Impact of an ecological factor on the costs of resource acquisition: fighting and metabolic physiology of crabs

1. Current game theory models and recent experimental evidence suggests tha t the strategy an animal adopts in agonistic encounters is determined by in dividual state. Therefore manipulation of an individual's state should elic it 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 affect ed. 2. Fights were staged between pairs of male crabs under normoxic and severe ly hypoxic (<15 torr) conditions to determine if the metabolic costs of fig hting and resource acquisition are affected by water P-O2. After fighting, blood and tissue samples from each crab were taken and analysed for metabol ites associated with anaerobiosis (L-lactate, glucose and glycogen). 3. The spectrum of behavioural acts performed during contests was unaffecte d 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 grea ter influence in the contests staged under hypoxia with 93% of the victors being of a larger size compared to 78% in normoxic conditions. Fight durati on and intensity had no relationship with relative size in either treatment s. 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 increa sed 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 w as energetically more expensive under hypoxic conditions.