Central place foraging for non-food items : determination of the stick size-value relationship of house building materials collected by eastern woodrats

Up to now optimal foraging theory has dealt exclusively with animals foraging for food. The currencies used to measure the benefit of food items, energy and nutrients, are inappropriate measures of the benefit of nonfood items collected by animals. This paper proposes that "value" can be used as a currency for nonfood items and presents a procedure for determining the size-value relationship of nonfood items. This procedure was used to determine the stick size-value relationship of sticks collected by eastern woodrats as house building materials. Simulations of profitability [(V-C)/T] versus distance curves for five general stick size-value relationships predict that regardless of the stick size-value relationship, the number of stick sizes above the minimum profitability value decreases with distance from the central place. This prediction was supported in a laboratory experiment. Woodrats were also found to collect more selectively (decreased variance) as they collect farther from the house. Since the woodrats were foraging adaptively, taking only those sticks above the minimum profitability value, it was possible to use stick-size preference to select between the five general relationships. It was determined that there is an increasing relationship between stick size and value for woodrats initiating house building. Since it is possible to select between the five relationships it should often be possible to use this technique to determine the size-value relationship for nonfood items collected by animals when the object size is the most important factor determining value.