Natural Selection Of Optimal Foraging Behavior In Columbian Ground Squirrels.

Abstract

In a field study of Columbian ground squirrels (Spermophilus columbianus) in northwestern Montana, I determined whether optimal foraging behavior could be subject to evolution by natural selection. I tested whether (1) individuals vary in their ability to select optimal diet, (2) selecting an optimal diet increases an individual's survival and reproduction (fitness), and (3) the ability to select an optimal diet is heritable. These hypotheses represent the necessary criteria for determining natural selection of a trait. The optimal diet (maximizing daily energy intake) for each of 109 individuals was determined by constructing a linear programming optimization model for each individual. The parameters for each individual's model were either measured directly from individuals in the field or estimated from other studies in one of two ways: on the basis of the individual's body size using regression equations, or from average values. An individual's optimal foraging ability was determined by comparing its observed diet with its optimal diet, and using the difference between the two as a measure of foraging performance. Optimal foraging ability varied in the population, to the extent that 37% of individuals deviated significantly from their optimal diet. An individual's deviation from an optimal diet did not change significantly within a season or with the immediate foraging environment. As a result, optimal foraging ability can be viewed as an individual trait. Individuals with smaller deviations from their optimal diets had greater growth rates, yearly survivorship, and litter sizes as a result of their greater energy intake. A correlation of the optimal foraging abilities of mothers and offspring under two conditions, mother present after weaning or mother absent, revealed that offspring tend to resemble their mothers. Furthermore, this resemblance may be due to genetic similarity or to offspring learning from their mother. This study, therefore, demonstrates that optimal foraging behavior may be potentially subject to natural selection.