What is unique about reindeer among deer?

Both sexes grow antlers, which no other deer does. The timing matters: males shed theirs in early winter after the rut, while females keep theirs until spring, through the pregnancy - so any reindeer with antlers in late December is female, a fact that Christmas iconography has never taken on board.

Antlers are bone, grown and discarded annually, and they are the fastest-growing tissue of any mammal - up to a couple of centimetres a day in the largest species. The female's are thought to be for defending feeding craters in the snow, which is a serious matter when the food is lichen under a hard crust and the herd is competing for it.

Two other oddities. Reindeer eyes change colour with the season, from gold in summer to deep blue in winter, an adjustment of the reflective layer behind the retina that trades sharpness for sensitivity during months of darkness - the only mammal known to do it. And their tendons click audibly as they walk, which appears to help the herd stay together in a whiteout.