Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Convict cichlids (Cichlasoma nigrofasciatum) form monogamous pair bonds and have cooperative biparental care of their eggs, larvae and fry for up to six weeks. This fish is widely used in laboratory studies as a model of biparental care. However, very little is known of its habits in its natural habitat. I studied four natural populations of convict cichlids, in 'pool' and 'stream' habitat, in Costa Rica during the long dry season, December to June in 1990 and 1991. The number of pairs guarding broods increased from December to a peak in mid-March, then rapidly declined. Twenty-eight percent of 175 males bred more than once per season (range 1-4), often in the same territory. Females generally bred only once per season (range 1-2); only 5% of 230 bred twice. Evidence of inter- and intrasexual selection was provided by positive size assortative mating, different colour phases in females and marked differences in size between breeding and non-breeding fish in some sites. Female size was positively correlated with fry number at emergence at sites with a wide size range of breeding females. Parent size (female and male) was positively correlated with fry number at independence at sites in stream habitat. The most important factor determining reproductive success was habitat. Forty-seven percent (n = 126 broods) of broods reared in stream habitat survived to fry independence compared to 14.9% (n = 141 broods) in pool habitat. Males occasionally desert their mates and brood before the young reach independence, leaving the female to rear young to independence alone. I used a theoretical model to predict four sets of conditions when male mate desertion should occur most frequently: (1) when brood predation is relatively light; (2) when mating opportunities for males are relatively high; (3) when offspring are near independence; and (4) when brood size is below average for a given stage of brood development. Eight percent of 334 males deserted their mate and brood. Each of the predictions generated by the model was supported by the field data. The importance of the spawning site for brood survival at all brood stages, and as a future spawning site for males may constrain male mate desertion. The necessity to simultaneously guard the brood and a spawning cave may be an important factor maintaining biparental care in this species. Experimental male removals to test the effect of male loss resulted in young being transferred into the foster care of neighbouring conspecific broods. Brood adoption in the general population was widespread: 29% of 232 broods received foreign fry, 15% donated fry. In mixed broods, foreign fry were always similar to or smaller in size than the host fry. In field manipulation experiments, parents actively discriminated against foreign fry larger than their own. In a laboratory experiment on broods of mixed fry sizes, small fry were eaten more than large fry when exposed to two of their common predators. There are two ways that parents can increase the survival of their offspring by selectively accepting fry smaller than their own by: (1) a dilution effect; and (2) a differential predation effect. A lone female should transfer her fry when the chances of rearing the fry to independence are low.



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