Rorqual whales, including the humpback whale (Megaptera novaeangliae), are large and fast. But their massive bulk isn't particularly manoeuvrable; imagine trying to catch a mosquito with a passenger jet. For these animals that hunt small and evasive fish, their poor manoeuvrability poses a problem. Instead, whales must use stealth. But when you're 30,000 kg, how do you sneak up on your prey?
David Cade from Stanford University, USA, studies the behaviour of humpback whales. Together with colleagues from St Louis University (USA) and Roma Tre University (Italy), Cade wanted to understand how whales trick their prey. To do this, they studied how individual anchovies (Engraulis mordax) behave when confronted with the threat of a looming whale. Of course, there are some major logistical challenges precluding laboratory-based studies of whales and anchovies, so, to get around the challenge, Cade used a beautifully simple simulation of a whale looming over the fish. He mounted a TV screen showing a dilating black circle behind the anchovy's tank. This circle represented not only the shadowy looming shape of a whale accelerating towards the fish, but also the opening of its jaws. As the circle reached a particular ‘looming threshold’, the fish thrashed their bodies. En masse, this has the effect of dispersing the school, and reducing the chance of a mouthful of fish for the whale. The team then varied the patterns and speed of the circle dilation, to see which of the whales’ hunting strategies might be the most successful.
Analysing the anchovies’ reactions, the team found that the tiny fish were most threatened by the prospect of a whale approaching with opening jaws. However, the fish were less threatened when the simulated whale approached with its jaws closed, even when the whale approached at faster speeds. In short, the whales’ large size, combined with their long bodies, tricked the fish into downplaying the threat. Having figured out which aspects of a whale approach sent the anchovies fleeing, Cade and colleagues then wondered whether humpback whales in the wild wait until the final split-second before opening their mouths. Examining video footage from a previous study of anchovy-hunting humpbacks off the coast of Southern California, the team found that the anchovies did indeed disperse just as the whales opened their mouths, but by then it was too late.
However, it wasn't clear how humpback whales maximise their catch. Cade used a predictive model of how individual fish in a school would behave during different whale approach strategies. They found that the whales are able to make the largest catches by approaching at high speeds and when using their fins to confuse the fish. But with this strategy, timing is everything; if the moment when the whale begins to open its jaws is delayed even by an instant, the whole school will be lost. In addition, the team found that the whales depend on team work, by feeding alongside common dolphins and California sealions. The other mammals help by herding the fish into densely packed bite-sized schools, ready for the whales to plunder.
The humpback whales’ ability to sneak up on fish is part of an evolutionary arms race and, since whales are a relatively recent arrival, fish such as anchovies have not yet figured out how to mount an effective escape. Whilst this isn't great news for the fish in question, hiding in plain sight from their dinner might be one reason that humpback whale populations have been able to stage their spectacular recovery from near collapse. Their large body size, once a tempting target for whalers, is paradoxically now their greatest asset.