Public Release: 

Battle of the sexes leads to evolutionary arms race

University of Toronto

Boxes of chocolate and Valentine cards won't get you far in the animal world where courting is considerably tougher than it is for humans. New research by evolutionary biologists from the University of Toronto and the University of Uppsala in Sweden confirm earlier beliefs that an evolutionary "battle of the sexes" can lead to a biological arms race between males and females.

The study, to be published in the Feb. 14 issue of Nature, unveils for the first time a sexual "arms race" in a group of insects - the water strider. Professors Locke Rowe of the University of Toronto's zoology department and Goran Arnqvist of the University of Uppsala in Sweden offer a new and quite different look at the fundamental conflicts of interest between the sexes.

Photo credit: Locke Rowe of the University of Toronto

The male antennae (on the right) is far larger than the female's. For this species of water strider (Rheumatobates rileyi), the male uses his antennae to grasp the female near the eye during the pre-mating struggle in an attempt to subdue her.

Full size image is available through contact

"Males and females of most animal species look and behave very differently," says Rowe. "Males are often provided with various distinct traits like the bright colours of a peacock's feathers, for example. Females, however, aren't easily impressed. These elaborate traits in males have traditionally been explained as evolutionary consequences of females searching for good fathers for their offspring."

The key to their research lies in the fact that males and females play very different roles in reproduction. What is best for one sex is rarely best for the other, leading to a range of sexual conflicts, says Rowe. "Males of most animal species benefit from mating often with as many partners as possible while females, who are already mated, lose from mating too much. Males, therefore, seek to 'convince' females to mate while females evolve resistance measures to foil the male's mating attempts."

The traits which males use in these conflicts vary, ranging from elaborate ornaments to grasping structures that make it difficult for females to escape. The result of such sexual conflict is, in theory, an "arms race" between the sexes whereby male persistence is matched by female resistance. Such arms races are, however, very difficult to study, say the researchers. The fact that the male and female adaptations counterbalance each other means that the underlying conflicts often remain hidden. Thus, while both sexes may be frantically battling in an evolutionary sense, their match essentially remains at a standstill.

Photo credit: Jens Rydell & Ingela Danielsson of the University of Uppsala

A male and female water strider (Gerris lacustris) engaged in a violent, pre-mating "wrestling match."

Full size image is available through contact

Using a combination of comparative and experimental tools with 15 different species of water striders, the researchers show that the arms race is indeed balanced, but not perfectly so. In the case of the water strider, the male evolved grasping structures aimed at immobilizing the female as he attempts to mate with her. To counteract this, the female evolved spines that hold the male away from her body and foil his mating attempts.

In some cases with the water striders, males are much more successful in their quests and females suffer very high rates of costly and superfluous matings. In other species of the insect, females have a slight upper hand and males are able to mate very rarely.

The researchers say their study can also be applied to male and female animals as they move up and down this seemingly endless co-evolutionary spiral. The new research not only confirms that sexual conflict can shape males and females - in the case of water striders, males evolved grasping hooks while females developed spines - but also indicates that such conflicts can promote the generation of new species, they say.


This study was supported by the Swedish Natural Science Research Council, the Natural Sciences and Engineering Research Council of Canada, the Swedish Foundation for International Cooperation in Research and Higher Education, the Knut and Alice Wallenberg Foundation and the Magnus Bergvalls Stiftelse.

The University of Toronto, Canada's leading research university with 55,000 students, is celebrating its 175th anniversary in 2002. On March 15, 1827, King George IV signed a charter granting a university for Upper Canada, named King's College, forerunner of today's U of T. The university now comprises 31 divisions, colleges and faculties, including 14 professional faculties, as well as numerous research centres, galleries on three campuses and Canada's largest university library system - the third largest research library in North America.


Janet Wong
U of T Public Affairs

Locke Rowe
U of T Department of Zoology

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