Cache Retrieval

Many animals cache food items, leaving them for later retrieval. Some, such as moutain lions and ravens, leave spoilable food, such as deer carcasses, for relatively short periods (hours or days) before retrieval and consumption. Others, particularly rodents (squirrels and chipmunks) and birds (chickadees, jays) store seeds for use weeks or even months later. Seeds are particularly appropriate for long-term caching, and many seed-eating ants, birds and mammals cache seeds as they drop from plants. (For a picture of a caching squirrel, go to the page on hiding food.)

Caching animals often scatter hoard--hide food items throughout their home range. Dispersing food items in this manner has the advantage of not creating a large food resource, which might be quite attractive to other animals, but has the disadvantage of creating a need to retrieve food from dozens, or even hundreds, of locations. The alternative to scatter hoarding is to assemble the food items in a central place, such as a hollow tree or a cavity in the ground. Such a cache doesn't present great challenges in finding the food at a later time, but may require intensive defensive efforts.

Obviously, then, cached food is only valuable if it can be retrieved later. The most commonly expressed hypothesis for efficient cache retrieval invokes learning cache locations and returning later to those spots. Some animals actually do this, but two other hypotheses must also be considered, reforaging and search by rule. Each of these hypotheses can reasonably explain cache retrieval:

• Learned cache retrieval
• Reforaging
• Search by rule

Pilfering of caches by other animals--both of the same species and of other species--is an important problem for caching animals. Even in scatter-hoarders, pilferage may substantially reduce the food supply of the caching animal. Pilfering animals have two strategies. One is to search for food in another animal's home range. Interestingly, in rodents, weather conditions greatly affect the effectiveness of pilferage; moist conditions bring out scents from cached seeds, so that the pilferer can much more easily find them. When it is dry, yellow pine chipmunks (Tamias amoenus) and deer mice (Peromyscus maniculatus) use spatial memory to find their stored food, and a caching animal has a substantial advantage over a pilferer in finding food. Under moist conditions, pilferers can sniff out seeds, and the cacher has little or no advantage over the pilferer in discovering seeds.

Observational learning plays an important role in the behavior of pilferers. Scrub jays, Aphoelocoma coerulescens, when observed caching by another bird, are likely to recache the food at a later time (Emery and Clayton 2001). Similarly, Pinyon jays, Gymnorhinus cyanocephalus, and Mexican jays, Aphelocoma ultramarina, may watch other birds and learn their cache sites so that they can pilfer the sites later (Bedekoff and Balda 1996). Ravens, Corvus corax, display elaborate avoidance behavior to keep other ravens from observing their cache sites (Heinrich and Pepper 1998).

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