In the evolutionary biology of sexual reproduction, the operational sex ratio (OSR) is the ratio of sexually competing male to females that are ready to mate. It is different from the physical sex ratio in that physical sex ratio also takes into account sexually inactive individual organisms, and sexually non-competitive individuals. This concept is especially useful in the study of sexual selection since it is a measure of how intense sexual competition is in a species, and also in the study of the relationship of sexual selection to sexual dimorphism.
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