Metabolic consequences of agonistic behaviour: Crab fights in declining oxygen tensions

The energetic consequences of fighting, which may depend on environmental c onditions, can be an important factor shaping contest strategy and duration . Energy expenditure may be costly to fitness because it depletes reserves that could otherwise have been allocated to reproduction, and metabolites a re produced that may constrain subsequent activities. We examined the varia tion in the metabolic consequences of fighting in relation to hypoxia. Cont ests were staged between pairs of size-matched male shore crabs, Carcinus m aenas L., under a range of water oxygen tensions (between 10 and 100% oxyge n saturation) which crabs experience in their natural habitat. Fighting und er normoxic and hypoxic conditions resulted in significantly elevated conce ntrations of haemolymph metabolites (L-lactate an. glucose) compared with c rabs at rest. However, these concentrations were much lower than in crabs t hat had been walking on a treadmill. Glycogen concentrations differed only under hypoxic conditions: glycogen stores were reduced in crabs after fight ing and this reduction was similar to that after exercise on a treadmill. C ontests were shorter when they were staged below a water Po-2 of 6.7 kPa (s imilar to 30% normoxia). As water oxygen tensions were reduced, fighting cr abs had greater concentrations Of L-lactate and glucose in their blood and tissues whilst glycogen stores were reduced. Fights became shorter when cra bs were exposed to severe hypoxia (Po-2=2 kPa) for increasing lengths of ti me, and blood L-lactate concentrations increased. The results suggest that as fights progressed, crabs experienced an increasing metabolic debt, in th e form of accumulation of L-lactate and a reduction in energy stores, which was amplified by hypoxic conditions. (C) 1999 The Association for the Stud y of Animal Behaviour.