Why Can't They Just Get Along?

Sexual Conflict (Princeton 2005) is a mind-expanding book by Göran Arnqvist and Locke Rowe.

Arnqvist and Rowe begin by recounting blood-curdling examples of war between the sexes: female robber flies playing dead in order to avoid sex; female penduline tits hiding their eggs from their partners and attacking them violently to protect the hiding place; male funnel spiders anesthetizing their sexual partners in order to avoid being killed and cannibalized by them.

Why all this conflict?

Arnqvist and Rowe write: "Biologists and laypeople have long regarded mating as a largely harmonious event in which males and females cooperate in producting offspring. In fact, if we assume the entire result of is the production of shared offspring, it is not immediately obvious why we should think otherwise."

So why is copulation so difficult for some species?--the waterfowl shown above, for instance? Biologists discovered recently (Brennan et al, PLoS ONE, 2007) that male mallards have penises with a clockwise twist while females have cloaca that twist the opposite way:

 
As the witty Allan Gu writes: "How could the female have sexual organs that are so elaborately designed to hinder insemination by males? It seems counterintuitive that female sexual organs would not be adapted to best fit male reproductive organs."

The answer lies in what Brett Holland and William Rice (Evolution 1998) have called "chase-away selection" (a contrast with Fisher's "run-away" model of sexual selection). Males and females of the same species often have conflicting reproductive strategies: different frequencies, places, times. Each attempts to nudge the other off its optimum.

Suppose, for example, that females of a species (and possibly males too) have a pre-existing sensory bias toward red. This might have nothing to do with sex--maybe they feed on red plants. Males, seeking to induce females to mate more often than is optimal for them (i.e. for the females) develop a red coloured spot.

These overly attractive males induce females to mate in a suboptimal manner (e.g., too often, less-than-ideal time or place). This counter-selects females to evolve resistance to (i.e., decreased attraction), rather than preference for, the male display trait, for example, a higher requisite stimulatory threshold to induce her mating response. Males are now selected to evolve a more extreme display trait to overcome the increased receiver threshold . . . and cyclic antagonistic coevolution ensues. (Holland and Rice)

Antagonistic selection is the opposite of Fisher's "run-away" selection, which begins with the pursuit of traits that would be adaptive in offspring. Sexual conflict is a hot new paradigm in evolutionary biology.

Incidentally, it is worth noting that sexual selection, particularly antagonistic selection, undermines an idea that many philosophers have found attractive: namely, that species are collections of similar organisms. For sexual selection often leads to radical sexual dimorphisms, or dissimilarities between males and females, in appearance, behaviour, and reproductive role.