Deer, Reindeer and REM Sleep

Because of warming due to climate change, caribou are facing a threat from deer. Deer are likely to keep expanding northward. Areas with more deer typically have more wolves. Deer can handle high predation rates, but caribou cannot.

At first glance, geese and reindeer (called “caribou” in North America) may not seem to have much in common. But there is a link: in some places, they share diets and habitats.

One of those sites is in the frigid seas halfway between mainland Norway and the North Pole, where both geese and reindeer browse the vegetation of a high-tundra archipelago, munching on thick moss, cropped grasses and low-lying shrubs. Now, researchers are working to untangle the impacts that these two animals are having on a changing and vulnerable Arctic ecosystem.

Reindeer are also being scrutinized for their relationships with another animal: white-tailed deer. In the boreal forest of Western Canada, a warming climate and increased human alterations of habitats have enabled deer to push farther north, which might be bad news for caribou.

Svalbard, an Arctic archipelago, is home to a variety of wildlife, including arctic foxes, polar bears, seals, walruses, whales, reindeer—and, in the summers, migratory geese. A research team is studying how reindeer and geese will affect the islands’ ecosystems in the future.

There’s still a lot more we’re just beginning to learn about Santa’s favorite ungulates: scientists have recently discovered that the more time reindeer spend ruminating (re-chewing of partially digested food), the less time they spend in non-rapid eye movement (NREM) sleep, or deep sleep. Electroencephalogram (EEG) recordings revealed that the brain waves of reindeer during rumination resemble those present during NREM sleep, suggesting that the reindeer are more “rested” after ruminating. This multitasking might help them get enough sleep during the summer months when food is abundant, and they feed almost 24–7 in preparation for the long and food-sparse Arctic winters.

Migrating geese have more impact on delicate Arctic ecosystems than reindeer

Reindeer have been year-round residents on the islands of Svalbard for thousands of years, but at one point they almost went extinct there. Svalbard reindeer, unlike their southern cousins, tend to be docile and extremely sedentary, making them easy targets for hunters. Miners, overwintering sailors and trappers relied on reindeer for food; and by 1900, the animals were locally extirpated in places, although a few isolated areas had small populations.

Those few reindeer were important, however, because they provided a population for slow recolonization after the Norwegian government extended full protection to the animals, according to scientists from the Norwegian University of Science and Technology in Trondheim, Norway. Today, reindeer populations on Svalbard number more than 20,000 animals.

Reindeer have been year-round residents on the islands of Svalbard for thousands of years; but at one point, they were almost completely gone. Today, though, there are more than 20,000.

Barnacle geese make a temporary but essential home on Svalbard at the end of their long, summer migratory journeys. Their time on the islands is fundamental to their life cycles, as they take advantage of the seasonal, 24-hour light and nutrient-rich vegetation to raise young during a hospitable season in an otherwise very tough environment.

Both reindeer and geese—two, hard-hitting herbivores—influence the composition of vegetation on Svalbard in both direct and indirect ways, from natural fertilization to removal of certain plant species to compaction of soil through trampling. The way these animals interact with their landscape has complex repercussions for how the ecosystem will respond to climate change in the future, say researchers from the Department of Wildland Resources in the Quinney College of Natural Resources at Utah State University.

The climate is shifting faster in the Arctic than in other places because of global climate change, and Svalbard is one of the most rapidly warming regions on Earth. But life over the last half-century has been relatively good for populations of barnacle geese. Conservation policies in their overwintering home in Scotland and changes in climate have led to an impressive population expansion from less than 3,000 birds in 1960 to more than 40,000 birds today. Since this growing population has the potential to change the face of the landscape in Svalbard, it’s important to understand the ways that the geese can alter the land.

AdobeStock (Created by Candice Gaukel Andrews)

Medium-sized barnacle geese have grey-black wings, black necks and a black line between their eyes and bills. They breed in crowded colonies on the western coast of the island of Spitsbergen in the Svalbard archipelago.

Geese are voracious eaters and maintain very low levels of plant biomass where they graze in concentrated patches. The influence that geese exert on the Svalbard tundra appears to be increasing over time; in 2018, peak summer grass biomass in goose-grazed tundra was five times lower than researchers had found in 2008.

The geese, who eat and live in concentrated groups, had a bigger impact on the ecosystem than did reindeer, who are much larger animals but more widely dispersed. This likely reflects inherent differences in both habitat sensitivity and the way that the habitats are used by the different herbivores, as well as the way they eat and move.

Experimental, long-term removal of reindeer from a portion of the island had little evident effect on the ecosystem’s health, said the researchers, but the experimental exclusion of geese made a big difference for the soils and vegetation—even though the geese are obviously considerably smaller in size and are temporary residents.

Barnacle geese, who eat and live in concentrated groups, have a bigger impact on Svalbard ecosystems than do reindeer, who are much larger animals but more widely dispersed.

The researchers say these findings, reported in the Journal of Ecology in September 2023, provide important information to aid projections about the effects of shifting herbivore populations on the functioning of Arctic ecosystems and help to refine predictions on whether and where these shifts are likely to mitigate or further amplify the consequences of climate change.

Deer moving north create considerable trouble for caribou

As the climate changes, animals are doing what they can to adapt. White-tailed deer, for example, have greatly expanded their range in North America over the past century.

Climate change can create milder winters, while habitat disturbances from energy explorations and forestry create new food sources for deer. Now, researchers from the University of British Columbia–Okanagan Campus and their colleagues who have been monitoring white-tailed deer movements into the boreal forest of Western Canada for more than five years have found that both a changing climate and increased habitat alterations have enabled deer to push farther north. What is good for the deer, though, isn’t necessarily suitable for other species, such as threatened woodland caribou.

Milder winters due to climate change and habitat alterations by humans create new food sources for deer. White-tailed deer have greatly expanded their range in North America over the past century.

As global temperatures climb, the researchers note that deer expansion is uprooting existing predator-prey dynamics. Areas with more deer typically have more wolves, and these wolves are predators of caribou. Deer can handle high predation rates, but caribou cannot.

The researchers determined that the northern Alberta–Saskatchewan border provided a convenient experimental location. While both sides have a consistent climate, habitat alteration is, on average, 3.6-fold higher on the Alberta side. So, between 2017 and 2021, the research team maintained 300 wildlife cameras throughout the region to collect motion-triggered images of large mammals. These images were used to estimate white-tailed deer density.

Key findings from their study, published in the journal Global Change Biology in April 2024, include a significantly lower deer density in areas with colder, snowier winters. While human land use was associated with higher deer densities, the effect of human-caused habitat change was much smaller than that of climate.

Svalbard is one of the fastest warming regions on Earth, with temperature increases significantly higher than the global average due to a phenomenon known as “Arctic amplification,” where the loss of sea ice reduces reflectivity, leading to more solar radiation being absorbed and further warming.

Winter severity is expected to decline as climate change progresses. This means that white-tailed deer are likely to keep expanding northward and increase in abundance, adding an increased risk to caribou. When planning for caribou recovery, say the researchers, these new forest residents need to be considered. Now that we are well into the United Nations Decade on Ecosystem Restoration, we must account for new pathways of species interacting in the food web.

Understanding white-tailed deer populations continues to be one piece of the caribou recovery puzzle, say the scientists. Another facet is that climate and human land use are often intertwined. As you move northward, the climate becomes harsher and human land use decreases, making it difficult to isolate these two factors. The debate over the relative effect of climate or habitat change isn’t unique to deer in the boreal forest—it’s one of the most pressing issues facing applied ecologists around the world.

Ruminating results in restful reindeer “sleep”

Light–dark cycles are absent in the Arctic during the summers and winters. Previous studies have shown that Arctic-dwelling reindeer don’t display circadian behavioral rhythms during these seasons, although they tend to be more active during the daytime during the spring and autumn equinoxes, when light–dark cycles are present. However, it was unknown whether these seasonal differences also affected how reindeer sleep.

Reindeer sleep approximately the same amount of time during the summer, autumn and winter, despite the fact that they are much more active during the summer.

To investigate the influence of seasonal light–dark cycles on reindeer sleep patterns, researchers at The Arctic University of Norway in Tromso (UiT) performed noninvasive electroencephalography on Eurasian tundra reindeer (Rangifer tarandus tarandus) during the autumn equinox, summer solstice and winter solstice. The reindeer, who were all adult females, were part of a captive herd at UiT, and the experiments were conducted in indoor stables with constant temperature, controlled lighting and unlimited food.

Results, published in the journal Current Biology in January 2024, showed that reindeer sleep approximately the same amount of time during autumn, summer and winter even though they were much more active during the summer. This contrasts with other species who change the amount they sleep in response to environmental conditions. On average, the reindeer spent 5.4 hours in NREM sleep, 0.9 hours in REM sleep and 2.9 hours ruminating during a given 24-hour period, regardless of the season.

The fact that reindeer sleep the same amount during summer and winter implies that they must have other strategies to cope with limited sleep during the Arctic summer. One possible strategy is the opportunity for rest during rumination, which is an important component of digestion for reindeer and other ruminants. Domestic cattle, goats, lesser mouse-deer and sheep have all been observed to produce sleeplike brain waves during rumination, but it was unclear whether rumination could serve a similar restorative function to sleep.

Ruminating reindeer and sleeping reindeer behave similarly. They tend to quietly rest, sit or stand, and are less reactive to disturbances, such as the movements of a neighboring reindeer.

The reindeer EEG readings during rumination resembled brain wave patterns that are indicative of NREM sleep; and ruminating and sleeping reindeer also displayed similar behavior, tending to quietly sit or stand during both activities. They were also less reactive to disruptions, such as a neighboring reindeer sitting down or getting up. Reindeer directly responded to these disturbances (by looking toward the neighboring reindeer) 45% of the time if they were awake, but only 25% of the time if they were ruminating and 5% of the time if they were in NREM sleep.

Next, the researchers tested whether rumination could reduce the reindeer drive to sleep by depriving the reindeer of sleep for two hours and measuring their brain waves during sleep before and after this deprivation. Following sleep deprivation, the reindeer EEG readings showed increased slow-wave activity, which is indicative of a build-up of “sleep pressure,” the unconscious biological drive for more and deeper sleep. That suggests that reindeer experience deeper sleep following sleep deprivation.

However, when the reindeer ruminated, this slow-wave activity was decreased during subsequent sleep; and the more they ruminated, the more the slow-wave activity decreased. That could be because rumination reduces sleep pressure, which could benefit the reindeer because it means they don’t have to compromise on sleep recovery when they spend more time ruminating.

To study sleep in reindeer, scientists are thinking of looking at young reindeer, since we know that the need for sleep in humans is much higher in young children and babies compared to adults.

This is especially important during the summer, because the more they eat, the more time the reindeer need to spend ruminating. Rumination increases nutrient absorption, so it’s crucial for reindeer to spend enough time ruminating during the summer to gain weight in anticipation of winter.

Since reindeer appear to sleep while ruminating only some of the time, follow-up studies should compare the impact of rumination while sleeping with rumination while awake and would also ideally measure reindeer behavior and sleep in more natural outdoor conditions, the researchers say. Looking at young reindeer could be informative, too, since we know that the need for sleep is much higher in human babies and young children compared to adults. It might be the same for reindeer.