Evolutionarily stable kleptoparasitism: consequences of different prey types

We present two elaborations of the model of Broom and Ruxton that found evolutionarily stable kleptoparasitic strategies for foragers. These elaborations relax the assumption that the distribution of times required to handle discovered food items is exponential. These changes increase the complexity of the model but represent a significant improvement in biological realism. In one elaboration, handling takes a fixed interval, t(h), at the end of which the whole value of the food item is obtained. We liken this to peeling then consuming a small orange. The other elaboration also assumes that handling takes a fixed interval, t(h), but this time the reward from the food item is extracted continuously throughout the handling period. We liken this to eating an apple. Both models predict that increasing food density, the ease with which food items can be discovered, or the length of aggressive contests all act to make kleptoparasitism less common. The difference between the evolutionarily stable strategy solutions of the apple and orange models provides a clear prediction of our theory. When prey items require handling before yielding a lump sum at the end, then kleptoparasitic attacks will be focused on prey items near the end of their handling period. However, if prey items yield reward continuously during handling, then attacks should be biased toward newly discovered food items. Another key difference between the model predictions is that kleptoparasitism increases with forager density in the apple model, but decreases in the orange model.