Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [34]
Ultimately, DeCoursey proved with her squirrels that the timing originated internally, and it was no small irony that her best proof came from the squirrels’ small errors in timing. For example, Squirrel Number 131 on average started to run (in total darkness) every 23 hours and 58 minutes, plus or minus 4 minutes, while another under the same dark conditions in the same room ran 21 minutes later each day; i.e., it had an activity cycle of 24 hours and 21 minutes. That is, under constant dark conditions, one squirrel lost 2 minutes each day while another gained 21 minutes per day. Within ten days of “free running” in constant darkness, one squirrel started activity 20 minutes before external evening, while the other was then 210 minutes late, or 3.5 hours out of synchrony with the external world. If both squirrels had consulted an exogenous or external timer, then they both would have run as if responding to the same drummer; they would have kept the same time.
The squirrel’s clock-running speed is genetically determined, but the time at which the beginning of the animal’s running activity is read off is determined by frequently resetting the clock in reference to an external signal. In squirrels, the signal to which their internal clock is synchronized is the moment of lights-out. We now know, for day-active animals, that light hitting the eye causes the pineal gland of the brain to reduce its production of melatonin, a sleep-inducing hormone that is normally produced rhythmically, on an approximate (but not exact) twenty-four-hour schedule. Hence melatonin pills to combat jet lag. A flying squirrel would have to take them in the morning. When DeCoursey reintroduced a one day light-dark cycle in their environment, then the off-schedule squirrels that had been “free-running” in continuous darkness reset their activity regimen to again start running right after lights-out the next day. Ordinarily the squirrels therefore reset their clocks when subjected to the normally occurring light-dark cycle. Superficially they act as though they respond only directly to darkness or light, and without the experiments that is all one could know. DeCoursey could, of course, have waited to make her observations at total eclipses of the sun. But it would have delayed her conclusions because of difficulties for replicating of observations. Experiments involve making things happen and then applying keen observations of the results.
DeCoursey’s demonstration of internal time-keeping was simple, elegant, and irrefutable. It brought a closure to the debate of whether or not a mammal had an internal circadian clock, and it opened up an area of research in cellular mechanisms. We now have volumes of information on circadian clocks since DeCoursey’s experiments in the 1950s, and the information is becoming of tremendous relevance to medicine. For example, effective dosages of many drugs depend strongly on the time in our own circadian rhythms when they are administered. The molecular mechanisms whereby circadian clocks operate have lately been traced to a number of genes, and the most popular “model organisms” in which they are now studied are no longer flying squirrels but mice and fruit flies.
The circadian clock has many potential uses. It allows hibernating ground squirrels, for example, to measure the daily light-dark durations, and from that data the squirrel can derive information about the changing seasons. Correct seasonal responses are crucial for winter survival. Indeed, the circadian clock mechanisms are necessary for all organisms that must prepare for winter, whether by pupating (insects), migrating (insects, birds, some mammals), or hibernating and physiologically preparing (most northern organisms).
BEING ABLE TO GLIDE from tree to tree is a very efficient way of locomotion, but in flying squirrels