These include urine (indicating predators or competitors), lichens (a food source) and fur (as possessed by wolves, predators of reindeer).[16] Although reindeer do not possess a specific UV opsin, retinal responses to 330 nm have been recorded, mediated by other opsins.[17] It has been proposed that UV flashes on power lines are responsible for reindeer avoiding power lines because "...in darkness these animals see power lines not as dim, passive structures but, rather, as lines of flickering light stretching across the terrain."[18]