Operational sex ratio

In the evolutionary biology of sexual reproduction, 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, or alternatively the local ratio of fertilizable females to sexually active males at any given time. Its difference from the physical sex ratio, is that it does not take into account sexually inactive or non-competitive individuals (individuals that do not compete for mates).

The theory of OSR hypothesizes that the operational sex ratio affects the mating competition of males and females in a population. 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. Usually variation in potential reproductive rates creates bias in the OSR and this in turn will affect the strength of selection. 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, thus creating a male biased OSR. One aspect of gestation and recovery time would be clutch loss. Clutch loss is when offspring or a group of offspring is lost, due to an accident, predation, etc. This in turn effects how long reproductive cycles will be in both males and females. If the males were to invest more time in the care of their offspring, they would be spending less time mating. This pushes the population towards a female biased OSR and vice versa. Whether or not it is the males or females investing more care in their offspring, if they were to lose their offspring for whatever reason, this would then change the OSR to be less biased because the once occupied sex becomes available to mate again.

As aforementioned, another major factor that influences OSR is potential rate of reproduction (PRR). Any sexual differences in the PRR will also change the OSR, so it is important to look at factors that change PRR as well. These include constraints to environmental factors such as food or nesting sites. For example, if males are required to provide a nutrient high gift before mating (most likely food) then when nutrients available is high, the OSR will be male biased because there is plenty of nutrients available to provide gifts. However, if nutrients is low, less males will be ready to reproduce, causing the population to have a female biased OSR. Another example would be if, in a certain species, males provided care for offspring and a nest. If the availability of nesting sites decreased, we would see the population trend towards a more female biased OSR because only a small amount of males actually have a nest while all the females, regardless of a nest or not, are still producing eggs.

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