The series of experiments that comprise this thesis used shell fights in the hermit crab Pagurus hernhardus as a model system to address the proximate physiological and whole-organism correlates of contest behaviour. These correlates of fighting ability rangec from the metal ions magnesium (Mg ) and calcium (Ca ), through respiratory pigments and muscular proteins to whole-organism performance capacities and behavioural syndromes. The work presented here thus demonstrates that a suite of physiological and whole organism variables influence contest behaviour in hermit crabs, and that both aerobic and performance capacities are very important in determining agonistic success.