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.
Highlights
Animals living above the Arctic Circle experience extensive annual periods of permanent light and near-complete darkness in the summer and the winter sky, respectively
An adaptive mechanism to dim illumination found in some mammals is the presence of a reflective surface behind the central retina, the tapetum lucidum (TL, commonly known as ‘cat’s eye’) that resides under an unpigmented region of the retinal pigmented epithelium (RPE)
This study reveals a unique mechanism of visual adaptation in a large mammal whose visual environment is challenging
Summary
Animals living above the Arctic Circle experience extensive annual periods of permanent light and near-complete darkness in the summer and the winter sky, respectively. Winter, when the sun is below the horizon, has a deep blue hue owing to extensive Rayleigh scatter This extreme photic environment poses unique challenges and one adaptive response in Arctic reindeer (Rangifer tarandus) has been the extension of their visual range into the near ultraviolet (UV) allowing greater use of winter light that is relatively UV rich [1]. TL was dissected from animals killed in summer, as above, and the preparations containing the TL and underlying choroid were wet mounted on slides These were all photographed directly, and had a pressure applied similar to the intra-ocular pressure found in winter This was undertaken repeatedly in two separate specimens of central TL
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