Abstract
The diversity, variability, and apparent rapid evolution of animal genitalia are a vivid focus of research in evolutionary biology, and studies exploring genitalia have dramatically increased over the past decade. These studies, however, exhibit a strong male bias, which has worsened since 2000, despite the fact that this bias has been explicitly pointed out in the past. Early critics argued that previous investigators too often considered only males and their genitalia, while overlooking female genitalia or physiology. Our analysis of the literature shows that overall this male bias has worsened with time. The degree of bias is not consistent between subdisciplines: studies of the lock-and-key hypothesis have been the most male focused, while studies of cryptic female choice usually consider both sexes. The degree of bias also differed across taxonomic groups, but did not associate with the ease of study of male and female genital characteristics. We argue that the persisting male bias in this field cannot solely be explained by anatomical sex differences influencing accessibility. Rather the bias reflects enduring assumptions about the dominant role of males in sex, and invariant female genitalia. New research highlights how rapidly female genital traits can evolve, and how complex coevolutionary dynamics between males and females can shape genital structures. We argue that understanding genital evolution is hampered by an outdated single-sex bias.
Highlights
The tremendous diversity of male genitalia has been described as one of evolutionary biology’s greatest enigmas [1], and several hypotheses have been put forward to explain this diversity (Box 1). Hypotheses explaining this diversity include the lock-and-key hypothesis of species isolation [2], the pleiotropy hypothesis suggesting that genital morphology is due to pleiotropic effects of natural selection on other traits [3], female choice [4], sperm competition [5], and sexual conflict [6,7]
An influential 2004 review by Hosken and Stockley [1] concluded that the field has arrived at a general consensus that sexual selection plays an important role in the evolution of genitalia
Given that the field has greatly expanded since the publication of Hosken and Stockley’s review [1], we examined whether the initially observed male bias has changed by analyzing the number of studies that have investigated the evolution of male genitalia, female genitalia, or that of both sexes in each year from 1989 to 2013 (Box 2)
Summary
It has been proposed that this bias arises because male genitals are often more rigid and easier to study than female organs [11]. It may be that female genitalia with obvious and quite variable external elements encourage further investigation of internal components Female genitalia can even be polymorphic within species [15], clearly showing that female genital morphology is rapidly evolving and subject to active selective forces. Since it was first proposed, sexual selection theory has been influenced by cultural assumptions about males and females, such as Darwin’s initial proposal of females being generally ‘‘coy’’ [23]. In time and due to criticism from female perspectives, investigators have abandoned gender stereotypes, such as females being generally passive [25]
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