For a male hermit crab, sex is a dangerous business, says Stuart Blackman. With a penis tucked away safely within the coils of his shell, he must come out into the open to fertilise a female, which leaves him vulnerable to predators and to other hermits trying to steal his home – unless, that is, he has a very long reach.
Mark Laidre of Dartmouth College in the US has measured the relative penis size of nine species of terrestrial hermit crab. Some of these species modify their shell interior to their own requirements, which makes them particularly valuable to their owners and more attractive to thieves.
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Laidre found that species that put the most effort into remodelling their shells are also the most well-endowed.
“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,” writes Laidre.
He says it’s reasonable to expect that the best-endowed species are also less vulnerable to predators during their most intimate moments, although that will require more research.
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