The giant mole-rat Cryptomys mechowi exhibits an extreme reproductive division of labour. Reproduction in the colony is restricted to a single breeding pair. Non-reproductive individuals do not breed unless removed from the colony and paired with unfamiliar conspecifics. Circulating basal concentrations of luteinizing hormone (LH) as well as LH levels measured in response to a single exogenous gonadotropin hormone (GnRH) challenge are not significantly different between the reproductive and non-reproductive groups of either sex. Neither non-reproductive males nor females are physiologically suppressed at the level of the pituitary, as they do not exhibit a reduced pituitary secretion of LH or decreased sensitivity to hypothalamic GnRH. Rather, reproductive skew seems to result from an inhibition to mate with familiar conspecifics. Familiar individuals housed in the absence of the breeding pair did not breed, whereas pairing unfamiliar individuals of the opposite sex resulted in sexual activity and recruitment. The results from the giant mole-rat suggest that reproductive skew may result from a predominantly behavioural repression in non-reproductive females arising from an incest avoidance mechanism.