Biased biology: the case of the missing vaginas

The female genitalia of the water strider Gerris gracilicornis have a genital shield that can block forced mating. Han, C. S. & Jablonski, P. G. PLOS One 4, e5793 (2009).

Genitalia are a hot topic. Interest in their diversity and rapid evolution have seen research in the field balloon in the past decade. Stories on studies of the penises of ostriches, chickens, sea slugs and a variety of insectshave all madethe science pages. But where are all the female genitalia?

A study published in PLOS Biology this week has quantified their dearth. Analysing 25 years of research in the evolution of genitals, the authors found a strong bias towards studying male animals a disparity that has gotten worse, not better, over time. The bias, they say, is down to ingrained female stereotypes that lead researchers to see female genitalia as less important to evolution something that, they argue, is hampering our understanding.

Malin Ah-King, an evolutionary biologist and gender researcher at Humboldt University in Berlin, and colleagues analysed 364 papers published between 1989 and 2013 that address the evolution of genitalia, and categorised them by research question and species studied. They found that the largest group, 49%, looked at male genitals alone. Just 8% of the papers looked only at female genitals, while 44% studied both (see graph below).

Why the disparity? The study found that the bias is not restricted to male researchers, as papers by women biologists showed the same trend. Nor is the phenomenon something that can be blamed on old attitudes: it seems to grow stronger from 2000 onwards, even after a similar study in 2004 flagged the issue.

The authors also dismiss the notion that female genitalia are any less scientifically interesting than those of males, citing a range of studies where variations are biologically significant including the genital shields of water striders (Gerris gracilicornis, pictured above) and the elaborate, corkscrew vagina of the long-tailed duck (Clangula hyemalis). There are a number of studies showing large variation in female genitals, both within and between species. But theres a lack of knowledge and of studies, says Ah-King.

Another reason could be that that the bias comes from male organs being easier to study than female ones. Johan Hollander, an evolutionary biologist at Lund University in Sweden, says that at least in the species he studies, sea snails, it is hardly surprising that males are studied, as their genitalia are external. They also present easy-to-see characteristics that are specific to each species, making them useful for taxonomy, he adds.

The authors recognise that this plays a part, as is evident, for example, in their finding that the bias is reduced in species where the female sex organs are external, such as spiders. But it is not the whole story, they say. Plenty of new techniques, such as high-resolution X-ray scanning, make research of internal, soft-tissue organs possible, says Ah-King.

Instead, they argue that at the root of the problem are long-standing assumptions about the roles of the sexes in evolution, namely the assumption that the females role is passive and relatively unimportant. This dates back to Charles Darwins theory of sexual selection, which the authors say proposed that females are generally coy. Darwins contemporaries even cast doubt over whether females had the mental abilities to choose mates, they add.

While many of these assumptions have been overturned, evolutionary theory still emphasizes the male side of the equation, leaving studies of the females role to lag behind, say authors. We think assumptions about the dominant role of males and lack of variation in females have influenced how people have been looking at these questions, says Ah-King. The fact that the study found that the bias varied depending on which evolutionary mechanism researchers were tackling, suggests that certain questions steer researchers towards focusing on males, she adds.

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Biased biology: the case of the missing vaginas