Whales are big, but why aren't they bigger? AN new study says the main factor is how many calories they can ingest.
The researchers came to this conclusion after using small boats to hunt 300 whales of various species around the world. They reached out with long sticks to connect sensors to the suction cup creatures, allowing them to record what the animals were doing while searching for food.
The results suggest that the body size of whales is controlled by how animals catch prey and how much food is available, according to a report released Thursday in the journal Science.
The study included 90 blue whales, the largest animals of all time. Size estimates vary, but 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 largest toothed whales; They can grow to about 60 feet in length.
Hunting toothed whales catch their prey one at a time, searching for them in the dark depths with sonar-like detection. But as body size increases, the hunting style becomes progressively less efficient – they recover fewer calories from their calorie-burned meals with each dive. This is a result of prey availability limits.
"Looks like you can't be much bigger than a sperm whale," if that's how you hunt, he said. Nicholas Pyenson from the Smithsonian National Museum of Natural History, senior author of the study.
Blue whales are tagged during the summer of 2019 in the eastern Pacific, central California, by the study's research team.
(Elliott Hazen / NOAA)
Blue whales can be larger because they feed in a very different and highly efficient manner, the researchers report. These whales open their mouths wide to swallow a huge volume of water – even larger than body size – while swimming. Then they close their mouths and filter the water through a sieve filter, trapping large numbers of tiny shrimp-like creatures called krill.
"It allows them to exploit a very large prey resource that is very rich in a small amount of time," he said. Jeremy Goldbogen from Stanford University, another member of the research team. "They make a lot of money."
So why aren't they even bigger than they are? Perhaps blue whales are limited by the seasonal nature of the most nutritious krill fragments, which grow gigantic and dense in the summer, Goldbogen said. And the calculations indicate that to sustain a 115-foot-long blue whale, "you would need prey density not seen anywhere in the world," said Pyenson.
But perhaps there is also some kind of limiting factor in the functioning of their bodies, Goldbogen said. He observed a recent study that found that a blue whale's heart beats up to 37 times per second after deep dives. It seems to be the fastest the organ can pump.
Humpback whales being tagged by the research team off the Antarctic coast in 2018.
(Goldbogen Laboratory, Stanford University / Duke University Marine Robotics and Remote Sensing.)
Another limiting factor could be a whale's ability to process the food it receives, he said. Terrie williams, comparative ecophysiologist at UC Santa Cruz.
The study provided a comprehensive overview of large whale species, and successful data collection "boggles the mind," said Williams, who was not involved in the work.
"You try to go out and put a small tag on the back of the biggest animal in the world," she said.