All it's Clocked Up to Be: Circadian Rhythms

Sandhill cranes migrating.

Have you noticed that the days are getting shorter? You’re not the only one! We’re a little under two weeks away from the winter solstice, and the subtle seasonal shifts of approaching winter affect plants and animals (including humans!) in significant ways. Keep reading to learn about circadian rhythms and how animal and plant behavior changes in response to these stimuli.

~~~

Everybody uses their clock. I’m not talking about the clock on your computer or on your phone, nor am I talking about the alarm clock that wakes you up every morning. Not even the clock on your office wall that you stare at, counting down the minutes until you get to go home and watch your favorite TV show. This is a clock that is unique to each person, one much more ancient than the sundial itself. Every human, animal, plant, fungus—even a large amount of protists and bacteria—have a clock inside of them that regulates the timing of all of their functions. 

What are circadian rhythms?

Circadian rhythms are physiological processes within our bodies that are controlled by a variety of environmental factors, including light, temperature, sound, and sleep, and are often influenced by pheromones and other social cues. These rhythms can be considered a “biological clock” that regulates when certain behaviors and internal processes need to occur. 

The most common example of circadian rhythms are sleep cycles. A lot of our sleeping habits are dictated by our circadian rhythms and how we “set” our biological clocks. Say someone chooses to sleep at 10:00 PM every night and has an alarm set for 5:00 AM every morning, and they keep this schedule for a month. Doing this consistently sets their biological clock, causing them to get tired around 10:00 PM every night without thinking about it and waking up at 5:00 AM every morning, no alarm clock needed. 

It can be really easy to set your circadian rhythms, but it can be just as easy to reset them. If you were to get on a plane from Miami and go to France, you would experience a common phenomenon known as jet lag for the following days. The time zone shift would mess with your internal rhythms as the patterns of the sun don’t match the set rhythms you were used to in Miami. This would be even more drastic of an effect if you were to go to Alaska and live there for a few months during the summer (or winter). Since Alaska gets 21 hours of daylight and three hours of darkness in the summer (and vice versa in the winter), spending some time there and then traveling back to an area with more consistent days and nights would be a very strenuous shift for your internal clock. 

Circadian rhythms in nature

So how do our native flora and fauna use these circadian rhythms? There are several examples that can be observed during the winter months.

Leaf fall

The most easily visible and drastic example of circadian rhythms is the leaf fall. In autumn, deciduous trees start changing colors, green changing to a variety of reds, oranges, and yellows. This is commonly thought to be a reaction to cooling temperatures, but this year temperatures were still very warm compared to average autumn temperatures. And yet, the trees still started changing colors, proving that temperature isn’t the only factor at play—day length is just as significant.

While temperature does play a role in tree dormancy, trees primarily use their internal clocks to measure the amount of daylight they receive each day, which lets them know when winter is approaching so that they can prepare. These rhythms also signal when the days start getting longer so that trees can begin growing new leaves for the spring. 

Winter dormancy

Animals rely on circadian rhythms to determine when to start their dormancy periods, including hibernation and brumation. 

Hibernation occurs when an animal enters a prologed period of rest accompanied by a lower heart rate, decreased body temperature, and slower respiration. Black bears are often associated with hibernation, but in the Blue Ridge it doesn’t typicalls get cold enough for our black bears to go into true hibernation.

Brumation is a state of dormancy undergone by ectothermic vertebrates similar to hibernation. This period of dormancy involves a slowed metabolism and reduced activity levels during periods of cold weather, but unlike hibernation it does not entail entering a period of long and deep slumber.

migration

One overlooked use of circadian rhythms is their role in migrations. In response to the winter season beginning and ending, many animals embark on migrations to overwintering and breeding locations dictated by their internal clocks.

Each species that migrates has their own unique timing for their journeys based on their specific needs. This is how mass migrations of species like the monarchs or the nighthawks are formed, as each individual subconsciously knows when the migration is supposed to take place. 

Not only do they use the sun to determine when they need to migrate, they also use the sun to determine where they need to go. By following the sun’s path across the sky, rising in the east and setting in the west, migrating animals can figure out the direction they need to travel in to reach their final destination. So not only do animals have an internal clock, but many have an internal compass too!

breeding cycles

Another important use of circadian rhythms is the timing of breeding cycles. There’s a reason you hear so many frogs and birds calling and so many flowers blooming in the spring! The internal clocks of many plants and animals are set so that the spring is the peak time for procreation. But, you won’t hear the same calls or see the same flowers all spring.

These breeding seasons are split up across the spring and early summer, each determined by each species’ internal clocks. So you may hear spring peepers and wood frogs as early as February, but those calls will be replaced by green frogs and bullfrogs come May. Some species even have breeding seasons in the fall, either in addition to a spring breeding season or as their sole breeding season. This is most commonly seen in marbled and mole salamanders.

There are so many uses for circadian rhythms—even beyond what we discussed. Almost every behavior an organism has can be tied back to their internal clock. So if you ever feel like you’re too bogged down in your schedule, take some comfort in knowing that almost every other organism has a strict schedule too!