Aerobic capacity influences giving-up decisions in fighting hermit crabs: does stamina constrain contests?

Strategic decisions in animal contests have been increasingly understood in terms of the physiological consequences of fighting, in particular by investigating energetic costs. In addition to these costs, the scope for vigorous agonistic activity may be modified by the aerobic capacity of individuals, engaged in a fight, which can be linked to circulating levels of respiratory pigment. Greater concentrations of respiratory pigment should increase aerobic capacity, thus enhancing aerobic respiration, postponing the onset of anaerobia, and reducing the amount of metabolic acidosis incurred. We examined postfight levels of the respiratory pigment haemocyanin in the hermit crab Pagurus bernhardus. The concentration of haemocyanin was higher in winners than losers, but was also positively correlated with body size. Residuals calculated for how much each individual deviated from this relationship revealed that individuals that had more positive residuals were more likely to win aggressive encounters. Thus, greater levels of respiratory pigment appear to enhance agonistic performance via their influence on endurance and aerobic capacity.