When time is of the essence: choosing a currency for prey-handling costs

1. Heartbeat rate was measured in shore crabs Carcinus maenas (L.) feeding on mussels Mytilus edulis L. in order to estimate the energetic cost of han dling prey and to assess the relative importance of energy and time as cost ing currencies. 2. Energetic handling costs represented approximately 2% of corresponding g ains. 3. The tendency of profitability (gain per unit handling time) to increase with prey size was weakened by including energetic handling costs. 4. Time was judged to be a more appropriate currency than energy for costin g prey-handling behaviour. 5. The importance of time as a costing currency, either through the princip le of lost opportunity or through exposure to mortality risk, may extend to other behavioural systems, including aggression.