Operational sex ratio

In the evolutionary biology of sexual reproduction, the operational sex ratio (OSR) is the ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate.^[1][2] It is different from the physical sex ratio, which takes into account sexually inactive or 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.^[3] The OSR is closely linked to the "potential rate of reproduction" of the two sexes;^[1] that is, how fast they each could reproduce in ideal circumstances. According to sexual selection theory, whichever sex is more abundant is expected to compete more strongly and the sex that is less abundant is expected to be "choosier" in who they decide to mate with.

The OSR is said to be biased toward a particular sex when sexually ready members of that sex are more abundant. For example, a male-biased OSR means that there are more sexually competing males than sexually competing females.

The operational sex ratio is affected by the length of time each sex spends in caring for young or in recovering from mating.^[4] For example, if females cease mating activity to care for young, but males do not, then more males would be ready to mate than females.

References

1. ^ ^{a b} Clutton-Brock, T. (2007). "Sexual Selection in Males and Females". Science 318 (5858): 1882–1885. doi:10.1126/science.1133311. PMID 18096798.  edit
2. ^ Kvarnemo, C.; Ahnesjo, I. (1996). "The dynamics of operational sex ratios and competition for mates". Trends in Ecology & Evolution 11 (10): 404–408. doi:10.1016/0169-5347(96)10056-2. http://linkinghub.elsevier.com/retrieve/pii/0169534796100562. Retrieved 2008-03-19. 
3. ^ Mitani, J.C.; Gros-louis, J.; Richards, A.F. (1996). "Sexual Dimorphism, the Operational Sex Ratio, and the Intensity of Male Competition in Polygynous Primates". The American Naturalist 147 (6): 966–980. doi:10.1086/285888. JSTOR 2463187. 
4. ^ Clutton-Brock, T. H.; Parker, G. A. (1992). "Potential Reproductive Rates and the Operation of Sexual Selection". The Quarterly Review of Biology 67 (4): 437–456. doi:10.1086/417793.  edit

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