Evolutionarily stable kleptoparasitism: consequences of different prey types

We present two elaborations of the model of Broom and Ruxtonthat found evolutionarily stable kleptoparasitic strategiesfor foragers. These elaborations relax the assumption that thedistribution of times required to handle discovered food itemsis exponential. These changes increase the complexity of themodel but represent a significant improvement in biologicalrealism. In one elaboration, handling takes a fixed interval,t_h, at the end of which the whole value of the food item isobtained. We liken this to peeling then consuming a small orange.The other elaboration also assumes that handling takes a fixedinterval, t_h, but this time the reward from the food item isextracted continuously throughout the handling period. We likenthis to eating an apple. Both models predict that increasingfood density, the ease with which food items can be discovered,or the length of aggressive contests all act to make kleptoparasitismless common. The difference between the evolutionarily stablestrategy solutions of the apple and orange models provides aclear prediction of our theory. When prey items require handlingbefore yielding a lump sum at the end, then kleptoparasiticattacks will be focused on prey items near the end of theirhandling period. However, if prey items yield reward continuouslyduring handling, then attacks should be biased toward newlydiscovered food items. Another key difference between the modelpredictions is that kleptoparasitism increases with foragerdensity in the apple model, but decreases in the orange model.