One of the major factors that determine the type of mating system a population will have is the operational sex ratio (OSR), the ratio of sexually available females to sexually available males. This ratio is usually male biased, which is why most species are polygamous, in which either sex shares multiple mates. Polygyny, where males mate with multiple females, is the most frequent form of polygamy. However, some species are polyandrous, where females mate with multiple males. When both sexes of a species mate several times with different individuals, there is a mixture of polygyny and polyandry. These species are said to be promiscuous.

The most frequently observed monogamy is behavioral monogamy. The male helps to raise young and guard his mate. This increases his own fitness by raising the successful survivorship of his offspring. This case is most frequently seen where the OSR is close to 1:1. Males who desert their mates will have a hard time finding other females because they will most likely already be taken. Even among so-called monogamous species, monogamy is often not strictly followed. Extra-pair copulations are extremely common. DNA fingerprinting has revealed that some offspring were not sired by their "father".

In situations where more males are ready to mate than females, we have the typical, polygamous case. Females are choosy and males are desperate to mate as often as possible. The mating choices of choosy females are influenced by four main benefits: direct benefits, the health of their mate, good genes, and exploitation of female preferences.

Direct Benefits

In mating with a particular male, some females may receive direct benefits, such as good territories. This is known as resource defense polygyny, where males fight for territory early in the breeding season, and females are attracted to males based on their territory. Another direct benefit comes in the form of nuptial gifts. The male katydid, for example, produces a spermatophore, which is basically a ball of sperm with highly proteinaceous material. The protein boost is very valuable to females when food is scarce, and they will even compete for matings to win this nuptial gift.

Health of the Mate

The second benefit a female receives by being choosy is phenotypically healthy mates. A brightly colored male suffers a cost, and by the honesty principle (see Honesty Principle ) he is probably a high quality male. It is also easier to see parasites on a bright background, and so a female can avoid catching a disease from her mate.

Good Genes

If the female does not receive a direct benefit, she will probably strongly be influenced by the desire for "good genes". The genotypic quality of the male is not necessarily apparent, but the phenotype provides a good clue. In barn swallows, for instance, there is a strongly negative relationship between a male's tail length and the number of parasites present on his offspring. This suggests a male with a long tail is more resistant to parasites than males with short tails. Once again, recall the honesty principle , which tells us that low quality males cannot afford to display costly traits.