By Mara Grunbaum To find a mate, most animals must travel—up a tree, down a stream, across the street to the bar. But not barnacles, which spend their entire adult lives cemented firmly to rocks, boats, whales and the like. To compensate for their immobility, barnacles have evolved the longest penises relative to body size in the animal kingdom. The appendages can reach up to ten times the length of the barnacles' bodies to allow them to search of a partner. See a video—safe for work!—below. According to new research published in Marine Biology, the shape of barnacles' penises varies depending on their circumstances. Barnacles spaced far apart from each other develop stretchier organs, the better for reaching across the gaps, and barnacles exposed to rough waves grow wider ones to stand up against the tide. Study author Matthew Hoch took advantage of the fact that barnacles grow new penises every mating season (a good thing, since the genitalia sometimes break off in the waves). In July 2005, before their reproductive tissues developed for the year, Hoch identified populations of acorn barnacles at five sites in Long Island Sound, and selectively removed barnacles so that some were densely crowded together, and others had space between them. Then, just before the barnacles' mating season began in mid-November, he collected organisms from each plot and dissected and analyzed their fresh-grown penises. Though overall length didn't change, Hoch found that sparsely arranged barnacles grew penises with more annulations--ringed folds that allow them to stretch farther, like an accordion or a bendy straw. Barnacles living right next to each other developed fewer folds. Though it's unclear exactly how they sense it, other barnacles' proximity seems to dictate what kind of genitals they grow. "Barnacles must perceive something related to the density of crowding and physical contact with neighbors as a cue for estimating the distance over which the penis must reach," Hoch writes. "Increasing the number of annulations of the penis most likely allows these mating barnacles to reach more distant neighbors." In addition, when he compared barnacles from sheltered harbor sites to those exposed to rough Atlantic waves, Hoch found that average penis diameter increased with wave strength. The thicker shape must provide structural support and reduce the risk of breakage in choppier waters, he concludes. The farther the penis meanders, the greater the potential for wave damage, so penis morphology probably comes down to risk versus reward, Hoch writes. If there are potential mates close by, it's not worth the extra energy it takes to grow a stretchy penis and the extra risk it takes to use it. But if the only other barnacles around are distant, the investment could pay off with a chance to reproduce. Lesson for life? Perhaps. But for now we're probably better off leaving the house. Related Content: Discoblog: And the Prize for World’s Largest Testicles Goes to… the Bushcricket! Not Exactly Rocket Science: Ballistic Penises and Corkscrew Vaginas -- The Sexual Battles of Ducks Not Exactly Rocket Science: Horrific Beetle Sex – Why the Most Successful Males Have the Spikiest Penises 80beats: Scientists Make Rabbit Penis Replacement Parts; Male Rabbits Rejoice

Video: Vimeo / Casey Dunn