Circannual Rhythms Mammals

Orientation in time, which ranks among the most important of organismal adaptations, is apparent in the ubiquitous distribution of seasonal rhythms: mammals anticipate and take advantage of predictable changes in the external environment imposed by the 12-month geophysical cycle. Striking seasonal changes in temperature, humidity, and day length, as well as prominent seasonal variation in biotic factors such as food availability, presence of predators, and access to mates, place a premium on accurate timing. The array of mammalian functions that varies seasonally includes, among others, reproduction, food intake, sleep, body and brain mass, thermoregulation, affiliative and agonistic social behaviors, pelage characteristics, secretion of hormones, and neurotransmitter concentrations in brain (Bartness & Wade, 1985; Buijset al.1986; Dark, Dark, & Zucker, 1987; Ferkin & Zucker, 1991; Goldman & Nelson, 1993; Heldmaier & Steinlechner, 1981; Sadlier, 1969; Turek & Van Canter, 1994; Walker, Haskell, Berger, & Heller, 1980). Seasonal rhythms are evident at all levels of biological organization.