The enormous bodies of krill-eating minke whales actually represent their smallest possible body size, research finds.
The findings could inform which whale species are more vulnerable to future climate change impacts, like shifting food sources.
“Depending on how krill fare, certain whale species will either win or lose in future oceans.”
The study published in Nature Ecology & Evolution focuses on the “rorqual whales,” a lineage of filter feeders that includes the blue whale, the largest animal of all time. The group is characterized by a lunging maneuver where individuals engulf an enormous amount of water along with their prey, which they then strain through fringed structures at the roof of their mouth.
By examining the smallest living species in this group—the Antarctic minke whale—the researchers find that individuals need to grow to at least 4.5 meters (approximately 15 feet, or weighing 1-2 tons), the length of weaned minke juveniles, in order to eat enough food to survive.
“We’re working to define the upper and lower limits of life at physiological extremes,” says the study’s senior author Jeremy Goldbogen, an associate professor of oceans at the Stanford Doerr School of Sustainability who’s spent the last decade demystifying the basic biology of the world’s largest whales. “This is important given the pace of environmental change so we can determine which species are at greatest risk and how we manage populations into the future.”
Giant whales, tiny krill
To calculate the minimum size required for lunge-feeding whales, the team turned to a unique population of minkes that frequent bays along the west coast of the Antarctic Peninsula. Unlike other minkes, which prefer fish, this population primarily forages for tiny animals known as krill.
“Our motivation for going to Antarctica was to observe this specific population,” says David Cade, the study’s lead author and a postdoctoral scholar in the Goldbogen Lab. “Most minke whales are elusive and typically surface just one at a time, but this group exhibits more social behavior, giving us a higher chance of safely approaching them.”
The team uses suction cups to affix a “biologging tag” packed with sensors and cameras to an individual’s back so they can track its orientation and movement. By combining tag data with drone photography, they can document engulfment events and estimate a whale’s body length.
Based on 437 hours of biologging data, the authors calculated the minkes’ feeding rate compared to that of krill-feeding humpbacks and blues to identify potential size limitations. They found that minkes are about as energetically efficient as larger whales when they feed at night, since prey migrate to shallower depths and are therefore more accessible. However, they feed at far lower rates during the day.
“In the daytime, minke whales dove deeper than we predicted to find prey,” says Cade. “This means that they’re operating at maximum capacity, and can’t possibly forage any more given size constraints and the time it takes to lunge feed. At 4.5 meters in length, weaned minke whales are the smallest they can be while still eating enough food on their own to survive.”
How will climate change affect krill?
According to the fossil record, whales became gigantic rather recently in evolutionary terms, approximately three to five million years ago. But a fundamental question remains: Did whales evolve to filter feed due to their large size, or did gigantism result from filter feeding?
The authors determine that it was a bit of both. Once whales evolved from eating one prey at a time to filter feeding many at once, the rise of gigantism was likely driven by an abundance of krill in prehistoric oceans, which favored their feeding style. Previous research shows that wind-driven upwelling and intense glacial cycles fueled denser patches of krill and allowed smaller animals to filter feed more efficiently.
Larger filter-feeding whales, which can engulf proportionally more water per gulp than smaller whales, then had an evolutionary advantage over their smaller counterparts, giving rise to the largest animals in Earth’s history. But it remains to be seen how whales will respond to future environmental change. For whales constrained by body size and operating at physiological extremes, like minkes and possibly blues, rapid environmental change could put them at greater risk of extinction.
“The next 10 million-dollar question is how climate change might affect krill populations,” says Goldbogen. “Depending on how krill fare, certain whale species will either win or lose in future oceans.”
Now that the research team has defined the minimum size for these ocean giants, more research is needed to determine whether blue whales—on the other end of the spectrum—are also living at the edge of their means or if they’re in the midst of evolving ever-larger before our eyes.
Cade is also affiliated with University of California, Santa Cruz. Additional coauthors are affiliated with Hopkins Marine Station; Duke University; Oregon State University; the University of Queensland; and the Cascadia Research Collective in Olympia, Washington.
This research received funding from the National Science Foundation, the Office of Naval Research Young Investigator Program, and Stanford University’s Terman Fellowship.
Source: Katie Jewett for Stanford University