Ed Yong/The Atlantic
In frigid waters, their ability to keep their bodies warm gives them an edge over sharks and fish.
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.
Signs of that shift are already apparent. In the Barents Sea, off the northern coast of Norway and Russia, stocks of capelin and other small fish have been going up in recent decades. That should be a boon for predators such as cod and harp seals, but while the cold-blooded cod are indeed flourishing, the harp seals have declined. And that may be because the local water has become considerably warmer.
Grady’s team estimates that every time the ocean’s surface warms by 1 degree Celsius, populations of marine mammals will fall by 12 percent, and populations of seals and sea lions in particular will fall by 24 percent. “The threat of warming waters is a real issue for a lot of marine mammals and birds,” Grady says.
But “predictions are hard,” Hauser notes. There’s not a lot of data on how Arctic mammals are responding to changing climate, but what we have paints a complicated picture. Polar bears are the archetypal losers of a warming world, but some populations are still doing well—as are those of bowhead whales. Some groups of belugas have shifted the timing of their migrations; others are foraging in deeper, colder waters. These changes in behavior might make marine mammals more resilient to shifting climates than simple calculations would suggest. Maybe they just need to find the parts of the world where fish remain slow, dumb, and cold.