Metabolic costs of fighting are driven by contest performance in male convict cichlid fish

Abstract

One of the most important decisions that an animal faces is whether to persist or surrender in an aggressive contest. It pays to win these encounters, but it might be just as economical to evaluate fighting costs and adjust behavioural decisions accordingly. Relatively few studies have explored the physiological basis of contest costs and fewer have determined the internal signals that animals might attend to during combat. We explored relationships between physiological parameters, including energy substrates, anaerobic metabolites and steroid hormones, and both contest performance and contest success in male convict cichlids, Amatitlania nigrofasciata. Size-matched pairs were allowed to fight until one member of the pair submitted, and tissues were subsequently harvested for analysis. There were no significant status differences (winner, loser, control) in mean postfight physiological measures. Dominance reinforcement during a short postsettlement period buffered the winners’ stress response. In both winners and losers, muscle lactate concentrations correlated positively with contest intensity, while only losers showed a significant increase in plasma glucose concentrations as contests intensified. Larger cichlids used more costly fighting tactics than smaller conspecifics, and size asymmetries affected contest outcomes between larger animals significantly more than they did those between smaller animals. These findings suggest that fighting is metabolically costly and that the ramifications of these costs may differ in a size-dependent fashion. We use a recent literature on the scaling of anaerobic tolerance to evaluate the possibility that larger cichlids may be more tolerant of lactate accumulation, and thus able to both engage in more intense fighting tactics and persist longer in escalated fights than marginally smaller opponents. Contest intensity varied independently of mass asymmetries but varied positively with the absolute size of both the winner and loser, suggesting that convict cichlids use self-assessment strategies during contests with similarly sized opponents.