anolis lizard_2015_01_19
Image: Darkroom Daze, Flickr

Penises: they're not just for fun, they're also for science. It turns out that genitalia – especially male genitalia, for many species – are often a good way to distinguish closely related critters. Because they can be used to tell similar species apart, many have argued that sex organs evolve more rapidly than other parts of anatomy.

The problem with that line of reasoning is that there's no empirical proof that it's true, according to researcher Julia Klaczko of Harvard University's Museum of Comparative Zoology. Some researchers have described the differences in sexual anatomy between related species – but to really support the hypothesis a study would need to dig deeper into how quickly animal genitals evolved over time and how that rate of evolution stacks up with that of non-genital traits. "To our knowledge," write Klaczko and her colleagues, "no phylogenetic comparative study" has ever done that.

So the researchers turned to Anolis lizards, or anoles, for answers. They decided to look at these small, colour-changing lizards because they're so tremendously diverse: there are hundreds of closely related species with slight differences owing to the different ecological pressures they must contend with from island to island in the Caribbean. They have differences in their limb length and the size of their toepads, for example, depending on whether they live close to the ground or nearer to the treetops. They also have slight variations in their dewlaps, small flaps of skin under their necks that they use for social communication. Together, this allowed the researchers to compare the rates of evolution for the lizards' penises (genital traits) as well as for their limb length and dewlap size (nongenital traits). Their findings were published this month in the Journal of Zoology.

The penises of lizards, like other scaled reptiles, aren't really what you think of when you think of penises. They actually have a pair of organs called hemipenes (one of the pair is called a hemipenis). Klaczko describes them as "intromittent tubular structures" – which means they're cylindrical, more or less, and meant to be inserted into the females' sex organs, much like the penises with which you're likely more familiar. Rather than expelling semen from the interior, the surface of each hemipenis contains a groove called the sulcus spermaticus, which is used to direct the flow of semen. Depending on exactly which type of scaled reptile you're looking at, their shape "varies from cylindrical tubes to deeply bilobed structures, ornamented with calyces, papillae, flounces, and spines," the researchers explain. Sexy, right?

Hemipenis Anolis Lizard 2015 01 19
The hemipenes of some of the Anolis lizards studied, in all of their diverse glory. Image: Klaczko et al, 2015.

The team compared the hemipenes of 25 different Caribbean anoles, including the total length, the width at the lobes (the business end, so to speak) and the width of the main body (err, the shaft, sort of). In addition, they took measurements of limb length and dewlap length. 

Their discovery is actually quite straightforward: penises do evolve faster than non-genital traits. Not only that, but the three genital measurements the team took all evolved at similar rates – six times faster than limb or dewlap length.

The researchers are still not sure why the lizards' penises have evolved as fast as they have. It's likely thanks to sexual selection, but is it because females prefer some males over others? Or is it more antagonistic, evidence of a sort of genital arms race for control over reproduction, like the corkscrew penises and vaginas of ducks? Nobody knows yet, and the researchers offer several ideas for future research on the matter. "While the causes of genital evolution remain to be determined, the rapid rate at which hemipenes diversify is now clear," they conclude.

Anoles are already famous for their high rates of diversification. That their genitals evolve at (relative) hyperspeed "speaks to the significance of genital diversification in animal evolution," the researchers add.

Top header image: Richard Glor, Flickr