Why warm-blooded predators thrive in the coldest places on Earth
Life is good when their cold-blooded prey is “cold, stupid and slow.”
There are 17 different species of penguin living in the cold waters of the Southern Ocean. There are a couple of hundred species of Arctic bird. Seals, sea lions, whales and dolphins all seem to thrive in the coldest and least hospitable places on Earth — near the Arctic and Antarctic regions. Now, researchers think they know why.
For warm-blooded predators, the price of survival under these tough conditions is worth paying because around the poles, their cold-blooded prey is "cold, stupid and slow." This means, at least as far as hunting goes, life can be easy.
A biodiversity puzzle
The diversity and numbers of warm-blooded marine predators near the poles has been a bit of a biological puzzle. Ecologist John Grady, who led a study examining this question, told Quirks & Quarks' Bob McDonald that, generally speaking, biological diversity is something that declines from the tropics to the poles. Predators are the exception to this rule.
"There's this really big shift, from cold blooded dominance in the tropics to warm blooded dominance by the poles," said Grady. "The mammals are about 20 times more common than sharks and [predatory] fish in the coldest water."
This was a puzzle that Grady, currently a post-doctoral researcher at the National Great Rivers Research Education Center in St. Louis, Missouri, has been working for years to try and solve. And he now thinks he knows the answer, which has to do with the way temperature influences the behaviour of warm and cold-blooded predators and prey.
Water temperature and activity level
In the tropics, warm water temperatures make cold-blooded creatures nimble and quick. This applies to both predators — sharks and large fish — and the smaller fish they prey on.
But for a warm-blooded predator, this presents problems. Prey are harder to catch, and cold-blooded predators, like large sharks, might well be targeting you.
One example to show how life can be tough for warm-blooded predators in warm waters is the endangered Hawaiian monk seal, which can't catch fast prey.
"They like to go out into the ocean and they kind of creep around the bottom floor and they go off to a slow moving fish," said Grady. "They're really preyed upon a lot by sharks like the tiger shark or Galapagos shark that are much faster than a shark in cold waters."
However, as water gets colder, eating and avoiding being eaten both become less challenging issues for warm-blooded predators.
"Cold water depresses the metabolic rate and the speed of fish," said Grady. "For roughly every 10 degrees Celsius you drop the metabolic rates, speeds drop about two and a half times."
"Cold, stupid and slow"
The cold-blooded species aren't just slower, they're dumber. "Their brains actually operate at a slower speed as their neurons fire more slowly, their reaction times are slower," said Grady. "the rate at which they can process sensory information declines and they become effectively stupider."
In this case, warm-blooded predators are favored in terms of getting their food and escaping their own predators.
There are of course, costs to being a warm-blooded animal in a cold environment. You need insulation and a high-revving metabolism to maintain your high body temperature. The answer to the first problem is the thick layer of feathers and fat that penguins lay down, or the blubber that marine mammals like seals, whales and dolphins have evolved.
As to the second, "If you're able to eat enough food you can help compensate for any extra energetic costs," said Grady.
The kind of food advantages are so high in the cold, it really outweighs any the thermal cost of staying warm in these environments.
Global warming could change animal dynamics
There are, however, implications here as global warming transforms previously cold water habitats, said Grady. "As waters warm with climate change, it gets harder for many threatened marine mammal and bird populations to make a living to catch food."
His study suggests this may already be an issue for the harp seal, whose prey is increasing in numbers as the water in their range warms, but are not profiting by it.
"You get a shift in the competitive balance and warm water will basically favour larger fish and sharks at the expense of marine mammals and birds."