Brood mixing and reduced polyandry in a maternally mouthbrooding cichlid with elevated among‐breeder relatedness

Uniparental maternal brood care often coincides with multiple paternity and single maternity of broods, possibly reflecting benefits of polyandry and costs of uniparental care. Genetic data from the maternally mouthbrooding cichlid fish Simochromis pleurospilus revealed the opposite pattern – low polyandry and allomaternal care. More than 70% of the investigated females had mated with a single male, and 14% of the females had unrelated fry in their broods. Broods with foreign fry were in the late stage of brood care, in which females guard free‐swimming fry and recall the broods into their mouths for protection. With one exception, fostering females were related to their adopted fry at the level of first cousins (RQG > 0.12), but relatedness between fosters and adopted fry was not significantly higher than between fosters and fry tended by other females. Relatedness among breeders extended to the level of first‐order relatives. Mean relatedness among contemporaneously breeding dams (RQG = 0.08) was significantly higher than among dams breeding in different seasons (RQG = −0.04), which suggests a temporal or spatial concentration of mouthbrooding relatives. Indeed, females sometimes brood in small groups. This behaviour may reduce brood predation but will increase the risk of brood mixing, which is possibly mitigated by low costs of brood care and indirect benefits accrued by relatedness among the breeders in the group. Remarkably, the apparent inbreeding potential did not give rise to bet‐hedging polyandry or active avoidance of relatives, as half of the mated individuals were related at RQG > 0.13 and polyandry did not coincide with high within‐pair relatedness.