In areas where they live, the sun remains very low in the sky for long periods. This means the light is scattered such that the majority of light that reaches objects is blue or UV. Some parts of the environment absorb UV light and therefore to UV-sensitive reindeer, appear to be black, strongly contrasting with the snow.
These include urine (indicating predators or competitors), lichens (a food source) and fur (as possessed by wolves, predators ). Although reindeer do not possess a specific UV opsin, retinal responses to 330 nm have been recorded, mediated by other opsins. It has been proposed that UV flashes on power lines are responsible for them avoiding these structures because “…in darkness there animals see power lines not as dim, passive structures but, rather, as lines of flickering light stretching across the terrain.