Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [33]
It is likely indicative that all of the about thirty species of the world’s flying squirrels are nocturnal, while none of the one hundred or so species of day-active squirrels are adapted for gliding flight. The fact that none of the day-active mammals are fliers or gliders cannot be attributed to dietary specializations. Does it concern predation? Gliding flight saves much energy for moving around, yet it makes the animal conspicuous to predators, with the additional drawback that the wing membranes compromise agility. (Bats, the best mammalian fliers, are among the poorest runners.) Maybe squirrels flew to save energy, then had to become nocturnal to escape predators, then had to fly even more, because the noise of scampering on the forest floor would be the owls’ hunting cue. Hence, the need for energy economy in these mammals would positively reinforce a nocturnal lifestyle that encourages gliding and flying.
Flying squirrels don’t leave being active at night up to mere chance; their circadian clock ensures that they get up and going only after sundown. That does not mean that they do not heed light cues from the environment. They do use light cues to synchronize their internal clock to keep to the daily twenty-four-hour rhythm so that they can get up and go out of their dark daylight hiding places soon after it gets dark outside. How then do we know whether a night-active animal becomes active after dark because it is the right time, or merely because it is dark then (and vice versa in a day-active animal)?
Flying squirrels were important in answering this fundamental question. They are one of the first mammals that were shown to be able to become active at the right time independent of external cues. The pioneering and now-classic experiments that revealed the fascinating world of chronobiology in the southern flying squirrels (Glaucomys volans), and subsequently in almost all other organisms examined, were conducted by Patricia J. DeCoursey from the Zoology Department at the University of Wisconsin.
DeCoursey’s research was based on sixty-eight squirrels that she trapped and raised in Wisconsin. The squirrels were individually housed in cages, each equipped with a running wheel mounted on a bicycle axle. An eccentric cam attached to the axle momentarily closed a microswitch circuit at one point in each wheel revolution to leave a mark on a chart moving at a uniform rate of 18 inches per day. A continuous record of both the number and time of wheel revolutions were thus displayed for later analysis. The continuous record over many yards of paper was cut into daily strips that were aligned by time and then pasted one day beneath the other in sequences of days extending over months. From these records DeCoursey could determine within two minutes when the squirrel had run in the twenty-four-hour cycle and how the activity on one day compared with others.
From the stack of numerous twenty-four-hour records one on top of the other, she saw at a glance that, not surprisingly, the flying squirrels are night-active (unless, of course, they can safely come out to the dairy bar for ice cream). They began running shortly after dark, and then they ran either sporadically or almost continuously (depending on the individual) until dawn, when they ceased all running activity until the next night, or they have two activity periods, one right after dark, and another before daylight.
The above now and perhaps even then rather prosaic results were the preconditions for the real experiment, when DeCoursey next put the caged squirrels into constant darkness. Would they now run continuously or sporadically? The answer was: neither. Surprisingly to DeCoursey, each squirrel ran on the wheel at nearly the same times as it had previously when it had experienced a twenty-four-hour light-dark cycle. That is, the squirrel knew when it was time to be active because it apparently consulted an internal timer. Skeptics