AGGRESSIVE-BEHAVIOR AMONG CONVICT CICHLID (CICHLASOMA-NIGROFASCIATUM)FRY OF DIFFERENT SIZES AND ITS IMPORTANCE TO BROOD ADOPTION

Convict cichlids (Cichlasoma nigrofasciatum) exhibit extended biparent al care of their young. Parents will adopt unrelated (foreign) conspec ific young of similar size to or smaller than their own but reject lar ger foreign young. Adoption of smaller foreign young may benefit the p arents by reducing loss of their own young to predators by the dilutio n effect, which may be enhanced by differential predation on the small er young. Another factor influencing adoption is that larger foreign y oung may pose a direct predatory threat to the host parents' young. Me asures of aggression among free-swimming young of different sizes show ed that there was at least a 1 mm (standard length) size difference be fore larger young attacked smaller, a 3 mm difference before injury oc curred, and a 4-5 mm difference before predation occurred. Parents wer e initially more discriminating than their young with respect to the s ize of foreign young accepted. As their young grew and became stronger swimmers, parents less actively rejected larger foreign young; howeve r, they continued to reject them before they were large enough to pose a direct predatory threat. Parental rejection of relatively large for eign young is therefore based more on protecting their own young from differential predation than from aggression by larger adoptees.