Whales are just the right size to eat their prey as efficiently as possible
Whales are big, but why aren’t they bigger? A new study says the key factor is how many calories they can take in.
Researchers came to this conclusion after using small boats to chase down 300 whales of various species around the world. They reached out with long poles to attach sensors to the creatures with suction cups, allowing them to record what the animals were doing as they dove for food.
The results suggest whale body size is controlled by how the animals capture prey and how much food is available, according to the report Thursday in the journal Science.
The study included 90 blue whales, the biggest animals ever to live. Size estimates vary, but the marine mammals can grow about 100 feet long and weigh more than 100 tons, according to the American Cetacean Society. Researchers also included sperm whales, the biggest whales with teeth; they can grow to about 60 feet in length.
Toothed whales on the hunt capture their prey one at a time, seeking them out in the murky depths with a sonar-like detection. But as their body size increases, their hunting style becomes progressively less efficient — they recover fewer calories from their meals per calorie burned in each dive. That’s a result of the limits on availability of prey.
“It looks like you cannot be much bigger than a sperm whale” if that’s the way you hunt, said Nicholas Pyenson of the Smithsonian’s National Museum of Natural History, the study’s senior author.
Blue whales can be bigger because they feed in a much different and highly efficient way, the researchers reported. These whales open their mouths wide to engulf an immense volume of water — even bigger than its body size — as they swim along. Then they close their mouths and strain the water out through a sieve-like filter, trapping huge amounts of tiny shrimp-like creatures called krill.
That “allows them to exploit a very large prey resource that’s very rich in a small amount of time,” said Jeremy Goldbogen of Stanford University, another member of the research team. “They get a huge bang for the buck.”
So why aren’t they even bigger than they are? Perhaps blue whales are limited by the seasonal nature of the most nutritious krill patches, which grow gigantic and dense in the summer, Goldbogen said. And calculations indicate that to sustain a blue whale that’s 115 feet long, “you’d need a density of prey that is just not seen anywhere in the world,” Pyenson said.
But maybe there’s also some kind of limiting factor in how their bodies function, Goldbogen said. He noted a recent study that found a blue whale’s heart beats up to 37 times a second after deep dives. That appears to be about as fast as the organ can pump.
Another limiting factor could be a whale’s ability to process the food it takes in, said Terrie Williams, a comparative ecophysiologist at UC Santa Cruz.
The study provided a comprehensive look across big whale species, and the success in collecting the data “boggles the mind,” said Williams, who was not involved in the work.
“You try to go out and put a tiny tag on the back of the biggest animal in the world,” she said.
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