The temperature hovers around freezing, but the sun is up for 24 hours each day. How do animals living in the continuous light of the Arctic summer know when to sleep and when to be active? Do they maintain a 24-hour cycle of rest and activity, or does living in continuous light alter their circadian rhythm?
Answering these questions may improve our understanding of biological clocks -- the internal, genetically programmed cycle of rest and activity that affects the behavior, metabolism and physiology of all animals, including humans. A better understanding may also help solve problems -- such as shift-work fatigue, jet lag and even seasonal affective disorder -- that are associated with disruptions of biological clocks.
One scientist who has spent a lifetime pursuing these questions and finding answers that have helped build the field of biological clock research is G. Edgar Folk, Ph.D., emeritus professor of molecular physiology and biophysics at the University of Iowa Roy J. and Lucille A. Carver College of Medicine.
Folk notes that humans have a natural circadian rhythm of close to, but not exactly, 24 hours. Importantly, all biological clocks are adjustable and respond to environmental cues such as sunrise or sunset, which continuously reset the clock and keep us on a regular 24-hour schedule.
However, previous research, including studies in Folk's lab, has shown that lab rats kept in continuous light develop a 26-hour cycle of rest and activity, meaning their peak of activity travels around our usual daily 24-hour clock. This phenomenon is called the "Aschoff Effect" after a German scientist who first recorded it in the 1960s. Folk sometime ago set out to determine if this effect was also seen in wild animals during the continuous light of the Arctic summer.
"In continuous light in the lab, the animal's clock changes depending on the intensity of the light," Folk explained. "We thought that would also happen in the Arctic. Much to our surprise, the Arctic animals maintained a very crisp 24-hour period of activity."
Working at Folk's permanent Arctic field lab at Barrow, Alaska, the research team studied two types of Arctic rodent: nocturnal porcupines (below, left) and day-living ground squirrels (below, right).
Heart rates - a good measure of metabolism and activity -- from four porcupines and direct observation of nine squirrels' activity showed that both creatures retained a 24-hour rhythm of behavior, just as they would if they were living under a normal day/night situation. The study results are published in the Dec. 2006 issue of Biological Rhythm Research.
It seemed that although the scientists were very careful not to provide time cues of any sort, the animals had managed to latch onto something that gave them regularity.
"I have written for years that experimental animals seem to be hungry for cues, or time signals, to keep on a regular cycle," Folk said. "So we tried to figure out what cue the wild animals were using, and we could find only one thing that kept a 24 hour periodicity. At Barrow, the sun travels in a circle overhead for 82 days, but at midnight the circle is tipped to the north.
"We postulate that the animals are conscious of where the sun is in the sky and that the nearness of the sun to the horizon could be a clue to animals, and even plants, to keep on a 24-hour schedule."
Folk found that several other scientific teams have also proposed the same theory.
"Our work shows that clocks are important, and for me it means that you get surprises -- I thought that we would see drift in the times the animals, slept, but we didn't. The broad implication is that, when possible, animals like humans, like to have regularity."
Sixty years of study have not diminished Folk's fascination with biological clocks, and he says the field still produces surprising results and raises new questions.
"There is a lot more to be done," he said. "For example, birds haven't been studied enough - I first got interested in this study when I was listening to birds singing in the Arctic and trying to figure out if they always sang at the same time of day even when the light was continuous. I'm pretty sure that as a species they must learn to resist the Aschoff effect, but no one has studied it."
Although Folk has not visited his Arctic lab in several years, the heart rate data he has collected in Arctic animals, including hibernating bears, continues to provide a goldmine of information. Folk is currently involved in a collaborative project with Eric Dickson, M.D., UI professor and head of emergency medicine, examining the cardiology of hibernating animals and looking at what application that information might have in human emergency medicine.
Folk's UI research colleagues include, Diana Thrift, a research assistant in Folk's lab, Bridget Zimmerman, Ph.D., clinical associate professor in the UI College of Public Health, and Paul Reimann in anatomy and cell biology.-University of Iowa
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