Increased aggression during pregnancy comes at a higher metabolic cost.

Aggressive behaviour is linked to fitness, but it is metabolically costly. Changes in metabolic demand during the reproductive cycle could constrain activity and thereby modulate behavioural phenotypes. We predicted that increased metabolic demands in late pregnancy would lead to reduced aggression and a lower metabolic cost of behaviour in the mosquitofish Gambusia holbrooki. Contrary to our prediction, females became more aggressive in late pregnancy, but metabolic scope (i.e. the metabolic energy available for activity and behaviour) decreased. Consequently, late-stage pregnant females spent significantly more of their available metabolic scope on aggressive behaviour. Hence, as pregnancy progressed, females showed increasingly risky behaviour by depleting metabolic resources available for activities other than fighting. We argue that the metabolic cost of behaviour, and possibly personality, is best expressed with reference to metabolic scope, rather than resting metabolic rates or concentrations of metabolites. This dependence on metabolic scope could render reproductive success sensitive to environmental changes.