In many species of animals, males and females have a conflict of evolutionary interests. Males compete with each other for the opportunity to fertilize the eggs of females. Males use all sorts of strategies in these competitions. They fight with each other for territory, they scare off intruding males, they put scrapers into females to dump out the sperm from previous males, and they inject “anti-aphrodiasiacs” to make females unreceptive to other males.
A number of experiments suggest that females have to pay a steep price for these male shenanigans. Anti-aphrodisiacs are toxic to the females, shortening their lifetime. Why would males harm the females that carry their offspring? In many species, males can mate with many females. The long-term health of any one female doesn’t matter–in an evolutionary sense–to the male.
As natural selection favors increasingly deadly male mating strategies, this onslaught opens up the opportunity, in turn, for the evolution of counterstrategies in females. In some species, females may evolve antidotes to male poisons. The males, in turn, may evolve counterattacks to overcome these new defenses. Theoretically, this coevolution can become a never-ending cycle of sexual conflict, capable of producing some of nature’s greatest extravagances (like absurdly kinky ducks).