Effects of group size, space, and 3-D structure on behavior in captive Midas cichlids

Abstract

Animals may perform elevated levels of aggression in captivity, which may be a response to the modified costs and benefits of resource defense imposed by their artificial environments. The Midas cichlid (Amphilophus citrinellus) is a species whose patterns of aggression appear to fit predictions of resource defensibility. Two experiments were performed to test the effects of small-scale changes in group size, available space, and habitat complexity on aggression to determine if juvenile Midas cichlids modify behavior under different conditions of defensibility. Proportions of time spent in aggression were not associated with group size or available space, but submissive behavior performed by subordinates and the amounts of body damage they received were. Aggression was lower in the presence of 3-D structure. Behavior in the experiments was then compared to that observed in a large zoo exhibit (large group size) and in nature (large available space) to investigate the effects of large-scale differences in defensibility. Aggression was highest under the more defensible, experimental conditions. Midas cichlids increased aggression under defensible conditions, but were unable to maximize net benefits by adjusting aggression according to fine-scale changes in defensibility in artificially small group sizes and enclosures, which resulted in aberrantly detrimental effects on subordinates. Captive aggression in the absence of food or mating motivation suggests that space was defended as a resource, but it may result as a default due to restrictive artificial conditions that do not provide opportunities for alternative activities. Regardless, it has serious animal welfare implications. Some alternative housing tactics that do not promote aggression may nevertheless be suboptimal as they restrict behavioral diversity.