Populations of the Yellow-Pine Chipmunk, Eutamias amoenus

Abstract

A population of about 600 yellow-pine chipmunks in the Cascade Mountains in central Washington was investigated for approxi- mately 12 months in three different years. The animals were live-trapped on a 42.4-acre study area in yellow-pine forest. There was only one litter in May or June per year. From a base level of 1.54 per acre in May the population rose with emergence of young in June to a peak of 3.13 per acre in July and August based on 3- to 6-day censuses, then gradually declined to 1.64 per acre by September and October. The mean for 1947 was 2.26 per acre. Censuses employing 8 to 12 days of trapping resulted in a mean of 3.20 individuals per acre. By mid-November virtually all chipmunks were in winter burrows. New unmarked animals (most of them young that began appearing in early June) reached a high in July of about 35% of the population. By October 50% of the population were young of the year. An unusual and unexplained change in sex ratio occurred in 1947. The percentage of males which had ranged from 53.5% to 58% through 1946 and to the end of May 1947, steadily dropped through the summer of 1947 to a low of 36% by September. By far the safest time for Eutamias amoenus was the five-month hibernation period, November to April. Survival was nearly 100%. Highest mortality was among young chipmunks in their first weeks above- ground. One-year survival of July, August and September groups aver- aged 29%. Ten per cent of the chipmunks caught in mid-August 1947 were retaken in 1950. Although overlapping in their habitats and foods, the three diurnal sciurids (pine squirrels, Tamiasciurus douglasii; golden-mantled ground squirrels, Spermophilus lateralis; and chipmunks, Eutamias amoenus) all flourished together in considerable numbers. Population density of the chipmunks appeared to be controlled mainly by territorial behavior that spaced individuals a minimum of about 50 feet apart, and by predation from eight diurnal carnivores.