Sex, economics, evolution, and stuckness all play roles in this new study about the evolution of larger penises in hermit crabs:
“Private parts for private property: evolution of penis size with more valuable, easily stolen shells,” Mark E. Laidre, Royal Society Open Science, epub 2019. (Thanks to Thomas Michel for bringing this to our attention.) The author, at Dartmouth College, explains:
the importance of private property in driving penis size evolution has rarely been explored. Here, I introduce a novel hypothesis, the ‘private parts for private property’ hypothesis, which posits that enlarged penises evolved to prevent the theft of property during sex. I tested this hypothesis in hermit crabs, which carry valuable portable property (a shell) and which must emerge from this shell during sex, risking social theft of their property by eavesdroppers. I measured relative penis size (penis-to-body ratio) for N= 328 specimens spanning nine closely related species. Species carrying more valuable, more easily stolen property had significantly larger penis size than species carrying less valuable, less easily stolen property, which, in turn, had larger penis size than species carrying no property at all.
Abby Olena has an essay about the new study, in The Scientist: “Larger Hermit Crab Penises May Prevent Shell Theft.”