Cryptic impact: Visual detection of corona light and avoidance of power lines by reindeer

Abstract

Assessing the impact of human development on animals is complicated by the fact that overt effects may have covert causes. Cryptic impacts (sensu Raiter et al. 2014) can arise where sensory stimuli to which species respond fall outside the human sensory range. Ultraviolet (UV) light, which is detected by a range of nonprimate mammals, is a potential example. We review evidence that dark-adapted eyes of reindeer–caribou Rangifer tarandus can detect light at 330–410 nm emitted by electrical corona on highvoltage power lines, which is necessarily barely visible to humans. Based on this, we suggest that the superior ability of Rangifer to detect corona UV light may partly account for the tendency of the animals to avoid power lines. Rangifer has UV-permissive ocular media that transmit approximately 15 times more corona light than human eyes. Retinal irradiance under full dilation is in the order of 7 times greater in Rangifer compared with humans. Seasonal transformation of the tapetum lucidum substantially increases retinal sensitivity in this species in winter. Threshold distances of detection of corona by Rangifer are in the order of hundreds of meters. Displays of corona may catch the animals’ attention, and plume coronas, in particular, may induce the illusion of motion (the phi phenomenon), thereby falsely signaling the presence of potential predators. Both features are likely to increase wariness and cause animals to withdraw from the source of the stimulus. We suggest that spatial and temporal variability of corona contributes to substantial variation observed in the strength and persistence of avoidance responses in Rangifer at these structures.