The Influence of Operational Sex Ratio on the Intensity of Competition for Mates
Date
2011-02
Authors
Weir, Laura K.
Grant, James W. A.
Hutchings, Jeffrey Alexander
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Abstract
The evolution and maintenance of secondary sexual characteristics and behavior are heavily
influenced by the variance in mating success among individuals in a population. The operational sex
ratio (OSR) is often used as a predictor of the intensity of competition for mates, as it describes
the relative number of males and females who are ready to mate. We investigate changes in
aggression, courtship, mate guarding, and sperm release as a function of changes in the OSR using
meta-analytic techniques. As the OSR becomes increasingly biased, aggression increases as
competitors attempt to defend mates, but this aggression begins to decrease at an OSR of 1.99,
presumably due to the increased costs of competition as rivals become more numerous. Sperm release
follows a similar but not significant trend. By contrast, courtship rate decreases as the OSR
becomes increasingly biased, whereas mate guarding and copulation duration increase. Overall,
predictable behavioral changes occur in response to OSR, although the nature of the change is
dependent on the type of mating behavior. These results suggest considerable flexibility of mating
system structure within species, which can be predicted by OSR and likely results in variation in
the strength of sexual selection.
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Citation
Weir, Laura K., James W. A. Grant, and Jeffrey A. Hutchings. 2011. "The Influence of Operational Sex Ratio on the Intensity of Competition for Mates." American Naturalist 177(2): 167-176.