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The adoption of unrelated young by brood-guarding convict cichlid (Cichlasoma nigrofasciatum) parents is influenced by several factors. Three of these are the risk of intrabrood aggression, the risk of differential predation of their own young, and the trade-off between the benefits of brood dilution and the costs of differential predation. This study introduces the "adoptable fry size hypothesis" model, which synthesizes the findings of earlier studies on each of these factors and details the range of adoptable fry sizes that increase foster brood survival across fry development. The influence of predation risk on the behaviour of parents toward foreign fry of different sizes was examined. Brood-guarding adults approached and mouthed at foreign fry larger than the upper limit of the model least often, but bit at them most often, thus excluding them. As host fry size increased, the number of behaviours directed at foreign fry decreased. Predator number did not significantly influence the size of foreign fry toward which the adults directed their behaviour. However, the overall frequency of these behaviours increased with increasing predator number, confirming previous studies suggesting that alloparental care is motivated by high predation risk.

This study examines the effect of parent size, fry growth rate, and habitat-related brood predation pressure on two measures of reproductive success (percent brood survival and fry number at independence from their parents) of free-ranging convict cichlids (Cichlasoma nigrofasciatum). Four populations in Costa Rican streams were studied, two in "pool" habitat and two in "stream" habitat. The number of fry that emerged from the spawning site (as free-swimming young) was positively correlated with female standard length (SL) for sites with a relatively wide size range of breeding females. SL of males had no effect on fry number at emergence, nor did size of either parent affect the probability of a brood reaching fry independence. Large parents reared more fry to independence than small parents in stream habitat but not in pool habitat. There was a significant difference among sites in fry number at emergence but not at independence (mean = 27 fry), suggesting a maximum number of fry that two parents can economically defend. Fry growth rate had no effect on either measure of reproductive success. Reproductive success was largely determined by the effect of habitat. Adult Cichlasoma dovii in pool habitat may have reduced mean brood survival from 47% (n = 126 broods) in stream habitat to 14.9% (n = 141 broods) in pool habitat. Predators such as juvenile C. dovii may be more effective in the dim ambient light in pool habitat, overwhelming the effect of parent size on brood defence.

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.

In many biparental species a sex difference in parental investment in a brood before fertilization, such as establishing a breeding territory or preparing a nest, may be an important component of the overall pattern of parental investment. Prefertilization investment patterns have been described for several species of biparental cichlid fishes, but there are discrepancies in the descriptions for the convict cichlid, Cichlasoma nigrofasciatum. This study describes quantitatively the prespawning behaviour of male and female convict cichlids and examines the influence on their behaviour of limiting the number of available spawning sites. The results indicate that pair formation begins several days before spawning when the female begins to follow a male and chase other females away from him. The pair then begins to spend more time in a smaller area where spawning eventually occurs. The breeding territory is usually established by the pair on the day of spawning. There was little difference in the pattern of prespawning behaviour offish with abundant versus limited spawning sites. It appears that neither sex will establish a territory until the female is ready to spawn and the pair has formed.

We staged contests between pairs of convict cichlids (Cichlasoma nigrofasciatum) that varied in weight ratio (larger/smaller) from 1.02 to 4.86 to test the predictions of the sequential assessment model. As predicted, contests proceeded in a consistent sequence of behaviour (lateral display, biting, mouth wrestling, and circling), the duration of contests was inversely correlated with weight ratio, and the duration of complete phases of behaviour was independent of weight ratio (a phase is complete if the contest escalates to the next behavioural phase). However, there was temporal overlap in the occurrence of different behaviour patterns and variation in the duration of contests that are not explained by the model. Nevertheless, the structure of convict cichlid contests is consistent with the logic of the sequential assessment model.

Parental care theory predicts that investment in current reproduction should be increased when the prospects of success in current reproductive effort are relatively high, and reduced when they are poor relative to expected success from future reproductive effort. A number of studies have shown that levels of postspawning parental investment (brood defence, parents' willingness to risk predation) increase when the brood is augmented. However, few studies have shown a change in pre-spawning (gonadal) investment in response to indirect indicators of reproductive success, such as nest site quality. Convict cichlids (Cichlasoma nigrofasciatum) are small freshwater fish that have biparental care of their young. Pairs of convict cichlids were required to spawn in either "risky" or "secure" spawning caves in the presence of potential egg predators. As predicted by parental care theory, females laid significantly more eggs in secure spawning caves than in risky spawning caves. The ability of this iteroparous fish to adjust gonadal investment in this manner would serve to optimize the use of its resources under varying environmental conditions and ultimately allow it to realize maximize lifetime reproductive success.

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Female Cichlasoma nigrofasciatum presented with three potential spawning partners of different sizes spawned most often near the medium-sized male. When mate choice was restricted to males of two size classes, females consistently spawned near the larger male. In the former experiment, small males courted females more actively than medium or large males did. This may explain why females occasionally spawned near small males in both experiments. Larger brood-guarding males showed more intensive aggressive behaviour towards an adult conspecific confined near their offspring than did smaller brood-guarding males. Selection will therefore favour the female choice of relatively large mates because such males are likely to be more effective defenders of their offspring.

Basic reproductive and behavioral data were collected from pairs of Cichlasoma nigrofasciatum breeding under controlled conditions. Increasing the density of potential breeding pairs increased the level of territorial aggression and decreased reproductive success. Time spent with their young by parents did not vary consistently with density. The importance of the increase in aggression and its possible influence on reproductive success are discussed.

Young Cichlasoma citrinellum contact parents preferentially, depending on sex and color of the parents. The number of epidermal mucus cells on adults' scales is positively correlated with the amount of contacting by the young. The epidermal thickness of these scales is negatively correlated with the amount of contacting. These correlations are discussed as possible cause–effect relationships with the contacting preferences of the young, and are compared to similar behavior in other cichlid fish species.

Experiments to test the feasibility of using radioactive I131 as a technique for evaluation of conspecifïc predation on fry by adult convict cichlids (Cichlasoma nigrofasciatum) are described. The technique is useful for short-term quantitative studies of predator–prey interactions where it is not possible to observe and quantify predation directly or where stomach content analysis of adults is not feasible. Key words: radioactive tagging, predator–prey interactions, Cichlasoma nigrofasciatum