Researchers have long suspected that male genitalia evolved particularly rapidly, especially because they can be very different even among closely related species. Hemipenes—the paired male organ used by snakes and lizards—consist of a pair of tubular structures, and the surface of each hemipenis contains a groove through which semen is conveyed. The shape ranges from cylindrical to deeply bilobed, which can be ornamented with spines. A team studying the hemipenes of over two dozen lizard species have discovered that genital traits evolve much more rapidly than nongenital traits. The findings were published in the Journal of Zoology last week.

A trio of researchers led by Harvard’s Julia Klaczko examined the hemipenes of 25 Anolis lizards from the Caribbean. They measured the total length, width at the lobes, and width at the hemipenial body. They also measured three nongenital traits: thigh length, shank (or lower foreleg) length, and the length of the dewlaps, the loose skin that hangs down from their throat. For each of these traits, the team performed mathematical calculations to figure out their rate of evolutionary change. Some hemipenial morphological variation is pictured here: (a) Anolis litoralis, (b) A. evermani, (c) A. brunneus (d) A. cybotes, and (e) A. grahami, also used to illustrate hemipenial measurements (1: length; 2: width at the lobes; 3: width at the body). Scale bar = 1 mm.

Anolis hemipenes, they found, have been evolving up to six times faster than non-sexual traits. "That the differences were that high was a great finding," Klaczko tells New Scientist.

The team has at least two major ideas about why changes in hemipenes over time were so rapid. One possibility is that females have specific preferences for what fits and stimulates them better. Another less cooperative idea, Live Science reports, is that male and female lizards are locked in an evolutionary arms race, and both are trying to control reproduction: Males may be evolving genitals that give them an advantage when it comes to fertilizing the females, while females are evolving their genitals in an attempt to take that advantage back.

Images: Wikimedia (top), J. Klaczko et al., Journal of Zoology 2015 (middle)