Reciprocal pilferage and the evolution of food-hoarding behavior

Current theories of food-hoarding behavior maintain that hoarding can be adaptive if a hoarder is more likely than any other animal to retrieve its own caches. A survey of the literature indicated that the hoarder often has a recovery advantage when searching for items it has stored, but levels of cache pilferage are often so high (2-30% per day) that at least for some long-term food hoarders, the caching animal is unlikely to recover a significant amount of its stored food. Except in a few cases (acorn woodpeckers and beavers), kin selection cannot explain the high levels of pilferage observed. We suggest that some small solitary animals with overlapping home ranges (e.g., most rodents, chickadees, and tits) are able to tolerate high levels of cache pilferage. Pilferage is not as damaging to these animals as it might otherwise be because many interspecific and all intraspecific cache pilferers also cache food. These or similar food caches can be pilfered later by the original food hoarder. In other words, pilfering in these species is often reciprocal, and because it is reciprocal, it can be tolerated. We argue that caching systems based on reciprocal pilfering can be stable and are not necessarily susceptible to cheaters, animals that pilfer food but do not scatter hoard food themselves, and we introduce a model of food hoarding to support this argument. These food-caching systems based on reciprocal pilfering resemble cooperative behavior, but the behavior is actually driven by the selfish interests of individuals. This theory of scatter-hoarding behavior based on reciprocity has important implications for the ways that food-hoarding animals interact with inter- and intraspecific competitors.