with unremodelled homes presented the next longest penises, while those who shed their shells as adults were found to have the shortest of all three types.