A model of caching depth: implications for scatter hoarders and plant dispersal

The ability of foragers to detect stored seeds and nuts is known to decrease with increasing depth at which scatter hoarders bury these food items. Depth of burial of seeds and nuts is known to influence the ability of seedlings to emerge and establish if seeds are not discovered by foragers. These two patterns are combined in a model of caching depth, and implications of the model for food hoarders and plant propagules are explored. Optimal caching depth for hoarders is the depth where the probable recoverable energy in a cache discounted by the energy to make and retrieve a cache is greatest. Optimal burial depth for a propagule is where the probability of it escaping detection by a forager times the probability that it will germinate, emerge, and survive in the absence of predation is maximized. Certain aspects of the cache depth model were tested for yellow pine chipmunks caching the seed of antelope bitterbrush in field and laboratory experiments. The depths yielding the greatest probability of establishment of bitterbrush seedlings was 10-30 mm, with seeds buried 20 mm deep having the greatest success. Yellow pine chipmunks, the primary dispersal agent of bitterbrush seeds in this Sierra Nevada study area, cached most seeds between 5 and 20 mm deep, in the upper portion of the range yielding the greatest bitterbrush establishment. The partial agreement between chipmunk caching behavior and bitterbrush requirements may be largely fortuitous, but certain evolutionary adjustments by the plants may result in a closer match between seedling requirements and the behavior of seed-caching animals.