Feb. 20, 2002 Boxes of chocolate and Valentine cards won’t get you far in the animal world, where courting is considerably tougher. New research confirms earlier beliefs: an evolutionary “battle of the sexes” can lead to a biological arms race between males and females.
We all know that males and females of most animal species look and behave very differently. Males are, for example, often provided with various “weapons”, bright colours or other ornaments. Females are, however, not easily impressed, and such differences between males and females have traditionally been explained by females search for good fathers for their offspring. A new and quite different explanation is instead based on different and fundamental conflicts of interest between the sexes.
Males and females play different roles in reproduction. What is best for one sex is therefore rarely best for the other. Such differences lead to a range of different sexual conflicts. For example, males of most animal species benefit from mating often with as many partners as possible, while females who are already mated instead stand to loose from mating with too many males. Males should thus seek to “convince” females to mate, while females should evolve resistance to males’ mating attempts. The traits which males may use in such conflicts may be of varying kinds, ranging from elaborate ornaments to mate grasping adaptations which makes it difficult for females to escape persistent males.
The result of such sexual conflict is in theory an “arms race” between the sexes, where male persistence will be matched by female resistance. Such arms races are, however, very difficult to study. The fact that male and female adaptations counterbalance, means that the underlying conflicts will often remain “hidden”. Both sexes may be running frantically, in an evolutionary sense, but their interactions may remain at a standstill.
New research conducted by Göran Arnqvist at the University of Uppsala, Sweden, and Locke Rowe at the University of Toronto, Canada, has for the first time unveiled the arms race between the sexes. In a study published in this week’s issue of Nature, they illuminates sexual conflict in a group of insects, water striders. By using a unique combination of comparative and experimental tools, the researchers show that the arms race between the sexes in these animals is indeed balanced, but not perfectly so. Male armament is not quite matched by females in some species, and females suffer very high rates of costly and superfluous matings as a consequence. In other species, females instead have a slight upper hand in the arms race, and males of such species are only able to mate very rarely. It is this relative balance between the sexes which determines their ability to achieve their interests. The study also shows that males are females rapidly move jointly up and down this seemingly endless coevolutionary spiral.
The new research not only confirms that sexual conflict can shape males and females, but also indicates that such conflicts can promote the formation generation of new species.
Arnqvist, G. and L. Rowe. 2002. Antagonistic coevolution between the sexes in a group of insects. Nature 415:787-789.
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