Kinky Skinks Show That Size Matters in Speciation

By Veronique Greenwood | August 29, 2011 1:05 pm

skinks

When a male’s bits don’t fit with a female’s bits, you wind up with reproductive malfunction. But shape isn’t everything, as a team of researchers recently discovered while watching hundreds of skink lizards court and spark.

Most studies looking at how genitalia mismatch contributes to new species take the concept literally: if the bits don’t fit together like lock and key, matings will be unsuccessful. And if the mismatch between the gear of two groups is bad enough, they will form separate reproductive populations, and, eventually, species. But the idea, which was first tossed around more than 150 years ago, has been discounted as a possible source of new species. Differently sized or shaped genitalia is such a big change that it’s likely to come after many other speciation triggers, like mutations or long separations between populations divided by mountain ranges.

But, as this research team points out—and as anyone in the dating pool can tell you—there are other aspects of physical incompatibility that can have an effect on sex, and thus could get speciation started. If the mating posture, chemical cues, or timing are off, even having matching genitalia doesn’t mean a mating will work out. Thus, animals that have compatible gear could still start to split into two species if differences in courtship, size, flexibility, and so on between groups within a population are big enough.

That’s where the skinks come in. The researchers noticed that two species of skinks of different sizes—large and small—had arisen in a bunch of different places independently. In each case, the large skinks and the small skinks lived the same area, yet appeared to mate just amongst themselves. But in the lab, large and small would get busy with each other right away, going through all the motions of sex. No offspring, though, resulted.

The researchers wondered what was keeping the matings from being fertile and if it had contributed to the separation of the two species. They found that the body length difference was at the root of it: in skinks, the male coils himself around the female during copulation, and too big of a size difference between the pair means that the male’s hemipenis and the female’s cloaca, while technically compatible, can’t get close enough to touch (and if this is your first encounter with the wacky world of nonhuman genitalia, please, let us direct you hither).

Observing their lab population, the researchers crunched the numbers on the frequency of mismatched matings and matched matings, as well as how frequently large-small matings failed to happen for other reasons, like the female turning the male down. They found that size difference was about as likely to have contributed to the divergence of the species as behavioral differences, and in populations where the difference wasn’t especially large, size was especially likely to have begun the species split—a sign that size could be more of a driver in speciation that people realize.

Although if you’ve ever watched a daschund and a Great Dane sniff each other up, this may not be such a surprise after all.

Image courtesy of The American Naturalist

MORE ABOUT: mating, sex, skinks, speciation
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