For many ocean creatures, there’s safety in numbers. A school of small, fast-moving fish can confuse and distract larger, lumbering predators such as sea lions, which are generally out for a single snack. But that strategy doesn’t work as well when the fish are faced with a heftier threat such as a humpback whale that can engulf nearly two-thirds of a school in one gulp.
To find out how these whales are able to catch speedy fish by surprise, researchers snuck up on individual anchovies in the lab with animations of fake “predators” (really just appropriately sized dots) to see what would send the fish fleeing. As it turns out, humpback whale–size predators are so enormous that the fish don’t even register them as threats as they approached from afar, the team reports today in the Proceedings of the National Academy of Sciences.
The researchers also observed real schools of anchovies in Monterey Bay and off the coast of Southern California as whales approached the fish at different speeds. In the wild, the fish only started to flee when the whales opened their mouths—at which point it was too late.
The fish likely don’t have a good defense response to the whales because they primarily evolved to respond to smaller predators, which are more common. The whales have another advantage as well: If some fish around the edges of the group try to make a break for it, the humpbacks raise their huge, white-bottomed flippers around the school in a hug of death, as seen in the above photo, shepherding wayward anchovies into their gaping mouths.