The blue whale is considered the largest animal that has ever existed on earth. The largest specimens were caught in the frigid waters of the Southern Ocean and once measured over 30 meters and weighed 190 tons. They were able to reach this size thanks to the high-energy food, krill, and their filter-feeding style. So is this nature’s recipe for success? A research team has been studying why smaller whale species have not also switched to this feeding method. To do this, they looked at the smallest relative of blue whales, the Antarctic minke whale, and found out surprising things.
Filtering food such as krill, small fish, or salps from the water is a matter of body size, and minke whales have reached the minimum possible body size for efficient filtering. That’s the finding of the study by David Cade, Ph.D., postdoctoral fellow at the University of California Santa Cruz (UCSC) and principal investigator of the study. “Anything smaller than a minke could not achieve the foraging rates necessary to survive,” he explains. “They’re right at the edge of what’s possible.”. The study was published in the latest issue of the journal Nature Ecology and Evolution.
Exploring Antarctic minke whales is a real challenge. This is because the animals are fast, agile, small and also love to be close to the pack ice in Antarctic waters. That is why they are among the most poorly studied rorquals, one group of baleen whales. In order to conduct their study and obtain results, the team equipped 23 animals along the Antarctic Peninsula with transmitters using data loggers and video cameras to obtain diving and feeding behavior and physiological data at the same time. Surprisingly, this showed that minke whales go on their dives mainly at night and dive much more frequently than their larger relatives. “During the day, they feed at depths comparable to humpback and blue whales, but their feeding rate isn’t as high because they’re smaller,” David Cade says. “But their nighttime feeding rate is two to five times the daytime rate.” This is also due to krill staying at greater depths during the day, which would require a greater energy expenditure for smaller whales. That is why the agile and fast minke whales tend to hunt the small krill groups at the surface during the day and the large numbers only at night.
The strategy of being able to take in as much food as possible, equipped with a wide-open mouth and a foldable throat pouch, is actually a successful model of evolution. Blue whales can thus take in a volume of water up to 135 percent of their body weight and thus as much food as possible at once. But the smaller the whales become, the less efficient this form is. For a minke whale weighing 5 tons, it is only just 42 percent of the body mass that the animal can hold. That’s why they have to dive more often, which in turn costs energy. “When we calculate how much energy they use in foraging and what their overall intake should be based on their size, we find that minke whales are right at the threshold,” David Cade says.
He and his colleagues assume that in evolution, the first such filter-feeders were the size of today’s Antarctic minke whales about five million years ago. Thanks to favorable environmental conditions, feeding sites were formed that could be visited repeatedly by the whales of that time, providing more energy for accelerated growth. Climate change along the Antarctic Peninsula is reversing these favorable conditions and whales are finding less food. This is bad news for an animal that is already at its limits.
Dr Michael Wenger, PolarJournal
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