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Why Do Guys Have Spines on Their Genitals?

It's a clear, pristine midsummer night in the skies over eastern North America, and love is in the air... No literally, lovemaking is taking place in the air.

The sky is aflutter with the flapping wings of mating eastern red bats. Males and females join together in a deft, midair maneuver, then subsequently drift down to the ground, locked in coitus. It's a beautiful act between two majestic flying mammals, made possible by the male's gigantic, nightmarish penis spines.

Sorry to spoil the moment.

The spines are hypothesized to serve as copulatory clamps during aerial mating, but that's not all. They also may provide sensory feedback for the male bats, and believe it or not, stimulate the release of female sex hormones. How's that for a turn on, ladies?

journal.pone.0047586.g003.pngThe penile spines of adult male hoary bats.

Penile spines certainly aren't exclusive to bats. They're actually fairly widespread throughout the animal kingdom. Hamsters, parasites, mice, and beetles are but a few of the critters that sport varying forms of penile spines, some hardly noticeable, others positively horrific.

Fruit flies in particular have spines of extraordinary length (relatively speaking). The large barbs belonging to Drosophila ananassae are absolutely vital to male mating success.

The sex scene for this species of fly is best described as a scrambling free-for-all. Hundreds of fruit flies will gather on, say, a wayward orange, and manic feeding ensues, along with intercourse, lots of intercourse. Thus, males evolved penile spines to latch onto and mate with as many females as possible during the mayhem. Recently, researchers at the University of Cincinnati found that flies which have their penile spines removed were unable to copulate at all, demonstrating how important they really are.

Penis spines aren't merely present in animals on the lower rungs of the evolutionary ladder. Chimpanzees have them, too. The spines on the adult chimp member are described as hard, corny bumps about 0.35mm wide. They're certainly not intended for "hooking up" in the literal sense (like in fruit flies), so what's their purpose?

As David Kingsley of Stanford University School of Medicine told Discovery News, the spines "have been proposed to do many different things, including increasing

stimulation in males, increasing stimulation in females, removing

copulatory plugs left by other males or even inflicting minor damage

during mating so that females are less receptive to sexual intercourse

with other males."

Just in case you weren't sufficiently irked by this blush-inducing discussion of prickly phalluses, I've got one more detail that might very well rile up some indignity: Our ancestors were adorned with penis spines as well. Don't fret however; they were certainly the less pronounced "bumpy" variety, perhaps similar to those of modern-day primates. They weren't of the frightening variety, like those of eastern red bats. Scientists hypothesize that these bumps may have been used to remove the previously deposited sperm of other male suitors. They also believe that the trait was lost when our species eschewed instinctive promiscuity in favor of socially evolved monogamy.

If penile spines are linked to promiscuity, perhaps that's why wanton sex remains such a thorny subject?

(Photo Source: Cryan PM, Jameson JW, Baerwald EF, Willis CKR, Barclay RMR, et al.

(2012) Evidence of Late-Summer Mating Readiness and Early Sexual

Maturation in Migratory Tree-Roosting Bats Found Dead at Wind Turbines.

PLoS ONE 7(10):

e47586.

doi:10.1371/journal.pone.0047586)