Publications by authors named "K A Stokkan"

At temperate latitudes, the annual cycle of day length synchronizes circannual rhythms, and, in mammals, this is mediated via nocturnal production of the pineal hormone melatonin, proportional to the length of the night. Here, we studied circannual synchronization in an arctic species, the reindeer (), which ceases to produce a rhythmic melatonin signal when it is exposed to extended periods of continuous midwinter darkness and continuous midsummer light. Using food intake, antler growth and moult as endpoints, we demonstrate that when animals living at 70°N are transferred from natural photoperiods in late autumn to either continuous light or continuous darkness, they undergo a conspicuous acceleration of the circannual programme.

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Occurrence of 24-h rhythms in species apparently lacking functional molecular clockwork indicates that strong circadian mechanisms are not essential prerequisites of robust timing, and that rhythmical patterns may arise instead as passive responses to periodically changing environmental stimuli. Thus, in a new synthesis of grazing in a ruminant (MINDY), crepuscular peaks of activity emerge from interactions between internal and external stimuli that influence motivation to feed, and the influence of the light/dark cycle is mediated through the effect of low nocturnal levels of food intake on gastric function. Drawing on risk allocation theory, we hypothesized that the timing of behavior in ruminants is influenced by the independent effects of light on motivation to feed and perceived risk of predation.

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The mammalian visual range is approximately 400-700 nm, although recent evidence suggests varying ultraviolet (UV) extensions in diverse terrestrial species. UV sensitivity may have advantages in the dim, blue light shifted environment experienced by submerged marine mammals. It may also be advantageous when seals are on land as UV is reflected by snow and ice but absorbed by fur, enhancing visual contrast.

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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.

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