Effects of food value, predation risk, and pilferage on the caching decisions of Dipodomys merriami

Animals that scatter cache their food face a trade-off between the benefits of protecting caches from pilferers and the costs associated with caching. Placing food into a large number of widely spaced caches helps to protect it from pilferage but also involves costs such as greater exposure to predators. I predicted that animals would disperse food into a larger number of more widely spaced caches when caching (1) a preferred food versus a less preferred food and (2) under conditions of low predation risk versus high predation risk. To test these predictions, I examined the scatter-caching decisions of Merriam's kangaroo rats (Dipodomys merriami). D. merriami distributed caches in clumped patterns, regardless of food preference, but they showed a tendency to invest more in a preferred food by distributing caches more widely. Under the relative safety of the new moon, they did not disperse caches more widely, rather they partitioned the same amount of food into a larger number of caches than they did under the full moon, when predation risk is higher. To examine whether their cache spacing decisions had a significant impact on the success of cache pilferers, I measured discovery by pilferers of artificial caches of two food types at different caching distances. Results indicate that the cache spacing behavior of D. merriami functions to protect caches from pilferers, because increased spacing of artificial caches decreased the probability of pilferage for both types of food.