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  1. Current game theory models and recent experimental evidence suggests that the strategy an animal adopts in agonistic encounters is determined by individual state. Therefore manipulation of an individual’s state should elicit different behavioural responses. In this paper, mechanisms are examined that underlie state-dependent strategies using Shore Crabs, Carcinus maenas, and how, by altering the environment, behaviour and physiology are affected.
  2. Fights were staged between pairs of male crabs under normoxic and severely hypoxic (< 15 torr) conditions to determine if the metabolic costs of fighting and resource acquisition are affected by water PO2. After fighting, blood and tissue samples from each crab were taken and analysed for metabolites associated with anaerobiosis (L-lactate, glucose and glycogen).
  3. The spectrum of behavioural acts performed during contests was unaffected by hypoxic conditions. However, fight duration was significantly shorter in the hypoxic treatment.
  4. The phenomenon of being of a larger relative size and winning had a greater influence in the contests staged under hypoxia with 93% of the victors being of a larger size compared to 78% in normoxic conditions. Fight duration and intensity had no relationship with relative size in either treatments.
  5. The accumulation of L-lactate was significantly greater in the blood and tissues of crabs after fighting under hypoxia than in normoxic conditions. In addition, there was greater glycolytic activity in the tissues of these crabs, shown by elevated concentrations of glucose in the blood and increased breakdown of glycogen.
  6. This study demonstrates that the internal state of the crabs altered the length of time they were willing to engage in fighting and that fighting was energetically more expensive under hypoxic conditions.


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