Why Did Whales Get So Big?

SCIENCE

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Discussion Ideas

A new study analyzes the evolution of baleen whales. What are baleen whales?
- Baleen whales (Mysticeti) are huge, carnivorous marine mammals. Baleen whales are named for baleen, the thick, bristly fibers that line their upper jaw. Baleen whales feed by opening their mouths underwater, gulping in water … and whatever happens to be in it. Then, they use their tongues to expel the water through their baleen. The baleen filters out food such as krill and other crustaceans, which the whale eats by the ton. Watch this great explainer video to see blue whales feeding.
  - The 15 species of baleen whales have a cosmopolitan distribution, and can be found in every ocean basin on Earth.
  - Some baleen whales, such as North Atlantic right whales and blue whales, are endangered species. Some, such as humpbacks and gray whales, have relatively healthy populations.
  - Baleen whales split off from their toothed cousins about 34 million years ago.

When did baleen whales get so big?
- Well, not all baleen whales are giants. The smallest baleen whale, the pygmy right whale, is about 6 meters (20 feet) long.
- The big baleens, such as blue, fin, and sei whales, increased in size during an ice age, about 3-4 million years ago. (On an evolutionary time scale, that’s just recently.)

Why did baleen whales get so big?
- Climate change forced a radical alteration in their food supply.
  - During the glacial period, “the planet entered a cycle where large glacial sheets would form in the northern hemisphere, expand all the way down to the northern U.S., and then retreat again. These cycles triggered a shift away from continuous warmth toward seasonal climates that varied over the year. That seasonality reshaped the oceans. By strengthening winds that blow from the south, it intensified the upwelling currents that bring nutrients up from the depths, specifically near the coasts of continents. And those coastal waters were also hit by run-offs from melting glaciers on land, which brought even more nutrients with them.”
  - “At the time of this transition, [filter-feeding] baleen whales … were well equipped to take advantage of these dense patches of food. … What’s more, large whales can migrate thousands of miles to take advantage of seasonally abundant food supplies. So, the scientists said, baleen whales’ filter-feeding systems, which evolved about 30 million years ago, appear to have set the stage for major size increases once rich sources of prey became concentrated in particular locations and times of year.”
    - Baleen whales such as blues and humpbacks still migrate enormous distances every year.

Why are we only now gaining greater understanding about how whales evolved?
- The same reason we don’t have better understanding about the evolutionary history of most species—an incomplete fossil record. “We haven’t had the right data,” says one researcher. “How do you measure the total length of a whale that’s represented by a chunk of fossil?” (Good point.)

How did scientists conduct the study of the evolution of whale size?
- Researchers recently “established that the width of a whale’s skull is a good indicator of its overall body size.” With this correlation, they could conduct their analyses.
  - Researchers measured fossil skulls to estimate the size of 63 extinct species of baleen whale. They also gathered data from 13 extant (still-living) species of baleens.
    - Specimens ranged from the earliest baleen whales, about 30 million years old, to species that are alive and well today. This provided a broad sample of whale size before and after the last ice age.