The shortest day of the year just passed and residents of the Hill have more daylight hours in their future. Days when we don’t go to work and come home in darkness are coming. For humans, this isn’t all that profound because it happens every year and many of us have stepped away from what the solstice really means. For our local plants and animals though, it’s a vital turning point in their circadian rhythms.
Circadian what? Simplistically, any biological process that oscillates over a 24-hour period, brought on by factors within the organism. For example: most flowering plants open and close their flowers over the course of a day. However the trigger for those internal factors is external — in this case daylight.
Here comes another big word: photoperiodism. This mouthful is defined as the physiological reaction of an organism to the length of day and night. Most often it’s described in relation to plants, because they are the most easily studied, but animals, fungi, and cyanobacteria all react to the length of the days. What the shortest day of the year represents to the Hill’s plants and animals is an essential turning point in the countdown towards the switch from vegetative to reproductive phases of their lives. Reproduction is often the end result of photoperiodic processes. However, photoperiodism can encourage the growing of leaves (and their loss), the changing of color in animals, and a wealth of other reactions that aren’t purely reproductive.
You’re probably wondering how some species can distinguish day length. For animals, it’s a pretty simple process: you see the length of day with your own eyes. Of course, a gray squirrel in Volunteer Park doesn’t just decide it’s time to start shedding out of its winter coat. Its body reacts to the environmental cues around it, including light. Another example not happening too far from here is that of snowshoe hares. Their switch between snowy and brown colorations is based largely on the length of the day, always a reliable cue in comparison to temperature and snowfall. This is an inherent, automatic process that’s brought on by photoreceptors which are “counting” the length of the days in the background.
In general, plants have photoreceptors on branches and leaves and animals have photoreceptors in our eyes, but like almost any notion in biology, this is simplistic. There’s plenty of examples of “extraocular” photoreception in animals. Both insects and amphibians have light-sensory organs that guide their physiological and behavioral lives without their eyes. I find it amazing that plants can 1) measure light through photoreceptor proteins, 2) relate that to a 24-hour clock, and 3) increase and abate their growth and flowering on those measurements.
For whatever reason, I think of a particular bird in relationship to photoperiodism: A humble male Song Sparrow, Melospiza melodia, spends its entire year living on the Hill. However, as the days lengthen photoreceptors tell his testes to grow, which means they can produce more androgen. This sends him down a tunnel of extreme machismo, which he exhibits by singing more and more frequently as the days lengthen, showing all that he’s one tough and virile little dude. By the time the days have peaked at the Summer Solstice on June 21, he’s convinced every other Song Sparrow nearby of his territory around Broadway Hill Park and has reared a brood successfully, all because light informed his body it was time to amp up. As the days shorten again and fall sets on, his testes shrink and he cares less about singing, territory, and mating, and more about finding food, staying warm, and not becoming dinner to a Sharp-shinned Hawk. And every one of our birds do this, in one form or another.
Photoperiodism isn’t the only way organisms get environmental cues to spur growth and behavior, as temperature, humidity, and a variety of other things coalesce to inform. However, photoperiodism is an essential tool to our wild neighbors on the Hill; and also for us. It’s no coincidence that we’re seeing physiological issues connected to those shining screens we cart around and stare at well past bedtime. Despite arrogantly behaving as if we are beyond them, our bodies rely on environmental cues too. When we take those away, we’re a bit lost. So, maybe we shouldn’t bemoan the darkness of winter because without it we wouldn’t truly enjoy the light of summer.