Northern Prairie Wildlife Research Center
Patricia DeCoursey, University of South Carolina, Department of Biological Sciences, Columbia, SC 29208 USA
The majority of terrestrial animals have evolved striking circadian (daily) behavioral patterns. Daily time-programming is not merely a reaction to environmental cycles but involves an internal neural pacemaker, the suprachiasmatic nucleus (SCN) of the ventral hypothalamus, which is corrected to local time in most cases by the environmental light-dark cycle. An internal timer enhances survival by its wake-up function which allows preparation for physiological and behavioral responses in advance of actual need, and by its chronometer capacity which permits continuous time consultation as in time-compensated celestial navigation.
Circadian timing appears important since virtually all terrestrial organisms posses a "biological clock." Laboratory data show that rodents which have been made arrhythmic by SCN-lesioning or genetic knock-outs survive well in captivity. Responses of arrhythmic mammals in the wild, however, are totally unknown because of technical difficulties in long-term tracking of mortality and reproduction of animals in natural habitat.
A direct test of the value of SCN pacemaker in fitness was carried out over a 16-month period on a wild population of approximately 65 eastern chipmunks, Tamias striatus, resident on 1.25 ha at Mountain Lake Biological Station, VA. A cohort of arrhythmic chipmunks was created surgically by electrolytic lesions of the SCN and a comparison of the 10 lesioned, 13 intact controls, and 5 sham-lesioned controls was made in terms of behavior and mortality. The lesioned animals showed disrupted rhythms or complete arrhythmicity under constant laboratory conditions, while sham-lesioned or intact animals exhibited precise free-running patterns. A combination of visual observation, live-trapping, and radiotelemetric tracking of these animals in their natural habitat was used to assess seven measurements of fitness, including reclaiming of den site, daily activity patterns, survival, space maintenance, body weight maintenance, reproduction, and winter thermoregulation.
Project Animals were outfitted in summer with a radio collar weighing approximately 3.9 g (Advanced Telemetry Systems, Isanti, MN), with a unique frequency in the range of 150.000-151.999 MHZ. Transmitters detected location, and in some cases activity. Winter temperature collars were available for some of the Project Animals. These weighed approximately 4.9 g and were used for detecting location, body temperature, and mortality. A microprocessor extended battery life through the 5 winter months by turning on power for 0.5 hr in the morning and 0.5 hr after dark. Signals were picked up by a hand-held Yagi antenna and its affiliated ATS receiver.
Every animal returned to its den site upon release. A few den site changes were seen in both SCN-lesioned and control animals, particularly for juvenile males, but the data for SCN-lesioned chipmunks did not differ from the population norm. Animals had very similar core territories after lesioning except for extensions to food resources as the summer and fall progressed. All individuals of the three groups were strictly diurnal, and remained in their burrows during hours of darkness. A 36-hr trapping, visual, and telemetric census showed no animals outside of burrows at night. Three-month mortality included 5.6% of controls and 0% of SCN-lesioned animals. Losses after 16 months included 11.1% of controls and 40.0% of SCN-lesioned animals; differences in survival of the two groups were significant (P < 0.10). Body weight varied seasonally for both groups. Most adults in control and SCN-lesioned groups were reproductively active in the spring and fall breeding periods. Limited winter data from 8 radio-collared individuals suggested that all individuals remained normothermic throughout the winter 1995-96, but rarely ventured out of their burrows during cold weather. Thus, only slight differences were detected between the SCN-lesioned and control groups in the 1995-1996 record acorn crop years; selection against SCN-lesioned chipmunks may, however, be more severe in years of limited food resources.