THE DILUTION EFFECT AND DIFFERENTIAL PREDATION FOLLOWING BROOD ADOPTION IN FREE-RANGING CONVICT CICHLIDS (CICHLASOMA-NIGROFASCIATUM)

Parental investment in unrelated offspring is potentially maladaptive but may be promoted by natural selection if the presence of foreign yo ung enhances the survival of the parents' own young. We experimentally augmented broods of free-ranging convict cichlids (Cichlasoma nigrofa sciatum) to test whether survival of the adopting parents' young (fry) increases, in relation to that of control broods, after the addition of smaller foreign fry, and whether such an increase can be attributed to the effect of brood dilution acting alone or to a combination of b rood dilution and the effect of differential predation on adopted youn g. Total fry survival did not differ between experimental (E) broods a nd control (C) broods, but E broods had significantly more large (host ) fry after 5 days and 10 days than C broods did. In E broods, small ( foreign) fry suffered higher rates of predation than large fry, indica ting differential predation. In E broods starting at 7.0 and 7.5 mm st andard length (SL), observed fry mortalities did not differ significan tly from mortalities expected from the effect of brood dilution. Howev er, E broods starting at 8.0 mm SL had significantly lower mortalities than expected, indicating that parents that adopt smaller foreign fry can increase the survival of their own fry by the combined effects of brood dilution and differential predation. Within E broods, growth of smaller foreign fry was significantly slower than that of larger host fry, suggesting that intra-brood agonistic behaviour affects access t o food for smaller fry. Therefore, increased predation and reduced gro wth are two negative effects that act on fry of donor parents.