Evolutionary Biologists Should Study Female Genitalia More

Evolutionary biologists are unduly interested in male genitalia and don't pay enough attention to their female counterparts, according to a Perspective piece appearing in PLoS Biology. By focusing on just one side of the story, researchers are allowing preconceptions and biases to limit their picture of the world and shape the conclusions they draw.

So what's the big deal about genitalia, anyway? Evolutionary biologists are mainly interested in how the diversity of genital shapes sheds light on processes like sexual conflict and reproductive isolation. For example, the "lock-and-key" hypothesis suggests that genital morphology can act as a barrier to reproduction between species, since the male's "key" will only fit in species-specific "locks". The authors write that the idea has dwindling support thanks to a lack of variation in female genitalia, but they also note that "taxonomic data on female genitalia are scarce, which may explain why studies that have relied on such data have failed to find evidence for coevolution of male and female genitalia." In other words, it's hardly surprising that the "locks" all seem to look the same if you only look at a handful of them.

It's not the first time this has been pointed out. In 2004, a review paper concluded that sexual selection is a driving force behind genital evolution, but a response paper emphasized the lack of attention given to the female role in sexual dynamics. Distressingly, the problem seems to have gotten worse, not better. For their Perspective paper, the authors surveyed published studies from 1989 to 2013, classifying them based on which genitalia they examined (male, female, or both) and noting the taxonomic group studied and the evolutionary mechanism credited for generating the variation. Nearly half of the papers they studied focused solely on male genitalia and another 43.7% studied both males and females, leaving a scant 7.7% attending exclusively to female genitalia. Although the rate of publication has roughly doubled since the 2004 reviews, the proportions have remained stubbornly persistent.

Does this skew really reflect a bias? Maybe female genitalia are just much harder to study? The authors disagree, pointing out that the skew is different depending on which mechanism is being investigated. Studies of the lock-and-key mechanism (which have shown a disappointing "lack of variation among females") show a remarkably strong skew, with 70% of the studies focusing only on males! By contrast, investigations of "cryptic female choice", where female genitalia can store and sort sperm from multiple males, were much more balanced; genitalia of both sexes were studied in 86% of the papers, with the rest evenly split between the two sexes. Clearly, the practical hurdles can be overcome — if there's enough interest.

Well, maybe female genitalia just aren't interesting enough. That doesn't seem to be true, though. When we have bothered to look, female genitalia have proven just as elaborate and interesting as that of males. Female ducks, for example, have an elaborate corkscrew vagina to guide (and misguide) the ballistic, spiralling penis of the males. Female Drosophila have paired 'pocket-like' structures in their genitalia to cope with the paired spines on the male genitalia. In many species, male genitalia have evolved to remove sperm from previous matings — their competitors' sperm — from a female. "Too often the female is assumed to be an invariant container within which all this presumed scooping, hooking, and plunging occurs," wrote the authors. In fact, the shape of female genitalia plays a decisive role. Male earwigs, for example, have genitalia as long as their bodies, topped with a fringe of hairs at the tip, presumably to help get rid of their competitors' sperm; the female genitalia, however, are even longer, leaving sperm selection up to her. My favourite example is the water strider Gerris gracilicornis. Apparently the females have evolved a genital shield to block forced copulation, forcing the males to court them instead — wow! In response, the males have started tapping the surface of the water while mounted on a female; the resulting ripples attract fish, and since the female is under the male, she's more likely than him to become a meal. Females can avoid this grisly end by giving in to the male's intimidation and mating with him. What a bizarre system...

The authors argue that the main reason for the skew is an undue bias shaped by our cultural assumptions, and I'm inclined to agree with them. I've written elsewhere that science isn't conducted in a vacuum, but is shaped by (and shapes) our cultural heritage and other intellectual baggage. As the authors point out, sexual selection theory is heavily influenced by cultural assumptions, such as "Darwin's initial proposal of females being 'coy'" or his contemporaries' uncertainty regarding female choice since they doubted "whether females had the mental ability to execute male choice". Likewise, modern assumptions — about females being less eager and more choosy, for example — continue to shape the field. I wasn't at all surprised to read that the bias in these studies didn't depend on whether the researchers studying genital evolution were males or females. The same was true in a recent gender-inequality study; male and female scientists both offered the female applicant a lower salary. Cultural biases run deep, and implicit bias is hard to recognize and even harder to cope with. It's heartening to see some acknowledgement that science, like every other human endeavour, is not immune to such biases — after all, the first step is admitting you have a problem.

Reference
Ah-King M, Barron AB, Herberstein ME. Genital Evolution: Why Are Females Still Understudied? PLoS Biology 12(5): e1001851. (2014) doi: 10.1371/journal.pbio.1001851