Acting unthreatening, giant whales use stealth to hunt tiny fish
You wouldn't think it to look at them, but some species of giant whale sneak up on their tiny prey. A recent study by scientists at Stanford University has found that the humpback whale, which weighs in at about 30 tonnes, uses stealth and deception to get within lunging distance of the schools of anchovies and other fish on which it feeds.
It seems something of a paradox that the largest creatures that have ever lived on the Earth feed on very small sea creatures, but that is the case with the rorqual whales, which includes the giant blue whale. They live by swallowing massive amounts of krill and small fish in a maneuver known as lunge feeding.
In this, the whale opens its mouth and the oral cavity expands like a living parachute to take in a volume of water larger than its own body, scooping up its prey in job lots. It then closes its jaws and tightens up its mouth, expelling the water against the combs or baleen that take the place of teeth, filtering out its dinner.
It works, but small fish are notoriously alert, skittish, and fast, so the whale can't just shoot at the schools like a torpedo. The piscine prey would just scatter. Instead, the Stanford team found that whales use a bit of guile.
Using a combination of video data from tags attached to humpback whales off Monterey Bay and Southern California, along with computer models, the scientists were able to gain insights into cetacean hunting practices. Specifically, they modeled how anchovies escape from virtual whales based on their reaction times when the whale opens its mouth.
"One of the innovations of this study was to use predator data to inform the models we played back to fish," says David Cade, a graduate student in the lab of Jeremy Goldbogen, assistant professor of biology at Stanford. "This allowed us to discover that the range of values at which a fish responds to an oncoming predator are all passed nearly simultaneously at a point when the whale opens its mouth, suggesting that by precisely timing its engulfment, the whale can avoid triggering escape responses in fish."
They found that the slower, less maneuverable whale could scoop up the anchovies by keeping its mouth shut until it was right up on the school. However, they didn't need to do this when hunting krill because the little crustaceans don't frighten as easily as the more intelligent fish.
"This made sense when we realized that fish have been evolving to avoid being eaten by smaller predators for at least 100 million years, but lunge-feeding is a relatively new feeding strategy, evolutionarily speaking," says Cade.
In addition to learning how the whale stalks its dinner, the team says that the new study also provides estimates of how much fish a humpback eats in one meal or over time. It also provided a better understanding of how the whale can alter its lunge feeding between fish and krill to minimize energy expenditure. In the case of the humpback, it is seven times more energy efficient to lunge after fish than feed on krill.
The research was published in the Proceedings of the National Academy of Sciences.