My study was based on four breeding populations of convict cichlids in streams in Costa Rica. Males are most likely to desert when the number of young in the brood is small, when the young are getting close to independence, or when the brood is reared in an area where there are relatively few brood predators. The effect of brood predators is reduced by diluting the brood and creating the possibility that adopted young get taken by predators, thereby sparing young related to the adults providing the care. Small young are relatively weak swimmers and poor evaders of predators. During a predator attack, smaller, adopted young experience differential predation relative to the natural young.
Differential predation enhances the dilution effect.
In a field experiment, we demonstrated that brood defense in fish can be 'depreciable', i.e. efficiency of brood defense declines markedly with increases in the number of young in the brood.