Reindeer have a special digestive strategy
Reindeer are part of the Christmas season. This species of deer, which lives high up in the north, has a particularly sophisticated form of digestion. The animals chew again while they sleep. Experts explain that this is the best way for them to prepare for winter.
Reindeer seem to have a rather clever strategy for gaining enough weight and resting at the same time during the bright Arctic summers: they sleep while ruminating. This is suggested by a study led by researchers from the University of Zurich. In this way, the animals could eat almost around the clock when the food supply is plentiful and thus prepare themselves for the long and food-poor Arctic winter.
Reindeer lichen, which still grows in the freezing temperatures of the Arctic, is the main food on the menu of the animals from the deer family, as well as other lichens, grasses and shrubs. This hard-to-digest food is initially only chewed roughly and later regurgitated by the animals and chewed again in a similar way to cows before it is finally digested.
Digestive work during sleep
During this ruminating process, the animals appear to be at least partially asleep. This is indicated by the results of a study published in the journal "Current Biology". "The more reindeer ruminate, the less additional non-REM sleep they need," explains lead author and neuroscientist Melanie Furrer from the University of Zurich in a press release. This presumably allows the animals to save time while meeting their sleep and digestive needs - especially in the polar summer months when the sun does not set.
The European research group analyzed the brainwaves of a herd of Eurasian tundra reindeer(Rangifer tarandus tarandus) in Tromsø, Norway, during the autumn equinox, the summer solstice when the sun does not set in the far north, and the winter solstice when it does not rise. These were female animals from a captive herd at the Arctic University of Norway, which were studied in indoor pens with controlled lighting, unlimited food and constant temperature.
The researchers found that the reindeer slept roughly the same amount in winter, summer and fall, although they were much more active in summer. "The fact that reindeer sleep the same amount in winter and summer suggests that they must have different strategies to cope with the limited sleep time in the Arctic summer," explains Furrer.
Different sleep phases determined
On average, the reindeer spent 5.4 hours per day in non-REM sleep, 0.9 hours in REM sleep and 2.9 hours ruminating. REM sleep and non-REM sleep with its various stages are two different phases of sleep, which are also known in humans and are expressed in different eye movements and brain waves, for example.
The researchers believe that animals can actually use rumination to rest. For example, sleep-like brain waves have already been observed in domestic sheep, goats, cattle and dwarf deer during rumination. However, it was previously unclear whether rumination could have a similar restorative function to sleep.
Ruminating and sleep phases in comparison
The analysis of the reindeer's EEG readings now showed that their brain wave patterns during rumination were similar to those that occur during non-REM sleep. This included increased so-called "slow-wave activity" with slow brain waves and sleep spindles, i.e. short spikes in brain activity that are typical for this stage of sleep.
In addition, sleeping and ruminating reindeer showed similar behaviors, according to the research team: In both states, they tended to sit or stand quietly and reacted less to disturbances, such as from neighboring animals. For example, if a neighboring reindeer sat down or stood up, they reacted in 45 percent of cases when they were awake, but only in 25 percent of cases when they were ruminating, and in five percent of cases when they were in non-REM sleep.
Melanie Furrer's group also tested whether rumination could reduce the animals' sleep drive. To do this, they deprived the reindeer of sleep for two hours and measured their brain waves before and after. In this way, the scientists found that ruminating after a forced short sleep reduced sleep pressure. According to neuroscientist Furrer, this could be advantageous for the reindeer, as they would not have to compromise on sleep recovery if they spent more time ruminating.
This advantage would be particularly important in summer, because the more the animals eat, the more time they have to spend ruminating. "Ruminating increases nutrient uptake. It is therefore important for reindeer to spend enough time ruminating in summer so that they gain weight for the winter," says Furrer.
However, as the analysis showed, reindeer only spend part of the time sleeping during rumination. Follow-up studies should therefore compare the effects of rumination during sleep with those of rumination while awake and ideally also investigate the sleep of reindeer under more natural outdoor conditions. However, this would require surgically implanted EEG sensors, as opposed to the non-invasive surface electrodes used in the study.
And finally, it might be worthwhile to study young animals in particular, says Furrer: "We know that the need for sleep is much higher in infants and babies than in adults, so it would be interesting to study sleep in young reindeer."
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Source: www.ntv.de
