Observation, rest and digestion: reindeer secrets to last
Calories are needed to combat the cold, and the reindeer have to work so hard to store up energy for the winter that they have to sleep ruminating during the summer. In winter, all lichens are good to have, especially Cladonia rangiferina, whose colours stand out like snow when exposed to ultraviolet light.
Lush vegetation, constant daylight: in summer in the Arctic, the reindeer eat constantly and ruminate for much longer than in winter. By observing them closely, a Swiss-Norwegian team led by Melanie Furrer, a neurologist at the University of Zurich, has discovered that these animals enter a particular sleep phase when they start ruminating. These results were published on 22 December in Current Biology.
To understand the reindeer’s resting strategy, the researchers recorded the brain activity of Rangifer tarandus tarandus, the European reindeer (also found on the Kerguelen archipelago). The experiment took place in Tromsø in a stable where the team controlled the light intensity and provided plenty of food. The reindeer were all females from a herd of free-ranging farm animals. The scientists observed their electroencephalograms at the autumn equinox and then at the winter and summer solstices.
During rumination, the shape of the brain waves was very close to that of quiet sleep (NREM), i.e. a phase of “non-rapid” eye movement. This differs from paradoxical sleep, when we dream and our eyes move rapidly. By interpreting these brain waves, the biologists suggest that after ruminating, the animals also rested, and that they therefore had the capacity to do both tasks at the same time.
Reindeer living in the Arctic do not have a circadian rhythm during the summer, i.e. a 24-hour cycle passing from sleep to wakefulness, and would therefore sleep while ruminating. We think it’s very important for them to be able to cover their sleep and digestion needs simultaneously, particularly during the summer months,” explains the researcher. “Ruminating increases the absorption of nutrients. It is therefore essential that the reindeer ruminate long enough during the summer to put on weight in preparation for the winter.”
An ultraviolet landscape
In winter, food is scarce and – for some reason unknown until 15 December – the reindeer’s golden eyes turn bright blue. “If the colour of light in the environment is predominantly blue, it makes sense for the eye to accentuate the blue colour to ensure that the reindeer’s photoreceptors maximise these wavelengths,” explains Nathaniel Dominy, an evolutionary biologist at Dartmouth University and first author of a study published in the review i-Perception..
The team of Scottish researchers behind this discovery went further. They looked at the ultraviolet light emitted by lichens such as Cladonia rangiferina – a popular winter food for reindeer. These lichens are white to the human eye, invisible in snow, but unlike snow, they absorb ultraviolet light. So, in these wavelengths, to which the reindeer are not insensitive, the snow and the lichen appear one light and the other dark in the polar night. An excellent way to tell them apart.
Camille Lin, PolarJournal
Link to studies:
- Furrer, M., Meier, S.A., Jan, M., Franken, P., Sundset, M.A., Brown, S.A., Wagner, G.C., Huber, R., 2023. Reindeer in the Arctic reduce sleep need during rumination. Current Biology 0. https://doi.org/10.1016/j.cub.2023.12.012
- Dominy, N.J., Hobaiter, C., Harris, J.M., 2023. Reindeer and the quest for Scottish enlichenment. i-Perception 14, 20416695231218520. https://doi.org/10.1177/20416695231218520
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