In the distant reaches of northeastern Greenland, musk oxen amble across the tundra, grazing as they go. As Arctic creatures, they need to gather enough energy to make it through cold, dark winters. So when the bright summers come, they eat as if their lives depend on it — as in fact they do.
Their lives are so extreme, scientists have wondered: Do they have circadian clocks?
Most creatures on the planet live in lock step with the planet’s daily cycle of light and dark. There’s a time of day for eating, a time of day for sleeping, a time for digestion and so on. Scientists think 24-hour internal clocks help maximize an organism’s survival by keeping it from, for instance, wasting energy foraging at times of day when food may be hard to find. Evolution clearly favored this approach — circadian clocks exist in almost every life form.
However, the long night of Arctic winter and the endless day of its summer are very different from conditions in the rest of the planet. And researchers report in a paper published Wednesday in Royal Society Open Science that musk oxen behavior does not seem to follow a daily pattern year-round. The most prominent cycles in their behavior are instead those of alternating grazing and digesting, which repeat every few hours, and sometimes are abandoned when the sun doesn’t set in the summer.
The researchers used GPS collars to track 19 free-roaming musk oxen for up to three years, said Floris van Beest, an Arctic ecologist at Aarhus University in Denmark and an author of the new paper. By keeping track of the animals’ movements, they could tell whether they were eating, resting or moving from one area to another over longer distances. They then checked to see whether there were patterns in any of these behaviors — whether they repeated and if they did, how frequently.
“We don’t find very strong circadian rhythms,” Dr. van Beest said, meaning that the animals didn’t seem to be repeating themselves every 24 hours.
Instead, they went through repeated foraging bouts that lasted less than 12 hours. Rhythms were also very different in the winter than in the summer, with some oxen completely losing their patterns in the sunnier months and eating frequently but more or less at random.
To the researchers’ surprise, whether the musk oxen kept up their rhythmic behavior during the summer seemed to depend on the quality of food nearby. Those in places with lush foraging didn’t keep up their patterns. Those with slimmer pickings stuck to their patterns.
This suggests that keeping a rhythm helps maximize the energy musk oxen get from sparse food. But it’s a rhythm that repeats on the scale of hours, rather than daily.
The findings dovetail with earlier work on Svalbard reindeer, where researchers tracking the animals’ body temperatures and other measurements found that they did have a 24-hour cycle. But in the summers they disregarded it entirely, eating as much as they could whenever they could.
When it comes to the musk ox, “we want to tease out what effect this has on fitness,” Dr. van Beest said. When some animals go into a free-for-all mode come summer, “Are they in better shape than animals that don’t do that?” he asked.
Now the team is collecting information about musk oxen’s survival and their reproduction, to see whether breaking from their patterns in summer makes for a healthier life and more offspring in the extremes of the Arctic. It’s more evidence that while 24-hour clocks may be the norm, they might not be as important everywhere on the planet, or in every animal, as we might think.