Operational sex ratio

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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, [1] [2] [3] or alternatively the local ratio of fertilizable females to sexually active males at any given time. [4] This differs from physical sex ratio which simply includes all individuals, including those that are sexually inactive or do not compete for mates.

Contents

The theory of OSR hypothesizes that the operational sex ratio affects the mating competition of males and females in a population. [5] 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. [6] 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. Usually variation in potential reproductive rates creates bias in the OSR and this in turn will affect the strength of selection. [7] 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.

Some factors that affect OSR

The operational sex ratio is affected by the length of time each sex spends in caring for young or in recovering from mating. [8] 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, affects 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. [9]

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. [10] [11] [12] [13] 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. [10] [14] [15] [16] Another example would be if, in a certain species, males provided care for offspring and a nest. [17] If the availability of nesting sites decreased, we would see the population trend towards a more female biased OSR because only a small number of males actually have a nest while all the females, regardless of a nest or not, are still producing eggs. [18]

Some factors that OSR predicts

A major factor that OSR can predict is the opportunity for sexual selection. As the OSR becomes more biased, the sex that is in excess will tend to undergo more competition for mates and therefore undergo strong sexual selection. [4] [8] [19] Intensity of competition is also a factor that can be predicted by OSR. [2] 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. It would be expected that when an OSR is more biased to one sex than the other, that one would observe more interaction and competition from the sex that is more available to mate. When the population is more female biased, more female-female competition is observed and the opposite is seen for a male population where a male biased would cause more male-male interaction and competitiveness. Though both sexes may be competing for mates, it is important to remember that the biased OSR predicts which sex is the predominant competitor (the sex that exhibits the most competition). [10] [20] [21] OSR can also predict what will happen to mate guarding in a population. As OSR becomes more biased to one sex, it can be observed that mate-guarding will increase. This is likely due to the fact that rival numbers (number of a certain sex that are also ready to mate) are increased. If a population is male biased then there are a lot more rival males to compete for a mate, meaning that those who have a mate already are more likely to guard the mate that they have. [22]

Further examples of factors that affect OSR

See also

Related Research Articles

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Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with, and compete with members of the same sex for access to members of the opposite sex. These two forms of selection mean that some individuals have greater reproductive success than others within a population, for example because they are more attractive or prefer more attractive partners to produce offspring. Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males.

<span class="mw-page-title-main">Sexual dimorphism</span> Condition where males and females exhibit different characteristics

Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most dioecious species, which consist of most animals and some plants. Differences may include secondary sex characteristics, size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection. These differences may be subtle or exaggerated and may be subjected to sexual selection and natural selection. The opposite of dimorphism is monomorphism, when both biological sexes are phenotypically indistinguishable from each other.

<span class="mw-page-title-main">Lek mating</span> Type of animal mating behaviour

A lek is an aggregation of male animals gathered to engage in competitive displays and courtship rituals, known as lekking, to entice visiting females which are surveying prospective partners with which to mate. A lek can also indicate an available plot of space able to be utilized by displaying males to defend their own share of territory for the breeding season. A lekking species is characterised by male displays, strong female mate choice, and the conferring of indirect benefits to males and reduced costs to females. Although most prevalent among birds such as black grouse, lekking is also found in a wide range of vertebrates including some bony fish, amphibians, reptiles, and mammals, and arthropods including crustaceans and insects.

<span class="mw-page-title-main">Behavioral ecology</span> Study of the evolutionary basis for animal behavior due to ecological pressures

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<span class="mw-page-title-main">Sex ratio</span> Ratio of males to females in a population

A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites.

<span class="mw-page-title-main">Sperm competition</span> Reproductive process

Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. However, multiple mates for a female means each individual male has decreased chances of producing offspring. Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase male's chance of reproductive success. Sperm competition results in a sexual conflict between males and females. Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition. Offensive tactics of sperm competition involve direct interference by one male on the reproductive success of another male, for instance by physically removing another male's sperm prior to mating with a female. For an example, see Gryllus bimaculatus.

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References

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