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HOW COME WHALES ARE SO BIG?

“BLUE WHALE HEARTS May Beat Only Twice a Minute During a Dive,” wrote Cara Giaimo in The New York Times, November 27, 2019. Equally interesting was this article’s reference to an evolutionary tale: Nicholas St. Fleur’s “How Whales Became the Biggest Animals on the Planet,” The New York Times, May 24, 2017.

Here are tidbits gleaned from St. Fleur’s article on selective diet, climate change, and opportunistic migration, all of which encouraged Cetacea to make an optimal life of sea-faring.

A blue whale off the coast of California. This and a following image from Silverback Films/BBC in The New York Times, May 24, 2017.

Filter-Feeding. St. Fleur wrote that whales “began as land-dwelling, hoofed mammals some 50 million years ago…. Between about 20 million and 30 million years ago, some of these ancient whales developed the ability to filter-feed, which meant they could swallow swarms of tiny prey in a single gargantuan gulp. But even with this feeding ability, whales remained only moderately large for millions of years.”

Climate Change and the Ocean’s Food Web. Around 4 1/2 million years ago, Earth’s climate became colder, leading to glaciation covering the Northern Hemisphere. “Runoff from the glaciers,” St. Fleur wrote, “would have washed nutrients like iron into coastal waters and intense seasonal upwelling cycles would have caused cold water from deep below to rise, bringing organic material toward the surface. Together these ecological effects brought large amounts of nutrients into the water at specific times and places, which had a cascading effect on the ocean’s food web.”

All-You-Can-Eat Buffets. “Throngs of zooplankton and krill would gather to feast on the nutrients,” St. Fleur noted. “They would form dense patches that could stretch many miles long and wide and be more than 65 feet thick.”

A Northern krill, Meganyctiphanes norvegica, 0.4–0.8 in. long. Krill is Norwegian for “small fry of fish.” Image by Øystein Paulsen.

St. Fleur quoted Stanford comparative physiologist Jeremy Goldbogen: “Even though they had the anatomical machinery to filter-feed for a long, long time, it wasn’t until the ocean provided these patchy resources that it made bulk filter-feeding so efficient.”

Traveling from Buffet to Buffet. “Plentiful food everywhere isn’t going to get you giant whales,” said University of Chicago evolutionary biologist Graham Slater. “They have to be separated by big distances.”

St. Fleur wrote, “Because the ecological cycles that fuel the explosion of krill and zooplankton occur seasonally, Dr. Slater said the whales must migrate thousands of miles from food patch to food patch. Bigger whale ancestors that had bigger fuel tanks had a better chance of surviving the long seasonal migrations to feed, while smaller baleen whales became extinct.”

Blue whales are the largest animal to have ever existed. In a sense, they are now as large as animals can grow.

Were food patches not far apart, whales would have grown to a certain size commensurate to that environment and ceased evolving larger. By contrast, for example, a blue whale can reach 100 ft. in length and weigh as much as 380,000 lbs.

A blue whale, Balaenoptera musculus, with an accompanying human diver for comparison. Image by Kurzon.

A blue whale’s giant heart and circulatory system have evolved to allow this air-breathing mammal to dive 10,000 ft. below the surface and remain there for more than two hours. For more on this, I refer you to Cara Giaimo’s article that got me into this evolutionary tale to begin with. ds

© Dennis Simanaitis, SimanaitisSays.com, 2019

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