On the evolutionary pathway of parental care in mouthbrooding cichlid fishes

Evolutionary theory predicts that differences in parental care patterns among species arose from interspecific differences in the costs and benefits of care for each sex. In Galilee St Peter's fish, Sarotherodon galilaeus (Cichlidae), male care, female care and biparental care all occur in the same population. We exploit this unusual variability to isolate conditions favouring biparental versus uniparental mouthbrooding by males or females. We first review a gametheoretic model of parental care evolution, predictions of which we test experimentally in this paper. Manipulations of the operational sex ratio show that males and females desert their offspring more frequently when the costs of care are high (in terms of lost mating opportunities). Breeding trials with males of different sizes show that small fathers desert more frequently than large fathers. We attribute this to the associated difference in the fitness benefit of biparental care relative to femaleonly care. Our experimental results confirm that in St Peter's fish the probability of caring is determined facultatively according to current conditions at each spawn. The experiments and model together suggest that interspecific variation in remating opportunities and clutch size may be responsible for differences in care patterns within the subfamily Tilapiini. Our results support the hypothesis that biparental mouthbrooding was the ancestral state of both male and female uniparental mouthbrooding in cichlid fishes.