Summer World_ A Season of Bounty - Bernd Heinrich [6]
Using their circadian clock, the cabbage butterfly larvae begin measuring time at a specific signal: as in most other species, the time of day when darkness turns to light. They then “sample” for the presence or absence of light after measuring off a specific time period—say, about fourteen hours (the exact time differs in populations adapted to various geographical areas). If, for example, in midsummer the day lasts fourteen hours, then they would “see” light when they sampled at their twelve-hour “window,” and then their central nervous system would interpret that as a long day (summer), and continue the normal cascade of hormones to continue their development. However, as the season progresses and the days get shorter, there eventually comes a day when they would experience darkness at the twelve-hour sampling window, and then with no light at that point they would shut down hormone secretion from the brain—until the signal is reversed the next summer, when development would proceed.
Fig. 3. How an animal may determine the season by the length of day. Based on experiments with the cabbage butterfly caterpillar, using three different photoperiods.
Some organisms do not have access to photoperiod signals. For example, at the equator the photoperiod is an even twelve hours of light and twelve hours of dark all year long. Are animals there clueless about what season they find themselves in? Apparently not, because migrant birds who spend the winter in the tropics “know” when it is time to return north and breed in the summer. And contrary to folklore, the groundhog does not need to come out on 1 February to measure its shadow to decide whether or not to stop hibernating and begin its summer agenda. Even if it did, it would have to know when the first day of February is! Strangely, the groundhog probably does know the approximate date. In the 1960s and 1970s Eric Pengelley and coworkers showed that the ground squirrels (Citellus lateralis) can, in the absence of both light and temperature cues, go into and come out of hibernation according to an internal calendar-clock. Subsequently Eberhard Gwinner showed that European migrant warblers also timed their annual fattening, migration, and breeding schedules with reference to this “circa-annual” rhythm.
One of the most conspicuous and stunningly beautiful seasonal phenomena in the north temperate zone is the flowering and leafing out of the northern forest. Both the flowering and the leafing out determine the insect populations, which in turn make the summer world possible for the majority of the birds and most mammals.
Flowering and tree leafing are precisely scheduled events. By the end of January we’ve had three months of seeing all our trees starkly bare, and we’re still experiencing snowstorms and bitter cold. “Only four months to go” we think then, before the glorious time arrives when the buds break and the trees flower, and are again resplendent in the long-awaited and much-anticipated color, green!
Our impatient waiting is all the harder when we realize that most of the buds are ready-made all along, just biding their time to burst forth. Indeed, they were already fully formed on the trees the summer before, long before the brilliant leaf shows of early October and the shedding of the leaves a week or two later. Buds are embryonic stems with leaves in one package and embryonic flowers in another (as in alders, hazelnut, and birch), or young stems with leaves and flowers encased all together under the same protective leaflike scales (as in most species). All through winter the various types of buds experience and must survive snow, ice, storms, and thaws, and the tree must bear costs for them to have been produced so early. Grouse live for months almost entirely from eating the buds of trembling aspen and birch. Purple finches, pine grosbeaks, turkeys, and squirrels feed on the buds of maple, aspen, firs, and spruce.