Operational sex ratio
Encyclopedia
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. 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
. The OSR is closely linked to the "potential rate of reproduction" of the two sexes; 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. 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.
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
, 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. It is different from the physical sex ratio
Sex ratio
Sex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms....
, which takes into account sexually inactive or non-competitive individuals.
This concept is especially useful in the study of sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural 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
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...
. The OSR is closely linked to the "potential rate of reproduction" of the two sexes; that is, how fast they each could reproduce in ideal circumstances. According to sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural 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. 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.