Acceptance threshold theory can explain occurrence of homosexual behaviour

Abstract

Same-sex sexual behaviour (SSB) has been documented in a wide range of animals, but its evolutionary causes are not well understood. Here, we investigated SSB in the light of Reeve's acceptance threshold theory. When recognition is not error-proof, the acceptance threshold used by males to recognize potential mating partners should be flexibly adjusted to maximize the fitness pay-off between the costs of erroneously accepting males and the benefits of accepting females. By manipulating male burying beetles' search time for females and their reproductive potential, we influenced their perceived costs of making an acceptance or rejection error. As predicted, when the costs of rejecting females increased, males exhibited more permissive discrimination decisions and showed high levels of SSB; when the costs of accepting males increased, males were more restrictive and showed low levels of SSB. Our results support the idea that in animal species, in which the recognition cues of females and males overlap to a certain degree, SSB is a consequence of an adaptive discrimination strategy to avoid the costs of making rejection errors.

Increased Male-Male Mounting Behaviour in Desert Locusts during Infection with an Entomopathogenic Fungus

“…This finding is consistent with previous work on locusts housed in all-male groups 20 , and data from a wide range of taxa in biased sex ratio or single sex groups 21–24 . Such an effect is expected if an animal’s mate acceptance threshold is plastic, because the threshold should decrease when sex ratios are heavily biased and the costs of incorrectly rejecting a suitable mate are higher than those of incorrectly accepting an unsuitable one 9 . Consistent with this idea, male locusts emit a courtship-inhibiting pheromone that prevents rival males from displacing those guarding females, and may also help prevent same-sex sexual encounters.…”

A prison effect in a wild population: a scarcity of females induces homosexual behaviors in males

“…Overall, SSB may emerge and persist in natural populations because the costs for total SSB inhibition outweigh the costs of nonadaptive SSB (Thornhill and Alcock 1983;Burgevin et al 2013;Scharf and Martin 2013;Engel et al 2015). Mechanisms that eliminate SSB may negatively impact on HSB, for example, through a decrease of general sexual motivation if common neuroendocrine pathways are involved.…”

Experimentally evoked same-sex sexual behaviour in pigeons: better to be in a female-female pair than alone

“…The most common explanations include (i) social glue, which is a method to establish a strong social relationship, thereby reducing tension and conflicts 10 – 12 ; (ii) intersexual conflict, when the same-sex interaction establishes dominance in the hierarchy 13 or reduces the reproductive success of a competitor 14 ; (iii) practice, in which a young individual obtains mating experience 15 , 16 or gains the ability to improve their territory acquisition 17 ; or (iv) alloparenting, in which fluid sexuality of females may be a mechanism to possess allomothering investment from a female not related to the offspring 18 , 19 . On the other hand, non-adaptive explanations of same-sex sexual behaviour have been proposed, such as (i) mistaken identity, which explains this behaviour as an error due to lack of sex recognition 20 ; (ii) heterosexual deprivation when the population density is high and the sex ratio is skewed towards one sex 21 – 23 ; (iii) an evolutionary by-product, when selection acts on traits linked to sexual responsiveness 24 ; (iv) maladaptation if the individual is not well adapted to its environment 25 ; (v) intoxication, when methylmercury causes same-sex sexual behaviour and diminishes reproductive output as a result 26 ; or (vi) nutritional rewards, which is based on evidence that the brain network of the sexual response cycle pathway is similar to the pathway of the pleasure cycle during food acquisition, so that sex preference in later life depends on which sex was a caregiver during infancy 27 .…”