Sex pheromone

Sex pheromones are pheromones released by an organism to attract an individual of the same species, encourage it to mate with it, or perform some other function closely related with sexual reproduction. Sex pheromones specifically focus on indicating females for breeding, attracting the opposite sex, and conveying information on species, age, sex and genotype. Non-volatile pheromones, or cuticular contact pheromones, are more closely related to social insects as they are usually detected by direct contact with chemoreceptors on the antennae or feet of insects.

Insect sex pheromones have found uses in monitoring and trapping of pest insects.

Sex pheromones have evolved in many species. The many types of pheromones (i.e. alarm, aggregation, defense, sexual attraction) all have a common cause acting as chemical cues to trigger a response. However, sex pheromones are particularly associated with signaling mating behaviors or dominance. The odors released can be seen as a favorable trait selected by either the male or female leading to attraction and copulation. Chemical signaling is also used to find genetically different mates and thus avoid inbreeding. Females are often selective when deciding to mate, and chemical communication ensures that they find a high-quality mate that satisfies their reproductive needs.

Odours may be a kind of male "ornament" selected for by female choice. They meet the criteria for such ornaments that Charles Darwin set out in The Descent of Man, and Selection in Relation to Sex. After many years of study the importance of such chemical communication is becoming clear.

Males usually compete for scarce females, which make adaptive choices based on male traits. The choice can benefit the female directly and/or genetically. In tiger moths (Utetheisa ornatrix), females choose the males that produce the most pheromone; an honest signal of the amount of protective alkaloids the male has, as well as an indicator of the size of female offspring (females fertilised by such males lay more eggs). Male cockroaches form dominance hierarchies based on pheromone "badges", while females use the same pheromone for male choice. In oriental beetles (Exomala Orientalis), females release the pheromone and passively wait for a male to find them. The males with superior detection and flying abilities are most likely to reach the female beetle first which leads to a selection for genetically-advantageous males.

In most species, pheromones are released by the non-limiting sex. Some female moths signal, but this is cheap and low risk; it means the male has to fly to her, taking a high risk. This mirrors communication with other sensory modalities, e.g. male frogs croak; male birds are usually colourful. Male long-range pheromone signals may be associated with patchy resources for the female. In some species, both sexes signal. Males can sometimes attract other males instead, the sex pheromone acting as an aggregation pheromone.

It is likely that most externally fertilizing species (e.g. marine worms, sea urchins) coordinate their sexual behaviour (release of sperm and eggs) using pheromones. This coordination is very important because sperm are diluted easily, and are short-lived. Coordination therefore provides a selective advantage to both males and females: individuals that do not coordinate are unlikely to achieve fertilisation and hence to leave offspring.

The main selective advantage of outcrossing is that it promotes the masking of deleterious recessive alleles, while inbreeding promotes their harmful expression.