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Light sleepers: animals that show little or no sleep during migrations, in the postpartum period or throughout their lives. (a) Commerson’s dolphin; (b) bottlenose dolphin Tursiops truncatus; (c) and (d) killer whale Orcinus orca being born; (e) bullfrog Rana catesbeiana; (f) white-crowned sparrow Zonotrichia leucophrys. Rana catesbeiana photo courtesy of James Harding; killer whale photos courtesy of SeaWorld, San Diego.
