Whales are big, but why arenât they bigger? A new study says itâs basically about how many calories they can take in.
Thatâs the conclusion of researchers who used small boats to chase down 300 whales of various species around the world. They reached out with a long pole to attach sensors to the creatures with suction cups, allowing them to record what the animals were doing on their dives for food.
The results suggest body size is controlled by how whales capture prey and how much food is available, researchers reported Thursday in the journal Science.
The study included 90 blue whales, the biggest animal ever to live. Size estimates vary, but the American Cetacean Society says blue whales can grow about 100 feet (30 meters) long and weigh more than 100 tons (91,000 kilograms). The study also included the biggest whale with teeth, the sperm whale, which can stretch about 60 feet (18 meters) long, the group says.
When toothed whales hunt, they capture one prey at a time, seeking them out in the murky depths with a sonar-like detection. The study found that as body size increases in such whales, they become progressively less efficient as they hunt â they recover fewer calories from their meals per calorie burned in each dive. That results from limits on availability of prey.
âIt looks like you cannot be much bigger than a sperm whaleâ when hunting the way it does, said Nicholas Pyenson of the Smithsonianâs National Museum of Natural History, who was part of the research.
The blue whale can be bigger because it feeds in a much different and highly efficient way, the study says. It opens its mouth wide and engulfs an immense volume of water, even bigger than its body size, as it swims along. Then it closes its mouth and strains the water out through a sieve-like filter, which traps huge amounts of tiny shrimp-like creatures called krill.
That âallows them to exploit a very large prey resource thatâs very rich in a small amount of time,â said Jeremy Goldbogen of Stanford University, another researcher. âThey get a huge bang for the buck.â
So why arenât they even bigger than they are? Maybe 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 thatâs 115 feet (35 meters) long, âyouâd need a density of prey that is just not seen anywhere in the world,â Pyenson said.
But maybe thereâs also some kind of limiting factor in how their bodies function, Goldbogen said. He noted a recent study that found a blue whaleâs heart beats up to 37 times a minute after deep dives, and that appears to be about as fast as the organ can pump.
Another limiting factor could be a whaleâs ability to process the food it takes in, said Terrie Williams of the University of California, Santa Cruz, who did not participate in the research.
The study provided a comprehensive look across big whale species, and the success in collecting the data âboggles the mind,â she said.
âYou try to go out and put a tiny tag on the back of the biggest animal in the world,â she said.q
