Competition for shells among hermit crabs can be fierce, and well kept homes left unattended may be quickly snaffled by rival creatures shopping for an upgrade. In a new paper titled "Private parts for private property," a curious Dartmouth College biologist describes an evolutionary phenomenon through which a famously staunch home-owner of the crustacean world is able to protect its living quarters.

While acknowledging that evolution has driven great diversity among the private parts of the animal kingdom, Mark Laidre, an assistant professor of Biological Sciences at Dartmouth, wondered how the value of an individual's private property might factor into this.

His "private parts for private property" hypothesis builds on observations made by Charles Darwin midway through the 19th century, when he discovered that barnacles possess the largest penis size relative to body size in the animal kingdom, at around eight or nine times the length of its body. The famed biologist suspected that these longer organs had evolved to allow for insemination of potential mates bound to different, but nearby, locations.

It was this, combined with much more recent research describing the tendency of some animals to seek out and privatize resources as a means of survival, that motivated Laidre's unique line of enquiry. To test out his hypothesis, the scientist enlisted 328 hermit crabs across nine different species and measured their penises relative to body size.

This enabled him to tease out some useful correlations. He says crabs that spent time making space to grow or store goodies by shaping their shells' interiors (thereby crafting more valuable homes) were endowed with a "significantly larger penis size" than hermit crabs burdened with less desirable dwellings. Those crabs did, however, still have larger penises than those with no shells at all.

Laidre says these patterns could not be explained by other means and says that the results indicate larger penises had evolved to allow for greater freedom when mating. It is Laidre's conclusion that the longer crabs spend modifying their shells, the longer their penises will likely to be. This allows them to continue occupying their property while getting down to business, as opposed to their less fortunate rivals who have to temporarily vacate in order to procreate.

The research has been published in the journal Royal Society Open Science.