Let's imagine a biological function that responds to light. We would not expect such a function to continue these responses without external information. For example, in constant darkness. One could imagine more complex functions that to not simply and immediately react to light, but takes several hours to reach a peak. Let's say around mid day. Although such a function looks like a daily rhythm, we would not expect it to continue in constant conditions. But such functions would be rather poor representations of our temporal environment if they ceased to exist without external information, which is similar to orienting with closed eyes. The remarkable thing about the rhythms that represent tides, days, lunar cycles, and years, is that they do continue without external information. These rhythms are self-sustained. Since their periods in constant conditions may deviate a little from the original cycle length. They are also called circa-rhythms. Circa-tidal, circa-dian or circadian, circa-lunar, and circa-annual or circannual. Now I want to give you examples of the various biological rhythms that are shaped by environmental cycles. The oceans shore lines experience tidal changes and are the home of many organisms. For several of these, plants and animals, scientists showed self-sustained circa 12.5 hour rhythms when they are kept in the laboratory in tanks, without tidal changes. In other words, when they are kept in constant conditions. The scientific proofs for self-sustained circa-lunar rhythms are rare. One example is the larvae of an insect called the Ant Lion. They build little craters in their sandy environment and wait at the bottom for ants and other insects to tumble down the steep walls. These are their next meal. Scientists who investigated these larvae in the lab found that the size of these craters changes with full and new moon, and that this rhythm continuous approximately every 28 days in constant conditions. The examples for annual rhythms are much more numerous. Birds changing their feathers or deer shedding their antlers, do so in circa-annual periodicities. Even if they are kept in constant conditions in the lab, without being stimulated by summer and winter conditions. The figure shows an example of starlings whose reproductive physiology changes with a circa-annual rhythm. Normally starlings are exposed to longer, warmer days in the summer and shorter, colder ones in the winter. Which makes it necessary for them to structure their physiology accordingly. But even if they are kept in a constant 12 hour photoperiod throughout the year, the size of their testes increases and decreases and they regularly molt. The period of the circa-annual rhythm measured in the constant lab conditions is slightly shorter than 365 days. Day length or photoperiod, is a crucial signal for the annual timing system. The reproductive physiology of the hamster exemplifies this remarkably. When hamsters are kept in constant darkness, or photo periods shorter than 12 hours, their testes are small. But when they are kept in photoperiods longer than 12 hours, their size increases tenfold. In birds, it is not uncommon for testes to shrink during the non-reproductive seasons from almost a gram down to a few milligrams. Most annual biological rhythms are in some way or another connected to reproduction. Even humans used to show strong annual rhythms in birth rates. These rhythms actually reflect seasonal differences in successful conceptions. And they correlate both with photoperiod and ambient temperatures. In the Spanish example shown here, these annual cycles are remarkably robust across many significant social changes and events, such as wars. Notably, their strongest decline in rhythmicity follows a massive industrialization of the country in the late 1950s. The weakening of annual rhythms in humans is strongly tied to industrialization. And industrialization means that we move from working outside, to inside. And that we started shielding ourselves from bright daylight and ambient temperatures which are both important signals that tell our physiology what time of year it is. [SOUND]
