Acceptance threshold theory can explain occurrence of homosexual behaviour

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.

同性性行为（Same-sex sexual behaviour，SSB）已在多种动物类群中被记录，但学界对其演化成因尚未形成充分认知。本研究基于里夫接受阈值理论（Reeve's acceptance threshold theory），对SSB展开探究。当识别过程无法做到完全无错时，雄性用于识别潜在交配对象的接受阈值应被灵活调整，以在错误接受雄性的代价与接受雌性的收益之间最大化适合度收益（fitness pay-off）。本研究通过调控雄性埋葬虫（burying beetles）搜寻雌性的时长及其繁殖潜力，改变了它们对接受或拒绝错误所对应的感知代价。正如预期那般，当拒绝雌性的代价升高时，雄性会采取更为宽松的识别决策，展现出更高水平的SSB；而当接受雄性的代价升高时，雄性则会更为严苛，仅展现出较低水平的SSB。本研究结果支持如下假说：在雌雄个体识别信号存在一定程度重叠的动物类群中，同性性行为是一种适应性识别策略的产物，该策略旨在规避错误拒绝交配对象所带来的代价。