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This week on Nature League, Brit Garner explores the rhythms of life on Earth by breaking down a recent scientific journal article about circadian rhythms of reindeer in the Arctic.

Article citation:

W. Arnold, et al.

Circadian rhythmicity persists through the Polar night and midnight sun in Svalbard reindeer

Scientific Reports, 2018

Article link:

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Nature League is a weekly edutainment channel that explores life on Earth and asks questions that inspire us to marvel at all things wild. Join host Brit Garner each week to learn about, connect to, and love the amazing living systems on Earth and the mechanics that drive them.
On Nature League, we spend the third week of each month exploring a current trending article from the peer-reviewed literature.

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It just requires a bit of a break down. [CHEERY INTRO MUSIC]. For this month's De-Natured segment, we're going to look at an article released in September 2018 in the journal Scientific Reports. This month is all about rhythms and cycles, and in this month's Lesson Plan we explored some endogenous biological rhythms- ones that are regulated by internal signals.

We discussed how some of these endogenous rhythms are so strong that they can persist regardless of environmental changes in things like light and temperature. Some of the most common endogenous rhythms on Earth are circadian- that is, they operate on a cycle of about 24 hours. That 24 hours lines up with the rotation of the Earth on its axis, and the daily light-dark cycle is pretty hard to miss.

But, different species and individuals experience these daily light-dark cycles differently, especially depending on where they live. For example, growing up in Florida, there wasn't a huge difference between the length of days throughout the year. Sure, when the clocks changed due to daylight savings time it got darker earlier or later, but in general, there wasn't too much of a difference.

Fast forward to my life now in Montana and whoa. Maximum daylight time during the summer can be more than 7 hours longer than in the winter. I'm still getting used to the massive seasonal differences, and I've been here for about five years.

These experiences in my own life have made me wonder what daily light-dark cycles are like for species that live at the poles of the Earth, where there's the biggest difference between winter and summer days. In this paper entitled, “Circadian rhythmicity persists through the Polar night and midnight sun in Svalbard reindeer”, the researchers investigate a species very familiar with this: a reindeer that lives in the Arctic. So here's what's already known.

Circadian clocks are entrained, or calibrated and set, to daily 24 hour rhythms by external cues. These cues as called zeitgebers, and they include things like temperature and light. Constant, 24-hour zeitgebers aren't really present in polar regions where day/night cycles are so extreme.

Without a predictable night and day available throughout the year, scientists figured that circadian rhythms in Arctic animals would deviate from 24 hours. There's something else to consider as well when it comes to daily rhythms in the Arctic. For large herbivores like reindeer, food is hard to come by during mid-winter when the ground is frozen and there isn't as much plant growth.

They make up for this by fattening up during the summer and then living off these reserves during the winter. So maybe it's advantageous to give up the whole circadian rhythm thing in order to take advantage of turbo feeding in the summer and then minimizing activity during the winter. Some studies have reported a lack of circadian rhythms in several Arctic species, and even mention that this loss of 24 hour rhythm might just be a regular thing in vertebrates living at the poles.

However, these studies were based only on the timing of certain behaviors. In this new study, researchers not only looked at behavior, but also physiological variables. So let's check out the variables this team measured on this study group of Svalbard reindeer.

The team measured four main response variables- these are variables that may or may not change in response to something. In this study, the first response variable was resting heart rate. In addition to that, they also got a temperature reading.

Behavior was also a variable of interest, and they measured both the activity of the reindeer as well as the changes in their head positions, which indicated grazing. So these were the response variables the team measured. However, they wanted to see if these variables changed in response to something, and those somethings were the predictor variables.

In this study, the predictor variables included the daily changes in sun radiation, daily changes in outside temperature, and the average amount of vegetation on the landscape. What's cool is that they measured all of these variables over the course of an entire year, meaning that they could investigate what happens behaviorally and physiologically day to day, and also across seasons. These reindeer live in the Arctic, and that comes with a lot of unique challenges.

For example, during the mid-winter they experience a Polar Night where the sun doesn't actually rise above the horizon. And in the Arctic summer, there's a period of Midnight Sun where there's continuous daylight. The whole point of the study was to see if circadian rhythms persist in a species that's subject to such incredible differences in environmental cues like daylight and darkness.

So what did they find? Overall, they noticed seasonal changes in their response variables throughout the year. But what about daily and circadian rhythms?

Well, the team found significant evidence of daily rhythms in all four response variables. What's more, these daily rhythms existed throughout the year, even during periods of. Polar Night and Midnight Sun.

However, they wanted to know if these rhythms were influenced more by the availability of sunlight, or by the availability of vegetation. Basically, is this behavior because it's light outside, or because there's a lot of food? To check this out, the team compared months of Midnight Sun with little or no plant growth to months of Midnight Sun with new plant growth.

They observed strong 24 hour rhythms in the response variables during the period of Midnight. Sun without new vegetation. During months with constant daylight but new plant growth, these 24 hour rhythms were still present, but somewhat weaker.

These results imply that periods of constant light aren't the only thing affecting behavioral and physiological changes. In these reindeer, the weakening of daily rhythms is more likely due to vegetation and feeding instead. This article was published in a high profile journal, and I've come across it on several science news sites.

Here's why I think this article is making the rounds. All life of Earth is subject to the fact that the Earth rotates on its axis. Some organisms experience this rotation differently depending on where they live, especially in terms of daily light and dark cycles.

Amazingly, we see circadian, or daily, cycles in all kinds of organisms and in all kinds of places. So, how deeply entrained circadian rhythms are in organisms is sort of a big question in biology. And, to see evidence of circadian rhythms in organisms that live through Polar Nights and Midnight Suns is pretty incredible.

Another reason these results are a big deal has to do with the current changes happening on Earth. With climate change, some species are expanding their ranges into the Arctic, and this will affect the light-dark cycle they experience daily and throughout the year. As with any piece of new research, there are several areas of improvement that exist in this study.

My first issue is the sample size. While the team collected an incredibly large amount of data on these reindeer and their environment, the sample size of the reindeer themselves was only 4. Measuring variables on only 4 individual reindeer could lead to issues with generalizing the results to full populations.

Another thing that jumped out at me while reading this article is that the results from this paper are at odds with previous research on the same topic. Past publications have reported a lack of daily rhythms in Polar vertebrates, including reindeer. However, the authors of this paper note that their results are different from previous ones because of how often they collected data, and the statistical tests they did.

My critique here isn't with this new paper, but with our current acceptance of past results. The truth is that our analytical power is rapidly increasing, and the availability of greater statistical power means that we might need to re-do previous studies and keep an open mind about the veracity of currently standing results. Room for improvement aside, one thing I really love about this project is how they incorporated so many different kinds of data.

I mean, the datasets including things like reindeer head movements, remotely sensed changes in vegetation growth, and solar radiation. It's really neat to see the melding of living and non-living variables within a single study. Thanks for watching this episode of De-Natured here on Nature League.

It looks like some life on Earth keeps daily rhythms despite living in extreme environments like the Arctic, and we'll keep exploring the intricacies of rhythms next week during an episode of From A to B. To keep going on life on Earth adventures with us each week, make sure to subscribe at and share.