Circanian rhythms—II. Endogenous and exogenous factors controlling reproduction and hibernation in chipmunks ( Eutamias) and ground squirrels ( Spermophilus)

Abstract

1. 1. Circadian rhythms are demonstrated in field-caught populations of two species of ground squirrels ( Spermophilus) and four species of chipmunks ( Eutamias), and also in laboratory born Spermophilus. 2. 2. Influences of environmental stimuli on phase and period lengths of the circanian rhythms were investigated. Flexibility in phasing enables adjustment to environmental variations between years while constant period length ensures breeding at the optimal time. 3. 3. Attainment of breeding condition involves a commitment point from which another circanian period is measured. In nature factors influencing the exact date of the commitment point synchronize the rhythms. 4. 4. The characteristics of the circanian rhythm of each species are related to size of animal, predictability and availability of food, and predictability and rigor of winter environment.