Got my evo working (Image: James Christensen/Mindem)

THEY say size matters, but what of speed? In some lizards, penises have been evolving incredibly quickly – up to six times faster than other traits.

Fertilising eggs when they are still inside your mate is a tricky business. To rise to the task, male genitals can come in different shapes and sizes, even among closely related species of animals. This suggests penises evolve particularly quickly – something that has now been shown in Caribbean lizards for the first time.

Julia Klaczko of Harvard University and colleagues looked at 25 species of Anolis lizard and compared penis shape with two other, non-sexual traits: leg length and the size of the flap of skin on their necks.


Genital gallery: (a) Anolis litoralis; (b) A. evermani; (c) A. brunneus; (d) A. cybotes; and (e) A. graham. Scale bar represents 1 millimetre (Image: J. Klaczko, T. Ingram and J. Losos)

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 finding is extraordinary, agrees Kirsten Nicholson of Central Michigan University in Mount Pleasant. “The idea that genitals evolve more rapidly has been around for a long time. Now we have support.”

This fast evolution could be explained by female preferences for new male shapes that better fit or stimulate them, or by a gendered arms race. Wherever they have been examined, the female Anolis genitals match the shape of the corresponding penises, says Gunther Köhler of the Senckenberg Research Institute and Natural History Museum in Frankfurt. So it’s likely that female genitals are evolving just as fast.

Journal reference: Journal of Zoology, DOI: 10.1111/jzo.12178

This article will appear in print under the headline “Super-fast penis evolution seen in lizards”