We all had to spring forward recently, a move that extends the afternoon daylight.
In this week’s Wildlife Matters, the Masked Biologist examines the impact of photoperiods on living things including us.
The recent change to Daylight Savings Time got me thinking about animals and daylight. The creatures in the woods do not have to change their clocks, naturally, but the seasonal change in day length plays an important role in their lives. Biologists use the word “photoperiod” as a way of saying “number of daylight hours.” Here in the United States, we humans value the role of photoperiod in our lives and society so much that we chose to change our clocks twice a year to maximize our use of daily photoperiod. Basically, we fool with Mother Nature! Only a handful of nations in the world use Daylight Savings Time, and most are in the Northern Hemisphere.
Why the Northern Hemisphere? In our location on the globe, we see a dramatic change in photoperiod length. December 21st is the winter solstice, with an 8-hour photoperiod. In the summer, our longest day is June 21st, with over 15 hours of sunlight – almost twice that of the shortest December day! This photoperiod change definitely affects humans; in the winter, it is commonly referred to as Seasonal Affective Disorder, or SAD. We become sad, irritable, disinterested, become sleepless or oversleep, it’s a kind of sunlight-deprived depression basically throwing our well-being off kilter.
Different wildlife species cope with these changes in different ways. If you are a migratory bird, you head south, flying thousands of miles closer to the equator (and somewhat closer to the sun); maybe down to Central or Southern America. If you are a bear, you find a den to basically sit out winter, snoozing, keeping activity to a minimum, growling if anyone comes too far into your den without first sedating you. If you are a chipmunk, forget about it – you find a warm spot deep in a hole and conk out completely until spring. If you are a turtle, you might lay in the mud, or below the water, allowing parts of your body to freeze until spring thaws. If you are a deer, you slow your bodily functions down so you can get by on a minimal amount of food, then just kind of trudge through the snow until the south-facing slopes open up.
The lives of plants are linked to photoperiod as well. The sun warms the soil in spring, signaling plants to grow and seeds to germinate. As plants grow, they have a special adaptation that allows the shaded side of the plant to grow faster than the sunlit side. The result is that plants lean toward the sun. Leaf thicknesses adjust to take advantage of the heat and humidity of summer for creating food, which is stored in root systems as daylight shortens in fall. Duration of photoperiod helps many plants bloom or flower at the right time, so that their fruit sets after the frost of spring and is ready to spread its seeds before the ground freezes in late fall.
So, photoperiod triggers in wildlife the sense that it is time to change behavior patterns, maybe give birth to young, or to migrate back north. It triggers plants to grow, bud, flower, or shed leaves. We know that photoperiod is not the only trigger for plant and animal species life events, because air temperature also helps bring snow cover to an end, waking up insects, and stimulating plant growth in the spring. In fact, recent studies have shown that migratory birds are heading north days earlier than they did years ago. The sunrise and sunset times have not changed drastically enough to change this behavior, but global temperatures have. But temperature, ground frost, snow cover, and photoperiod are all still seasonally linked. These combined factors tell wildlife that it is time to resume their higher energy functions—and time for humans to be less sad.
Striving to make new things familiar and familiar things new, this is the Masked Biologist coming to you from the heart of Wisconsin’s great Northwoods.