When ecologists watch nature documentaries, sometimes they get ideas for research projects. John Grady, an ecologist from Michigan State University, kept seeing those inevitable scenes in which shoals of hapless fish are demolished by predators, and thinking about the differences between the cold-blooded killers—the tuna, the cod, and other big fish—and the warm-blooded ones. With a group of colleagues, he started tracking down their whereabouts, and soon found a surprising geographical trend.
The warm-blooded predators—the whales, the seals, the penguins of the world—bucked an almost universal pattern. Most groups of plants and animals are richer in species and more abundant in the tropics. In the ocean, that held for cold-blooded predators. But warm-blooded predators were more diverse toward the poles and conspicuously missing from several warm hot spots. For example, in the seas around Indonesia and Australia, which are among the richest in the world, marine mammals are virtually absent, as are penguins and other swimming birds.
Why? This riddle has a simple answer, Grady argues in a new study—but one with chilling implications for the future of seals, penguins, and whales.
It’s not about food. Grady and his team considered the possibility—warm-blooded animals need a lot to fuel their gas-guzzling metabolism. Perhaps colder waters are just richer in algae, plankton, and small fish? But they found that at higher, colder latitudes, there isn’t actually much more food around. It’s more that warm-blooded animals are eating a much bigger share of it than their cold-blooded rivals.