Circadian Locomotor Rhythms of Rodents in the Arctic

Abstract

In both the arctic and subarctic regions of Alaska during the spring and summer pronounced daily periodicity was observed in the field pnd running-wheel activity of the following diurnal and ]"nocturnal" rodents Spermophilus undulatus, Clethrionomys rutilus, Lemmus trimucronatus, Microtus miurus, and M. oeconomus. Animals whose phases were shifted to abnormal times of the day by artificial light cycles regained their normal phases when exposed to the local field conditions. However, difficulty in retaining proper phase was observed in several of these nocturnal species at the time of the summer solstice, and the usually nocturnal L. trimucronatus centered their activity about noon then. Temperate zone rodents (Ammospermophilus leucurus pnd Peromyscus leucopus) taken to Point Barrow, Alaska, also showed pronounced daily periodicity under arctic conditions, although the latter species did not show locomotor activity until August, when the sun moved below the horizon near midnight. All of the species studied from both the arctic 8nd temperate zones have endogenous circadian rhythms of activity. Laboratory experiments showed that those rhythms could be entrained by square wave and approximately sinusoidal cycles of illumination even when the amplitude of the entraining light was as small as that encountered at Point Barrow at the summer solstice (maximum:minimum = 23:1). However, individuals differed in their ability to entrain successfully to some light cycles. These experiments and field observations support the conclusion that in the arctic, even during the summer, the environmental light cycle is the principal entraining agent of the endogenous rhythm. When the amplitude of the light cycle was insufficient to entrain an animal, "oscillatory free-runs" were frequently observed, i.e., as an organism''s overt rhythm on successive days scanned across the 24-hr. light cycle, due to its non-24-hr. periodicity, its frequency changed in response to the light intensity. The functional significance of a pronounced circadian locomotor rhythmicity in the arctic may be attributed to 2 factors: its usefulness during that restricted part of the year when light and dark occur every day; and its historical derivation from temperate zone ancestors, its ability to entrain to low amplitude light cycles being a part of that inheritance.