Practiced by humans since prehistoric times using wooden ski-shaped contraptions, cross-country skiing was popularised as a sport by the Norwegian military in the mid-1700s and was first imported to North America by Scandinavian immigrants in the 1850s.

BBC: Cross-country skiing in Washington��s Methow Valley

It has even been suggested that the link may hark back to prehistoric times, when humans stored up on fat during the summer months when food was plentiful and they slept less as a result of the shorter nights.

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Other caves in the Highlands include the Inchnadamph Bone Caves, where the prehistoric remains of animals, including bears, and humans have been uncovered.

BBC: Excavation at High Pasture Cave

Plant-eaters of prehistoric origin, rhinos have no known predators other than humans.

WSJ: 3 charged in smuggling horns of endangered rhinos