title: Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer.
abstract: Arctic reindeer experience extreme changes in environmental light from continuous summer daylight to continuous winter darkness. Here, we show that they may have a unique mechanism to cope with winter darkness by changing the wavelength reflection from their tapetum lucidum (TL). In summer, it is golden with most light reflected back directly through the retina, whereas in winter it is deep blue with less light reflected out of the eye. The blue reflection in winter is associated with significantly increased retinal sensitivity compared with summer animals. The wavelength of reflection depends on TL collagen spacing, with reduced spacing resulting in shorter wavelengths, which we confirmed in summer and winter animals. Winter animals have significantly increased intra-ocular pressure, probably produced by permanent pupil dilation blocking ocular drainage. This may explain the collagen compression. The resulting shift to a blue reflection may scatter light through photoreceptors rather than directly reflecting it, resulting in elevated retinal sensitivity via increased photon capture. This is, to our knowledge, the first description of a retinal structural adaptation to seasonal changes in environmental light. Increased sensitivity occurs at the cost of reduced acuity, but may be an important adaptation in reindeer to detect moving predators in the dark Arctic winter.
authors.affiliation: Department of Arctic and Marine Biology, University of Tromsø, , Tromsø, Norway, Institute of Ophthalmology, University College London, , 11-43 Bath Street, London EC1V 9EL, UK, Moorfields Eye Hospital, , London, UK.
authors.forename: Karl-Arne
authors.initials: KA
authors.lastname: Stokkan
authors.forename: Lars
authors.initials: L
authors.lastname: Folkow
authors.forename: Juliet
authors.initials: J
authors.lastname: Dukes
authors.forename: Magella
authors.initials: M
authors.lastname: Neveu
authors.forename: Chris
authors.initials: C
authors.lastname: Hogg
authors.forename: Sandra
authors.initials: S
authors.lastname: Siefken
authors.forename: Steven C
authors.initials: SC
authors.lastname: Dakin
authors.forename: Glen
authors.initials: G
authors.lastname: Jeffery
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D001110:Arctic Regions; D003624:Darkness; D007429:Intraocular Pressure; D012053:Reindeer / Q000502:physiology*; D012160:Retina / Q000502:physiology*; D012621:Seasons; D014785:Vision, Ocular / Q000502:physiology*
keywords: retina; scotopic vision; seasonal adaptation