Circadian rhythms of locomotor activity in captive birds and mammals: Their variations with season and latitude. Oecologia

Summary. 1. The seasonal variations in time of daily onset and end of locomotor activity are described for 3 species of mammals and 5 species of birds kept in captivity at the arctic circle and at lower latitude. These variations are most pronounced at high latitude. 2. The duration of daily activity plotted versus the photoperiod can be described as an S-curve in all species tudied so far, both in nature and in captivity. In both male and female fringillid birds activity times were longer before the summer solstice (spring) than after the summer solstice at equal photoperiods. 3. The seasonal changes in activity time result from roughly mirror-image changes in the times of onset and end of activity relative to sunrise and sunset, cancelling out each other. Therefore the midpoint of activity stays relatively stable; remaining minor changes in the midpoint of activity do not produce a general seasonal pattern. 4. At high latitude, a large seasonal f uctuation in the day-to-day variability (or precision) of activity timing is detected. These patterns of precision of the rhythm cannot be attributed to a single Zeitgeber property without complex assumptions. Onset and ends of activity become more precise when occurring during the civil twilight, i.e. at times of day with mos