Rethinking whale appetites

This exercise is a part of Educator Guide: Whales Eat More Than We Thought / View Guide

Directions for teachers: Ask students to read the online Science News article “Baleen whales eat (and poop) a lot more than we realized,” which details scientists’ efforts to accurately estimate how much certain whale species eat and what that means for ecosystems, and answer the questions below. A version of the article, “Whales eat more than we thought,” appears in the December 4, 2021 issue of Science News.

1. What did researchers recently discover about the diets — and bathroom habits — of baleen whales?

Baleen whales eat, on average, about three times as much food as previously thought. More food in means more poop out.

2. How do these habits shape ocean ecosystems? What does the discovery indicate about whales’ roles in ocean ecosystems?

Whale poop serves as a source of crucial nutrients for ocean ecosystems. The volume of food that whales eat, and then excrete, suggests that the animals play a larger role in shaping ocean ecosystems than previously thought.

3. How much does a blue whale eat in a day and how much energy does that translate to? What analogy do scientists use to describe their estimate and why might they use an analogy?

A blue whale can eat on average about 16 metric tons of krill in a day, which translates to roughly 10 to 20 million calories. That is equivalent to eating about 30,000 Big Macs in a day. Scientists use the analogy of Big Macs to help readers better understand the magnitude of a blue whale’s food intake.

4. What three questions did scientists need to answer in order to estimate whale food intake?

How often do whales feed? How big are the gulps that whales take when feeding? How much food is in each gulp?

5. How did scientists go about answering those questions? What technologies and techniques did scientists use?

Scientists used sensors attached to the backs of 321 whales representing seven species to monitor when whales lunged for prey — a sign of feeding. Aerial drones helped the team estimate gulp size for 105 whales. Sonar mapping provided information about krill density in feeding areas.

6. How do the scientists’ methods compare with methods used in previous research?

The scientists’ methods relied on newer technologies, which provided a more detailed view than previous methods such as inferring whales’ energy needs based on their size and dissections of dead whales.

7. What happened to the world’s populations of giant whales over the last century?

Whale hunting decreased populations of certain species by up to 99 percent.

8. What impact did scientists expect the decline of whale populations to have on krill, tiny crustaceans that are a source of food for whales?

Researchers expected that krill populations would grow since there were fewer whales around to eat the krill.

9. What actually happened to krill populations? How does the recent discovery help explain what happened?

Krill in the Antarctic declined by more than 80 percent in areas where whales were heavily hunted. The research revealed that whales eat more than previously thought, which means their poop plays a bigger role than previously realized. Fewer whales mean less poop overall, and that means less iron is available for the phytoplankton blooms that krill feed on. Less iron results in shrinking blooms, leading to declines in krill populations.

10. How might the rebounding of whale populations to levels observed in the early 1900s affect the Southern Ocean’s ecosystem?

The ecosystem’s productivity could increase by 11 percent, the researchers estimate. That boost would translate to 215 million metric tons more carbon stored in the ocean each year.