Reindeer sleep by ruminating, a strategy that could help them in summer

The researchers report December 22 in the journal Current biology that the more time reindeer spend ruminating, the less time they spend in non-REM (non-REM) sleep. EEG recordings revealed that reindeer brain waves during rumination resemble brain waves present during non-REM sleep, and these brain wave patterns suggest that reindeer are more “rested” after ruminating.

Researchers believe this multitasking could help reindeer get enough sleep during the summer months, when food is plentiful, and feed almost 24/7 in preparation for the long, rare Arctic winter.

“The more reindeer ruminate, the less additional non-REM sleep they need,” explains first author and neuroscientist Melanie Furrer of the University of Zurich. “We think it is very important that they can save time and simultaneously cover their sleep and digestion needs, especially during the summer months.”

Light-dark cycles are absent in the Arctic in winter and summer, and previous studies have shown that reindeer living in the Arctic do not exhibit circadian behavioral rhythms during these seasons, although they tend to be most active during the day during the spring and autumn equinoxes. when light-dark cycles are present. However, it was unclear whether these seasonal differences also impacted the duration and quality of reindeer sleep.

To study the influence of seasonal light-dark cycles on reindeer sleep patterns, researchers performed non-invasive electroencephalography (EEG) on Eurasian tundra reindeer (Rangifer tarandus tarandus) in Tromsø, Norway (69° N), during the autumnal equinox, summer. solstice and winter solstice.

The reindeer, which were all adult females, were part of a captive herd at UiT, the Arctic University of Norway in Tromsø, and the experiments were conducted in indoor stables with controlled lighting, unlimited food and a constant temperature.

They found that reindeer slept about the same amount of time in winter, summer and autumn, despite the fact that they were much more active during the summer. This contrasts with other species that change their sleep duration in response to environmental conditions. On average, reindeer spent 5.4 hours in non-REM sleep, 0.9 hours in REM sleep, and 2.9 hours ruminating in a given 24-hour period, regardless of season.

“The fact that reindeer sleep as much in winter as in summer implies that they must have other strategies to cope with the limited sleep time during the Arctic summer,” says Furrer.

One possible strategy is to rest during rumination, that is, to re-chew partially digested food, which is an important part of the digestion of reindeer and other ruminants. It was previously observed that domestic sheep, goats, cattle, and mouse deer all produced sleep-like brain waves during rumination, but it was unclear whether rumination could serve a similar restorative function to sleep.

The researchers found that the reindeer’s EEG readings during rumination resembled brain wave patterns indicative of non-REM sleep, including increased slow-wave activity and sleep spindles.

Sleeping and ruminating reindeer also displayed similar behavior, tending to sit or stand quietly during both activities, and were less responsive to disturbances such as a neighboring reindeer sitting or standing – the reindeer responded directly to these disturbances (looking towards 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 non-REM sleep.

Next, the researchers tested whether rumination could reduce reindeer’s sleep craving by depriving them of sleep for 2 hours and measuring their brain waves during sleep before and after this deprivation.

Following sleep deprivation, the reindeer’s EEG readings showed increased slow wave activity, indicating a build-up of “sleep pressure” – the unconscious biological drive for longer, deeper sleep – suggesting that the reindeer experience deeper sleep after sleep deprivation.

However, when the reindeer ruminated, this slow-wave activity decreased during subsequent sleep, and the more they ruminated, the more the slow-wave activity decreased. “This suggests that rumination reduces sleep pressure, which could benefit reindeer because it means they do not have to compromise on sleep recovery when they spend more time ruminating,” explains Furrer.

This is especially important in summer, because the more they eat, the more time reindeer need to spend ruminating. “Rumination increases nutrient absorption, so it is crucial that reindeer spend enough time ruminating during the summer in order to gain weight in preparation for winter,” says Furrer.

Since reindeer appear to sleep by ruminating only occasionally, follow-up studies should compare the impact of rumination during sleep with rumination while awake and would also ideally measure reindeer behavior and sleep in more natural outdoor conditions, researchers say. However, such measurements would require surgically implanted EEG sensors rather than the non-invasive surface electrodes used in this study.

“Another thing we could add is observing young reindeer,” says Furrer. “We know that the need for sleep is much higher in young children and babies than in adults, so it would be interesting to examine the sleep of young reindeer.”