Why are larger convict cichlid (Cichlasoma nigrofasciatum) fry sometimes adopted into brood of smaller fry?

Convict cichlids (Cichlasoma nigrofasciatum) are substrate-brooding fish, native to Central America, with extended biparental care of young. Parents in the field and laboratory readily adopt foreign young of similar size to or smaller than their own, but usually reject larger foreign young. Under certain circumstances, the adoption of unrelated young has been shown to be adaptive to both donating and foster parents through (1) the dilution effect: the probability that their own young will be captured by a predator is reduced as the brood size increases: and (2) the differential predation effect: smaller young are more easily caught by brood predators than are larger young. These effects are not mutually exclusive. It has been demonstrated that parents of young with a standard length (SL) of 8 mm or bigger reject larger foreign young less often than do parents guarding smaller young, suggesting that the differential predation effect, and interfry aggression, may only be at work at smaller young sizes. In this study relative vulnerability of young to small gape predators (sub-adult conspecifics) decreased with increasing size of young. When young were most vulnerable to predators (less than 8 mm SL) there was little or no benefit from dilution with larger young. However, as the young grew, the benefit of dilution with larger fry increased.