Local Adaptations in Chipmunk (Eutamias amoenus) Populations and Evolutionary Potential at Species' Borders
Authors: States James B. 
Published: April 2006

A study of four yellow pine chipmunk (Eutamias amoenus) populations distributed along a steep environmental gradient in central Oregon revealed differences verifying a prediction of high evolutionary potential at distributional borders of species occupying unstable environments. Population density was lowest on the coniferous forest edge of the ponderosa pine transition zone (Spiders population), highest near the center of the transition zone (Main Grid), and intermediate in populations at the desert periphery of the ponderosa zone (Cabin Lake) and isolated in a disjunct ponderosa forest on the Oregon High Desert (Lost Forest). Attrition of adult @M @M was greatest during the oversummer periods, and was more likely due to predation than to dispersal. Attrition of adult @V @V was greatest during the overwinter periods. During this period, winter food caches may be exhausted before new supplies appear; these shortages, combined with increases in @V energy demand associated with gestation and rearing of young, may have contributed to overwinter starvation. Significantly higher adult @V body weight at the edge of the dense coniferous forest suggests an adaptive response to spring food shortages caused by late snowpacks and by cool weather inhibition of foraging. At the desert edge of the ponderosa forest, increased body weight may instead be an adaptive response to spring food shortages resulting from fewer seed—producers and from competition with least chipmunks (Eutamias minimus). Competitive advantage among chipmunks is commonly associated with larger size, and E. amoenus @M @M at Lost Forest, which were nearest to E. minimus in body weight, showed the greatest increase over central population weights. Observations at the Main Grid, Cabin Lake and Lost Forest showed behavioral characteristics of the populations to be generally similar, although significant populational differences in activity budgeting were observed. Higher population density at the Main Grid led to a much higher level of positive social interactions with a much higher percentage of aggressive chasing and fighting than at other sites. At Cabin Lake weather had a strong influence on activity budgeting and habitat orientation. Lost Forest chipmunks exhibited a greater proportion of protective behavior (especially alarm vocalizations), suggesting an adaptation by which the population may reduce danger from predation due to low protective cover without restricting the area over which individuals may safely forage. This same population appeared to have adjusted activity budgets to avoid high desert temperatures by foraging more in early hours and orienting more toward the shade of trees and shrubs in later hours. These trends were less pronounced in the peripheral forest—edge population and were reversed in the central population. When foraging, individuals from all populations oriented most strongly toward shrubs, then open ground, slash piles, and trees; they oriented least strongly toward rocks, stumps, or logs. Chipmunks involved in maintenance or protective activities showed a nearly reversed order of preference for the same habitat features. These and other observations suggest that, contrary to recent literature, presently observed lines of contact between allopatric chipmunk species do not represent dynamic tension zones established and maintained in delicate balance by aggressive interactions. Rather, they represent extremely stable distributional limits established through habitat selection as a mechanism by which smaller species may avoid fruitless as well as energetically costly competitive encounters with larger species.