UC Berkeley

Caching is the movement and storage of food items by animals for future use. Caching facilitates survival during periods of scarcity, may reduce foraging time during future searches for food, and allows animals to take advantage of periods when available food exceeds current needs. Scatter-hoarding animals store one item per cache, and must employ cognitive strategies to protect their caches. These strategies include assessing the relative value of food items, carefully hiding food items, deceptive behaviors to thwart potential pilferers, and remembering each cache location. Such decisions should be driven by economic variables, such as the value of the individual food items, the scarcity of these items, and competition and risk of pilferage by conspecifics.

My dissertation begins with a general overview of the food-storing literature and the natural caching behavior of the scatter-hoarding fox squirrel (Sciurus niger). I then describe several experiments that explored the decisions fox squirrels make when storing food. A study examining how fox squirrels adjust effort assessing and caching food based on the food item’s value (weight, perishability and nutritional content) using two different foods, hazelnuts and peanuts, is described in Chapter 2. Squirrels (n = 23) were observed during natural periods of food scarcity (summer) and abundance (fall). Assessment and investment per cache increased when resource value was higher (hazelnuts) or resources were scarcer, but decreased as experimental sessions continued. This study showed that fox squirrels’ assessment and caching behaviors were sensitive to both daily and seasonal resource abundance.

Another important problem facing scatter-hoarding animals is how to maximize the retrieval of stored food items while minimizing the risk of pilferage by competitors. One defense against theft could be the spatial placement of caches. I describe a study examining whether the spatial distribution of caches is dependent on nut species in Chapter 3. I measured four key variables of the cache decision: distance and direction traveled, the use of distinct cache areas by nut species, and density of caches. Fox squirrels (n = 48) were tested in 50 sessions, and the geographical coordinates of over 900 cache locations were recorded. Results suggested that squirrels distribute caches using three heuristics: matching the distance traveled before caching to the value of the food item, systematically covering a caching area, and matching cache density to minimize pilferage risk to the highest valued food items. Squirrels spatially chunked their caches by nut species, but only when foraging from a single location. This first demonstration of spatial chunking in a scatter-hoarder underscores the cognitive demand of scatter-hoarding.

I describe a final field study in Chapter 4. A pilot study revealed that there was a high level of pilfering (25%) among a population of fox squirrels. Nineteen fox squirrels cached 294 hazelnuts with passive integrated transponder tags implanted in them. Variables collected included assessment and cache investment and protection behaviors; cache location, substrate, and conspicuousness of each cache; how long each cache remained in its original location, and the location where the cache was finally consumed. polymer chain reaction (PCR) analysis of hair samples obtained from 14 of the subjects was used to determine relatedness among this group of squirrels, and its potential impact on behavior. Results suggest that cache protection behaviors and the lifespan of a cache are dependent on the conspicuousness of a cache. Squirrels may mitigate some of the costs of pilfering by caching closer to the caches of related squirrels than to those of non-related squirrels.

In Chapter 5, I describe a model of the antagonistic relationship between food storing animals and their competitors using agent-based simulations where caching, memory size, and pilfering co-evolve. During periods of food abundance and scarcity, individuals could consume or store found items, retrieve old caches, or pilfer others’ caches. In the absence of pilfering, selection is strongest for longer memory. As pilfering increases, shorter memory may be more adaptive, because old caches are likely to be depleted. Contrary to common thought that social interactions enhance cognition, these findings demonstrate how competition may constrain rather than promote some cognitive abilities.

Finally, in Chapter 6, I argue that my research demonstrates that food assessment and cache investment strategies of fox squirrels represent a complex suite of behaviors. These behaviors allow squirrels to maximize the benefits of periods of excess food in the environment, while increasing the likelihood of retrieving nuts later, when food in the environment is scarce. Competition via pilfering influences these food-storing decisions and outcomes, and in some cases, may impair the cognitive abilities of food-storing animals. I discuss the overall implications of this work, and potential directions for future research.