Inbreeding tolerance predicts mechanisms of socially induced infertility in vertebrates.

In cooperatively breeding species, reproductively capable individuals often forgo breeding to assist others. This socially induced infertility is typically viewed as a consequence of cooperative breeding, yet the remarkable diversity of its mechanisms remains unexplained. We propose that cooperative living inevitably increases opportunities for reproduction among relatives, selecting for reproductive suppression to regulate the costs of inbreeding and reproductive conflict. Species-specific variation in inbreeding tolerance subsequently determines how suppression evolves. Where inbreeding tolerance is high, increased intragroup reproductive conflict favours strong physiological suppression of the hypothalamic-pituitary-gonadal axis to maintain social cohesion. Where inbreeding tolerance is low, behavioural mechanisms regulate reproduction while preserving reproductive competence. This framework explains mechanistic diversity and generates testable predictions across cooperative vertebrates.