Abstract
Optimal Diet Theory suggests that individuals make foraging decisions that maximise net energy intake. Many studies provide qualitative support for this, but factors such as digestive constraints, learning, predation-risk and competition can influence foraging behaviour and lead to departures from quantitative predictions. We examined the effects of intraspecific competition within a classic model of optimal diet – the common shore crab, Carcinus maenas, feeding on the mussel, Mytilus edulis. Unexpectedly, we found that breaking time (Tb), eating time (Te), and handling time (Th) all decreased significantly in the presence of a conspecific. Reduced handling time in the presence of a competitor resulted in an increased rate of energy intake, raising the question of why crabs do not always feed in such a way. We suggest that the costs of decreased shell breaking time may be increased risk of claw damage and that crabs may be trading-off the potential loss of food to a competitor with the potential to damage their claw whilst breaking the shell more rapidly. It is well documented that prey-size selection by crabs is influenced by both the risk of claw damage and competition. However, our results are the first to demonstrate similar effects on prey handling times. We suggest that crabs maximise their long-term rate of energy intake at a scale far greater than individual foraging events and that in order to minimise claw damage, they typically break shells at a rate below their maximum. In the presence of a competitor, crabs appear to become more risk-prone and handle their food more rapidly, minimising the risk of kleptoparasitism.
Highlights
Prey choice has a significant impact on individuals and may have consequences for both predator and prey populations
Since the pioneering work of Emlen [1] and MacArthur and Pianka [2], optimality theory has been used to explain prey selection as decisions involving the trade-off between costs and benefits in order to maximise the rate of net energy intake [3]
Optimal Diet Theory [4], [5] predicts that foraging predators that aim to maximise their long-term average energy intake should always accept prey items into their diet with a profitability higher than their long-term average intake rate
Summary
Prey choice has a significant impact on individuals and may have consequences for both predator and prey populations. Elner & Hughes [6] used the shore crab, Carcinus maenas, feeding on the mussel, Mytilus edulis, to test the predictions of Optimal Diet Theory. They showed that when prey availability was unlimited, crabs chose mussel sizes close to the predicted optimum, but that as the optimal mussels become depleted, crabs chose progressively less valuable mussels, both above and below the optimal size. Numerous studies have shown that these constraints can be very important in determining diet choice, for example: digestive constraints cause large mammalian herbivores to select for digestive quality over quantity [8], [9]; learning, recognition time and prey misidentification may affect energy maximisation [10]; and risk of predation often results in a trade-off between energy return and predator avoidance or vigilance, resulting in sub-optimal prey being eaten [11,12,13,14,15]
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