Marine ecologists love to make sense of the patterns they observe in nature and one of the most intriguing of these is how different species are distributed across our global oceans. Which animals live where – and why? A well-accepted observation is that diversity generally increases as one moves from the poles towards the equator. It has been supposed that a reason for this pattern is that the warmer waters support lots of cold-blooded fish that rely on their surroundings to regulate their internal temperature. However, birds and marine mammals (seals, whales, dolphins and other ocean species that, like us, breathe air and nurse their young with milk) throw a spanner in the ecological works: their diversity is highest in colder, temperate waters.