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Sexually antagonistic coevolution (or SAC) is the biological phenomenon in which one sex of a species first develops an antagonistic adaptation toward the other, to which the latter sex responds by evolving a resistant counter-adaptation. If the two sexes continue evolving and compensating for one another, the species is said to be locked in an evolutionary arms race.

As there can be rapid developments over the course of generations, particularly in the sexual functions of a species, there are a number of circumstances which can induce this coevolution. Male seed beetles, for example, have been found to possess spiny genitalia, a sexual characteristic that is harmful to the female. The spines, by helping the male to more firmly anchor itself within the female, allow the male to transfer more sperm. This initial difference between reproductive priorities is known as a sexual conflict; though the male's spiny genitalia causes injury and a loss of fitness in the female, it provides the male with a competitive edge over its rivals, and so optimizes the male's potential for fathering offspring. However, despite the potential benefit of the male's adaptation, were the female's genes not to competitively coevolve, the consequence would be a decrease in female fitness, possibly endangering the survival of the species. And so, as a responsive counter-adaptation, the female seed beetle develops a thicker tissue along her reproductive tract; the predictable result being that as the male and female genitalia competitively evolve, neither sex loses substantial fitness, and the species population is maintained.

The duck is another animal recently associated with SAC, though the circumstances differ substantially from those of the seed beetle. Because 97% of all birds species lack a phallus at all, the duck was already considered sexually unusual by the scientific community; and scientific curiosity was only compounded further by the considerable lengths these phalluses could reach—the largest known phallus (relative to size) belonged to an Argentine Lake Duck, which at 17 inches was twice the size of its body. The initial explanation was similar to the seed beetle, that the duck's large phallus was a competitive evolutionary response to other male rivals. That was until recently; new research into the size and shape of female duck genitalia suggests that the duck's phallus evolved not as an adaptation but as a counter-adaptation. Similar in shape to the long, corkscrew phallus, the female duck's oviduct was discovered to be a length of spirals and dead ends. Additionally, by examining different duck species, a precise correlation was found between the lengths of the male and female genitalia. Dr. Patricia Brennen, the ecologist responsible for the discovery, suggests the theory that male sexual aggression caused the female oviduct's elaborate development. Male ducks are known to rape females, sometimes injuring or even killing them in the process. Therefore, the complexity and length of the oviduct, Brennen reasons, is most likely an anatomical check against undesirable mates; the dead ends and pockets of the oviduct potentially allow the female duck to collect and then eject unwanted sperm. Brennen supported her theory by pointing out previous studies which claim that while 1/3rd of duck matings are forced, those encounters only account for 3% of total duck offspring. The relative failure of forced mating suggests that the evolution of the oviduct necessitated a counter-development in the entire male population for mating, forced or otherwise.

And then we have the African bat bug. Like their relatives the bed bugs, male bat bugs are known for their sharp, hypodermic genitalia. Ignoring the female's reproductive tract entirely, the male bat bug instead pierces directly into her abdomen, a process called traumatic insemination. Though the sperm continues to find its way to the ovaries, the damage to the female bat bug is considerable. And so, as an evolutionary response, the female bat bugs have developed paragenitalia (known as the "spermalege") on their abdomen. This paragenitalia prevents injury by guiding the male organ into reservoirs of spongy tissue. However, while this is strange, researchers were more surprised to find the effect it had on the male bat bug population. Because male bat bugs would commonly pierce the abdomens of other males as well, male bat bugs began adapting after the example set by the female bat bugs, developing similar paragenital defenses. And stranger still, in some populations, the female bat bugs were observed to have mimicked the male paragenitalia, resulting in greater efficiency and less stabbings.



  1. "Evolutionary arms race" Wikipedia
  2. "Sexual conflict" Wikipedia
  3. "Coevolution between harmful male genitalia and female resistance in seed beetles" by Johanna Rönn, Mari Katvala & Göran Arnqvist, Proceedings of the National Academy of Sciences (May 14th, 2007)
  4. "In Ducks, War of the Sexes Plays Out in the Evolution of Genitalia" by Carl Zimmer, the New York Times (May 1st, 2007)
  5. "The Largest Penis in the World" by Stefan Anitei, Softpedia (January 5th, 2007)
  6. "Bat Bugs Evolved Fake Genitals to Avoid Sex Injuries" by Anne Minard, National Geographic News (September 25th, 2007)
  7. "The Most Bizarre Sex: Abdominal Stabbing Penetration, Sperm Through Blood, Defensive Para-Genitalia and Female-to-Male Transsexuality" by Stefan Anitei, Softpedia (September 22nd, 2007)
  8. "Traumatic insemination in the plant bug genus Coridromius Signoret (Heteroptera: Miridae)" by Nikolai J Tatarnic, Gerasimos Cassis & Dieter F Hochuli, Biology Letters (November 2nd, 2005)
  9. "Afrocimex constrictus" Wikipedia

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