Polar bears (Ursus maritimus) are carnivorous marine mammals that prowl the ice sheets and coastal waters of the Arctic region. They have developed several key adaptations to thrive in the harsh cold of their northern habitat, including a thick, insulating layer of fat and a coat of white fur that not only provides camouflage in snow and ice, but also extends to the bottom of their paws to provide additional grip. Polar bears are excellent swimmers because of their elongated bodies and strong front paws, and are able to cross distances of up to several hundreds of kilometers in water. The animals have no natural enemies and primarily hunt for seals along the edges of the ice where their prey surface to breathe, but they are also known to feed on land mammals or the carcasses of whales. After mating in spring, female polar bears set out to gain weight and dig a den in the snow, where they enter a hibernation-like state and give birth in winter, generally to a litter of two cubs. They raise their young for a period of approximately two and a half years without the help of males, boldly protecting them against the elements and threats, such as wolves and other polar bears.
Information sources: National Geographic, WWF
Image: Polar bear roaming sea ice in Nunavut, Canada. Credit: Ansgar Walk, Wikimedia Commons.
New research performed by biologists from different universities has shown that zooplankton migrate vertically through the waters of the Arctic Ocean by using moonlight during dark winter nights. By studying acoustic data recorded by moored instruments, the scientists found that the vertical migration patterns of zooplankton in winter are driven by lunar illumination across the entire Arctic, in fjord, shelf, slope and open sea environments. During the Arctic winter, zooplankton shift their known diel vertical migration (DVM) periods from a solar day (24 hours) to a lunar day (24.8 hours) when the moon rises above the horizon. In addition, mass sinking of zooplankton from the surface waters to a depth of approximately 50 meters occurs during periods of full moon (every 29.5 days). The scientists suggest that lunar vertical migration (LVM) may enable zooplankton to avoid visual predators, such as carnivorous zooplankters, fish and birds, which use moonlight to hunt during the polar night. The discovery of LVM in the Arctic indicates that light-mediated patterns of biological migration may occur even without the presence of sunlight and has important implications for the exchange of carbon between the surface waters and deeper waters during the Arctic winter.
Journal reference: Last, K. S., Hobbs, L., Berge, J., Brierley, A. S. & Cottier, F. (2016). Moonlight Drives Ocean-Scale Mass Vertical Migration of Zooplankton during the Arctic Winter. Current Biology, 26, 1-8.
Image: Icebergs in the Arctic near Cape York, Greenland. Credit: Brocken Inaglory, Wikimedia Commons.