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The behavior and ecology of the firemouth cichlid were investigated in southern Mexico. Observations were conducted primarily at Laguna Bacalar, Quintana Roo. The fish bred throughout the year, nesting in small rocky crevices. Territories were established either by wandering pairs or by lone males which subsequently attracted mates. Stationary territories were maintained for two to three weeks by both pair members until the young were free-swimming, after which the parents defended the young as they foraged. Brood care lasted approximately three months. The distances at which fish were attacked and the behavior performed varied with the age of the defended young and the sex of the parent, as well as the species and size of the intruder. The sexes differed in their reproductive and agonistic activities. Predation on young was rapid if the parents were diverted from their defense. Serious injuries to adults were not observed to result from intraspecific contests. Such interactions were very common, but were ritualized with physical contact between animals rare. Color patterning varied with breeding interval, sex and behavior.

The agonistic behaviour of pairs of hermit crabs (Pagurus bernhardus) was observed in the laboratory. It was found that relative size was a reliable predictor of the outcome of a contest only when the crabs differed by at least 100% in weight. However, weight differences of 10 to 30% could be sufficient to cause differences in behaviour. It was concluded from this that whereas crabs can accurately assess small differences in weight, these differences may not be sufficient to reliably determine the outcome of contests. Other factors affecting RHP are relevant at intermediate size differences. Two possible cues for size assessment were tested: the size of the major cheliped and the size of the opponent's shell. The presence and size of the major cheliped were found to have a strong effect on contest initiation and outcome, and therefore this is likely to be a cue used in RHP assessment. The major cheliped was also found to play an important part in defensive behaviour, crabs lacking this appendage being less successful in defending their shells from attack. However, the apparent size of the opponent's shell did not seem to affect the initiation or outcome of contests. Thus it seems that crabs assess their opponents independently of the shells they occupy. It is concluded that large size differences are sufficient to decide contests. For smaller size differences other factors of RHP are important, and can override size differences. There is no evidence indicating that the defending crab can assess the quality of its oppo- nent's shell and make contest decisions based on this evidence. These data do not support the negotiation hypothesis.

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