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.
The real explanation for that pattern, the team says, is deceptively simple. An animal’s speed, agility, and mental prowess depend on its metabolism, which in turn depends on its temperature. Since birds and mammals can keep heating their bodies in frigid conditions, they remain fast and alert. By contrast, the fish they hunt become slower and dumber. At some tipping point of temperature, seals, dolphins, and penguins start outswimming their prey. They become more likely to encounter targets, more likely to catch them, and more likely to outpace cold-blooded predators of their own.
In Grady’s words, “Warm-bodied predators are favored where prey are slow, stupid, and cold.” That’s why sharks and other predatory fish dominate near the equator, but colder waters are the domain of whales and seals. By monopolizing food in the poles, these creatures can then specialize on specific types of prey, which makes them more likely to split into separate species. The killer whales of the North Pacific, for example, include mammal-eating transients and fish-eating, year-round residents, who don’t interbreed.
The team’s conclusions about “the thermal constraints of marine predators seem to fit with observations in nature, as well as theory,” says Donna Hauser from the University of Alaska Fairbanks. Consider the mammals and birds that, bucking the trend, do thrive in the warm tropics. To Grady, these exceptions simply prove the “slow, stupid, and cold” rule. The penguins of the equatorial Galápagos Islands, for example, feed in areas with cold currents. Sperm whales and their relatives forage by diving into frigid depths. Monk seals in Hawaii go after slow, bottom-dwelling prey. Giant whales, like blues and humpbacks, have evolved a style of foraging—lunge feeding—that allows them to engulf massive shoals of prey in fast surprise attacks.
And dolphins—the only group of marine mammals that have really diversified in the tropics—make up for any physical disadvantages with intellect. They can corral fish into balls using curtains of bubbles, herd them toward one another with tail slaps, and even drive them onto shorelines. When you’re as smart as a dolphin, perhaps everything seems slow and stupid, even when it isn’t cold.
But the world is changing. It’s likely that the surface of the oceans will warm by 2 to 3 degrees Celsius within this century. As that happens, marine mammals and birds should disproportionately suffer, as warmer water robs them of the advantages that they’ve historically enjoyed over 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.