Active inbreeding in a cichlid fish and its adaptive significance

Levels of inbreeding are highly variable in natural populations [1, 2]. Inbreeding can be due to random factors (like population size), limited dispersal, or active mate choice for relatives [3]. Because of inbreeding depression [4], mating with kin is often avoided [5], although sometimes intermediately related individuals are preferred (optimal outbreeding [6, 7]). However, theory predicts that the advantages of mating with close kin can override the effects of inbreeding depression [8-12], but in the animal kingdom, empirical evidence for this is scarce. Here we show that both sexes of Pelvicachromis taeniatus, an African cichlid with biparental brood care, prefer mating with unfamiliar close kin over nonkin, suggesting inclusive fitness advantages for inbreeding individuals. Biparental care requires synchronous behavior among parents. Since parental care is costly [13], there is a conflict between parents over care [12, 14], which can reduce offspring fitness [15]. Relatedness is expected to enhance cooperation among individuals [16]. The comparison of the parental behavior of in- and outbreeding pairs showed that related parents were more cooperative and invested more than unrelated parents. Since we found no evidence for inbreeding depression, our results suggest that in P. taeniatus, inbreeding is an advantageous strategy.