Wrinkles in Time Temperature-Circadian Interactions
Temperature compensation... was deliberately sought as a
functional prerequisite of a good clock.
Colin Pittendrigh, "Circadian Rhythms..." Cold Spring Harbor Symposia
on Qualitative Biology, 1960.
In the 1950s, as interest in biological clocks intensified, scientists sought
general principles and mechanisms that might explain biological timing systems.
Colin Pittendrigh used fruit flies to investigate the role of temperature in the
circadian clock. He systematically raised fruit flies at different ambient
temperatures to see if these changes would either speed up or slow down the
clock. Pittendrigh found that the time of emergence of adult fruit flies from
their pupal cases, which normally occurs at dawn, was unaffected when the ambient
temperature was reduced by 10 degrees Centigrade. Pittendrigh thus showed that
biological clocks were temperature-compensated, at least in the case of fruit
flies, a finding that has since been extended to mammals.
Temperature compensation of the biological clock intrigued Pittendrigh and others
because most biochemical reactions, including most pathways of basic cellular
metabolism, vary with changes in temperature. For homeothermic (warm-blooded)
animals who regulate their body temperature, temperature compensation is still
necessary since regulation is not perfect, and since body temperature fluctuates
during the course of a day.
Are You Running Hot or Cold?
Numerous factors can affect your body temperature, including ambient temperature,
exertion, the presence of infection, and stress. Generally, however, your
temperature should show the same overall circadian pattern of highs and lows as
is shown in this chart.
Quantifying Data An Oral History.
The internal temperature of humans, unlike that of Drosophila, does not vary
greatly with external temperature changes. It does, however, vary according to a
circadian pattern. The development of instruments capable of providing precise
quantitative measurements has been critical to human temperature studies.
Santorio Santorio is believed to have designed the first oral thermometer. Surface thermometers generally
provide a less accurate measure of internal body temperature because heat is
continually released from the surface of the body.
A. Oral thermometer, with ivory scale bar, ca. 1875
B. Oral thermometer, with looped mercury tip, ca. 1877
C. Oral thermometer, ca. 1891
D. Oral thermometer, ca. 1891
E. Surface thermometer, which measures skin temperature, pre–1930
Weighing In An Early Measurement of Daily Rhythms.
Over a 30-year period, Santorio Santorio (1561–1636) collected physiological data
using himself as a subject. This apparatus a large "weighing chair" in which
he frequently took his meals was used in his experiments. He reported
circadian variation both in body weight and in amounts of perspiration.
Bat Mobile Testing Temperature-Compensation in Bats.
Verifying the existence of temperature compensation in mammals is problematic
because their internal temperatures are relatively constant. Thus, researchers
turned to bats, mammals whose body temperature can be made to conform to ambient
temperature. Using the apparatus diagrammed here, Kenneth Rawson demonstrated
that the period of bats' biological clocks was unaffected by varying ambient
temperature, thus extending Pittendrigh's conclusions about temperature
compensation to mammals.