Reindeer are multitaskers. Researchers have discovered that they can sleep and actively digest their food at the same time by ruminating their cud. During this time, your sleep is even deep and restful at times and is similar to deep sleep. At the same time, the more time the reindeer spend slumbering while ruminating, the less time they spend in normal deep sleep. Thanks to the digestive naps, the animals are generally more rested than without these sleep phases and thus gain valuable time in the summer to eat enough winter fat.
Some mammals such as cows, sheep and deer have multiple stomachs. As their name suggests, these ruminants chew their food, swallow it, regurgitate the half-digested food, and chew it again. This means they utilize the ingredients of their purely plant-based food more efficiently than with just one chewing session. This strategy is also used by the reindeer, which are widespread in the tundra and taiga of the northern polar regions. During the forage-rich summer months, they graze there almost around the clock in order to accumulate enough energy in the form of body fat for the long, dark winter when they find little food left. Because it never gets really dark in the Arctic in summer and never really light in winter, reindeer do not follow day or night patterns during this time, as previous studies have shown. Whether and how the animals get enough sleep and recovery during the intensive feeding period in summer has been a mystery until now.
Reindeer sleep the same amount in summer and winter
A research group led by Melanie Furrer from the University of Zurich has now examined the sleeping behavior of reindeer in more detail for the first time. To do this, the neuroscientists measured the brain waves of four adult female Eurasian tundra reindeer (Rangifer tarandus tarandus) using EEG. These belonged to a herd living in stables at the University of Tromsø in Norway. To find out whether the light-dark rhythm has an influence on the reindeer's sleep quality and sleep patterns, Furrer and her colleagues carried out the measurements three times for four days each: around the summer and winter solstices and during the equinox Autumn. The experimental animals always had unlimited access to food.
Surprisingly, the EEG recordings showed that the reindeer slept the same amount overall under these conditions, regardless of the time of year. On average, they spent 5.4 hours a day in so-called non-REM sleep, which also includes deep sleep phases, 0.9 hours in REM or dream sleep and 2.9 hours ruminating. At the same time, the animals were much more active in the summer and ate more and for longer periods of time, just like in the wild, as the scientists observed. How does that fit together? “The fact that reindeer sleep the same amount in winter and summer means that they must have different strategies to deal with the limited sleep time in the Arctic summer,” says senior author Gabi Wagner from the University of Tromsø. She and her team suspected that the reindeer rest while ruminating - similar to what has already been observed in other ruminants. When they do this, they lie or stand as still as when they sleep, and react significantly less to their surroundings.
Reindeer sleep and digest at the same time
To test this theory, Furrer and her colleagues evaluated the EEG recordings in more detail and observed the consequences of targeted, temporary sleep deprivation. In fact, it was shown that the reindeer's brain waves during rumination were at times similar to those during non-REM sleep phases. According to this, the animals not only rest when ruminating, but even sleep deeply at times while they digest their food. By meeting their sleep and digestion needs at the same time, the animals are more “rested” after ruminating than if they remained awake, the researchers report.
The reindeer also save time: “The longer reindeer ruminate and sleep, the less additional non-REM sleep they need,” explains Furrer. This is especially important in summer when the animals eat more and have to digest more. In summer, the reindeer suffered less from the consequences of targeted sleep deprivation than in winter because they compensated for this more often with digestive sleep in summer. However, the data also shows that reindeer only sleep temporarily while ruminating. Follow-up studies must clarify why this is the case and whether rumination may be less efficient when the animals sleep. These could then also study the behavior and sleep of younger reindeer as well as reindeer under more natural conditions.
At the same time, the findings explain why reindeer need large areas in which they can graze undisturbed. “Our data illustrates reindeer’s need for undisturbed rumination and rest and supports traditional reindeer herders’ repeated calls for grazing peace near humans,” says Wagner.
Source: Melanie Furrer (University of Zurich) et al., Current Biology, doi: 10.1016/j.cub.2023.12.012