Lizard penises evolve at a rapid rate—likely in an effort to satisfy the changing shape and size of female lizard genitals.
The finding doesn’t come as a huge surprise; biologists have long assumed that the male genitalia of species which fertilize internally evolve more quickly than other physical characteristics. But this study is the first quantitative analysis of genital differences within a cluster of related lizard species.
The study , published this week in the Journal of Zoology, confirms that sexual selection is a powerful driving force of physical change in the sex organs—an idea that makes sense in theory but has never been confirmed in practice.
The scientists investigated male genitals in 25 Caribbean Anolis lizard species and, compared the penis rate of evolution to that of other physical traits using a phylogenetic, or family, tree. The genus is a favorite of evolutionary biologists since their members speciate—or branch into new species—rapidly when their habitat changes, for example, or an offshoot group becomes separated from the main population. The morphology of their penises can vary significantly, so much so that it can help scientists classify an individual.
Here’s Penny Sarchet, writing for New Scientist:
Hemipenes—as the genitals are known—come in a wide variety of shapes, but Klaczko’s team focused on their length, width and lobe size. They looked at how quickly these features changed on the Anolis family tree. The results surprised them. Anolis hemipenes have been evolving six times faster than the other traits. “That the differences were that high was a great finding,” says Klaczko.
The researchers haven’t yet scrutinized the corresponding female parts, but they appear to match the shape of their corresponding penises, suggesting that they’re evolving just as quickly. Perhaps, the authors write, the females aren’t just being choosy about their partners. A theory called sexually antagonistic coevolution—an “evolutionary arms race for control over reproduction,” as the study’s authors put it—could explain why both male and female genitals are evolving so rapidly.