Responses of a scatter-hoarding squirrel to conspecific pilfering: a test of the reciprocal pilferage hypothesis

Abstract

The reciprocal pilferage hypothesis argues that, despite the occurrence of pilferage, scatter-hoarding behaviour could evolve if cache loss is compensated by gains pilfered from others. However, this model has not been strictly tested because of the difficulty associated with tracking caches, including ownership, over long periods. Using infrared radiation cameras, we tracked caching of Juglans regia seeds by groups of two Pere David's rock squirrels, Sciurotamias davidianus, within a large natural enclosure. Our goal was to quantify how squirrels responded to the presence of a conspecific when seed hoarding and pilfering and test the reciprocal pilferage hypothesis. We found that the numbers of seeds harvested from the seed sources and pilfered from conspecifics was initially low, increasing as seeds were removed and then dropping off once few seeds remained at the seed sources. Additionally, the number of seeds scatter hoarded increased with a decreasing number of seeds remaining at the sources. Seeds harvested from the source and pilfered from competitors were at first cached randomly within the enclosure and then centralized to the low-competition area near the nest as seed sources declined. Overall, pilferage was not high. The proportions of seeds pilfered did not vary between conspecifics and were positively correlated with each other over trials. More seeds were harvested from the seed sources than pilfered by competitors at the early stage of hoarding. These results suggest that, under conspecific competition, squirrels appeared first to compete for food at the sources, then for caches with each other as food sources decreased, and finally cache ownership became relatively stable. Squirrels compensated for cache loss by both harvesting food from the sources and pilfering caches from its competitor. The amount of seeds gained from pilfering was sufficient to replace pilfered caches, supporting the reciprocal pilferage hypothesis.