Monogamy and Aggression

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This project explores the relationship between a male and a female in a monogamous pair of fish, the Midas cichlid of Nicaragua. As is typical of monogamous species, males and females look alike and have the same behavior patterns. Pair formation is accompanied by high levels of threat display as each sex probes to determine that the other has the wherewithal to obtain and protect a breeding territory, and to defend the eggs and young against predators. When arbitrarily selected, only about 50% of pairs put together successfully mate; aggression gets out of control in the others. Some fish lose their dark pigments and become strikingly gold in color. The gold-colored fish inhibit aggression in their opponents; thus nature has provided a unique experimental tool for exploring pair relationships. The experiments are of two types. In the first, Dr. Barlow will test three alternative hypotheses in aquaria. Each makes different predictions about the success of pairing depending on the aggressiveness of the individuals. In the second experiment, Dr. Barlow will explore the relationship between color, aggressiveness, and sex, using a large pool situation. In the field, the fish mate predominantly with those of the same color, but experiments have proved that they have no preferences by color that would predict such an outcome. The effect of gold color on aggression probably produces assortative mating in the field; if so, this can tell us something important about mate compatibility. Dr. Barlow will also make a field trip to Nicaragua to test conclusions and predictions. This project departs from the prevailing vogue. Current theory on mating has led to an emphasis on polygynous species, in which the sexes look strikingly different from one another, mate quickly, then separate. Mates in monogamous species remain together and have a greater need to adjust aggressiveness. The Midas cichlid offers the possibility of being a model species for understanding intersexual aggression in monogamous species in general. Note that spousal abuse is exceedingly common in humans; in the USA, a spouse is abused on average every 15 seconds. Finally, the Midas cichlid is the most important food fish in Nicaragua; information about its reproductive behavior is useful in managing and culturing this species.