Mouthbrooding in the black-chinned tilapia, Sarotherodon melanotheron (Pisces: Cichlidae): the presence of eggs reduces androgen and estradiol levels during paternal and maternal parental behavior.

The black-chinned tilapia (Sarotherodon melanotheron) is considered a paternal mouthbrooder in which the male churns the eggs in his mouth for 14-18 days after they are fertilized. We previously showed that plasma androgen and estradiol concentrations increased significantly between the beginning and end of mouthbrooding. Here we provide evidence that androgen and estradiol are relatively high in prespawning male tilapia, decrease significantly with the initiation of mouthbrooding, and return to and slightly surpass prespawning levels by the end of mouthbrooding. We then test the hypothesis that the initiation of mouthbrooding causes a decrease in androgen and estradiol. We raised single pairs of tilapia in seawater aquaria and conducted two experiments. In the first, we removed eggs from males on the morning of Day 1 of mouthbrooding, followed changes in plasma androgen and estradiol in the afternoon of Day 1 and on Days 6, 11, and 16, and compared them to levels in control males who kept their eggs. Removing eggs from mouthbrooding males on the morning of Day 1 eliminated the decrease in plasma androgen and estradiol observed in the afternoon of Day 1 of mouthbrooding. This demonstrates that the eggs are necessary for the decrease. We observed that, infrequently, the female of the pair would pick up the eggs to brood them in her mouth. In a second experiment, in the afternoon of Day 1, we sampled both the male and female of pairs in which the female brooded eggs. In female-brooding pairs, the females had significantly lower androgen and estradiol levels than prespawning females. The black-chinned tilapia in captivity exhibits both paternal and maternal parental behavior. In both sexes the initiation of mouthbrooding triggers a decrease in androgen and estradiol concentrations. We conclude that the presence of eggs inhibits the pituitary-gonadal axis in both males and females and hypothesize that a chemical signal from the eggs is delaying the initiation of the next brood. Copyright 2000 Academic Press.