Fighting animals must make a series of decisions, and understanding the proximate causes behind these decisions can give insight into how they are made. For example, fights have been analysed with respect to energetic costs and endocrine changes associated with engaging. However, another mechanism for the control of demanding activity, such as fighting, is the modulation of aerobic capacity by divalent metal ions. Here we examine post-fight haemolymph concentrations of magnesium (Mg2+) and calcium (Ca2+) ions in the common European hermit crab Pagurus bernhardus. Hermit crabs fight over the ownership of gastropod shells, where they adopt two very different roles during the encounter: attacker and defender. Despite the two roles performing different activities, we found that Mg2+ and Ca2+ affected them similarly, with concentrations of these ions being highest in successful individuals. Haemolymph concentrations of Mg2+ and Ca2+ were also found to increase as a result of fighting, and these elevated levels will, via allosteric interactions, increase the oxygen affinity of the respiratory pigment haemocyanin, enhancing respiratory capacity and therefore fighting ability. Furthermore, the present study revealed that seasonal changes in circulating levels, along with the ability of competitors to respond to them, may ultimately influence an individual’s success in aggressive interactions.