Aggressive behaviour among convict cichlid (Cichlasoma nigrofasciatum) fry of different sizes and its importance to brood adoption

Convict cichlids (Cichlasoma nigrofasciatum) exhibit extended biparental care of their young. Parents will adopt unrelated (foreign) conspecific young of similar size to or smaller than their own but reject larger foreign young. Adoption of smaller foreign young may benefit the parents by reducing loss of their own young to predators by the dilution effect, which may be enhanced by differential predation on the smaller young. Another factor influencing adoption is that larger foreign young may pose a direct predatory threat to the host parents' young. Measures of aggression among free-swimming young of different sizes showed that there was at least a 1 mm (standard length) size difference before larger young attacked smaller, a 3 mm difference before injury occurred, and a 4–5 mm difference before predation occurred. Parents were initially more discriminating than their young with respect to the size of foreign young accepted. As their young grew and became stronger swimmers, parents less actively rejected larger foreign young; however, they continued to reject them before they were large enough to pose a direct predatory threat. Parental rejection of relatively large foreign young is therefore based more on protecting their own young from differential predation than from aggression by larger adoptees.