As you may be aware, the biggest mammal on Earth feeds on one of the smallest creatures in the ocean. In order to make their diet energetically viable, blue whales need to be smart when hunting for krill, and new research published in the journal PeerJ has identified how surface feeding can conserve energy.
Moving around as the biggest animal the planet has ever seen is no mean feat, and for blue whales foraging involves some pretty costly maneuvers such as diving, holding their breath, and opening their immense mouths against the pressure of the surrounding water. Their hydrodynamic bodies and broad flippers make them masters in the water, but any speed they build up is abruptly lost as their open mouths act as pretty efficient brakes.
To afford such an energy-expensive lifestyle, blue whales need to aim for quantity when it comes down to snacking on krill. Opening your mouth for a single crustacean is a fool’s errand if you’re going to waste more calories than your meal possesses just getting your jaws open.
"People think about whales having to dive deep to get to the densest prey patches, but if they can find their prey in shallow waters, it's actually more energetically profitable to feed near the surface," said Leigh Torres, an assistant professor and director of the Geospatial Ecology of Marine Megafauna Laboratory at OSU's Marine Mammal Institute, in a statement. "In this population of whales in New Zealand, they foraged more in areas where their prey was dense and shallow.”
By aiming for the surface schools of krill, the whales are able to dive for shorter periods of time and hold their breath less, which uses up less energy. Torres and her team also found on their field trips off the coast of New Zealand that krill patches were denser the closer they were to the surface.
Their findings also showed that the blue whales near New Zealand had relatively short dive times compared to blue whale populations off the coast of California, diving for 2.5 minutes compared to 10 minutes, respectively. When surface foraging was observed, the dive time of the New Zealand blue whales dropped even more to just 1.75 minutes.
Co-authors of the study captured fantastic drone footage of the feeding strategy, which enabled them to observe how the whales approached their dinner. They found that the whales were making decisions about whether or not a patch of krill was worth surfacing for. The whales used their right eye to target the prey and would rotate in order to scoop up the most krill with their cavernous mouths.
"The video allows us to describe a lot of really cool kinematics and body movement coordination by the whale that we haven't been able to see before," Torres said. "The footage also allowed us to see the prey response in a new way. We can see when the krill begin to flee as the whale approaches, which is really amazing. At the whale's fastest speed and acceleration, the krill begin to jump away just eight-tenths of a second before the whale strikes at the krill patch."
The authors note the findings based on the drone footage were only taken from recording the behaviors of just one whale, though that individual did provide four feeding events in the time it was being filmed. While this means that the findings can’t necessarily be applied to the wider blue whale population, it highlights how drone footage is an effective and exciting new avenue for data collection in behavioral observation studies on whales.