Male water striders of the species Gerris gracilicornis aren't the most romantic bunch. Their mating strategy of choice can best be described as a "take-her-by-surprise" approach. Like a fencing champion with sword held horizontal and straight, they typically lunge at females, hoping to score a copulatory strike. But the ladies have evolved an even better defense. It's not in their interest to mate with every single male water strider that trickles past, so evolution has seen fit to erect a "genital shield" for the vagina akin to a castle drawbridge (see the picture above), so only the male they choose is allowed to enter. Now, instead of forcing themselves upon females, males have started advertising to attract mating opportunities. Advantage females.

The diversity of female genitalia within the animal kingdom is truly fascinating. Male seed beetles have spines on their penises, so females responded by padding their vaginal tracts with lots of connective tissue. Females of the earwig Euborellia plebeja have sperm storage organs. Female kangaroos have three vaginas!

Evolutionary biologists, however, seem preoccupied with the penis. According to a new analysis published in PLoS Biology, of the estimated 364 studies that have been conducted on genitalia since 1989, 48.6% (177) focused on male genitalia, while only 7.7% (28) examined female genitalia. The remaining reports studied both male and female genitalia.

Why the discrepancy? Authors Malin Ah-King, Andrew Barron, and Marie Herberstein, elucidate a few of the many possibilities. For one, male organs are overwhelmingly external, which makes them easier to study. Additionally, it's long been thought that female genitalia don't vary nearly as much as male genitalia, and thus offer fewer insights into animal reproduction, an assumption that burgeoning biological data is now showing to be false.

One explanation Ah-King, Barron, and Herberstein ruled out is gender bias from the investigators themselves. Their data shows that both male and female authors are just as likely to focus on the penis.

The authors believe that the most plausible explanation for why female genitalia are understudied is that antiquated stereotypes concerning sexual selection still haven't been completely shorn off. Dating all the way back to Darwin, it's been assumed that males enact sexual selection through competing for females. The females are just there to accept sperm.

"Too often the female is assumed to be an invariant container within which all this presumed scooping, hooking, and plunging occurs," the authors say.

But if female genitalia aren't considered, then researchers run the risk of overlooking key pieces of the evolutionary puzzle. Thus, Ah-King, Barron, and Herberstein urge evolutionary biologists to study male and female genitalia in tandem.

Early on in the paper, the authors offered up a gem of an analogy. Brimming with scientific sexual innuendo, it serves as a fitting conclusion:

Just as Darwin predicted reciprocal evolution of pollinator proboscis length and floral tube length, genitalia in internally fertilizing species ought to involve an evolutionary dynamic between both sexes.

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

(Images: PLoS Biology, Stellenbosch University)