How do humpback whales hunt with bubble nets?

Dawn has barely broken over the icy waters of south-east Alaska, and ten humpback whales are about to perform one of the most astonishing choreographies in the animal kingdom. Within minutes they will coordinate their dive, their breathing, their sound production and their ascent in order to trap an entire school of herring inside a cylinder of air bubbles, then swallow several hundred kilos of it in a single collective mouthful. First filmed underwater in 1983, the spectacle has fascinated biologists and the wider public ever since. Bubble net feeding (bubble netting) is today regarded as one of the finest documented examples of animal culture.

The choreography in detail

It all begins with locating the prey. Several humpback whales, usually between four and twenty individuals, patrol an area known for its concentrations of herring, capelin or krill. They dive together, descend to a depth of thirty to fifty metres, then gather beneath the target school. Once in position, the orchestration begins.

One designated whale, the bubble blower, starts a rising spiral around the school, releasing air steadily from its blowhole. With each exhalation a chain of bubbles climbs towards the surface, forming a cylinder that can reach twenty to thirty metres across. Seen from above it looks like a ring of bubbles closing in. Underwater it is a translucent wall rising. For the fish, that wall is impassable: the bubbles scatter the light and disturb their perception, confuse their lateral line sense (which detects movement in the water), and create an acoustic barrier. They stay inside.

At the same time another whale in the group, the caller, produces a powerful feeding call. First recorded in 1985 by Cynthia D’Vincent and her team in Glacier Bay, this call is a low, frequency-modulated sound of around five hundred hertz, lasting several seconds and reaching one hundred and eighty-five decibels at source. Its function remains partly debated, but the leading hypothesis is that it disorients the fish and concentrates them towards the centre of the spiral. The panicked herring bunch into a tight ball. There may also be a coordinating function among the whales themselves: a signal marking the moment to act.

The wall of bubbles reaches the surface. The school is as concentrated as it will get. This is the moment. All the whales, positioned beneath the school, rise vertically with their mouths wide open. They pass through the school at full speed, their throat pleats stretching like a concertina to take in an impressive volume of water and fish. They surface, close the jaw, push the water out through the baleen and swallow the catch. Each whale can take in several tens of kilos of fish in one coordinated mouthful.

A culture only some populations share

Bubble net feeding is not a universal behaviour in the humpback whale. It is particularly well documented in south-east Alaska (Glacier Bay, Frederick Sound) and around the Antarctic Peninsula. It has been observed more rarely in the North Atlantic, and remains exceptional elsewhere. Even within a single geographical area, not all individuals use the technique. Some hunt alone, others in pairs, others only in coordinated groups.

This non-random distribution suggests that the bubble net is a cultural innovation, passed on by social learning rather than a genetically programmed behaviour. A long-term study published in Science in 2013 by Jenny Allen, Mason Weinrich and their colleagues made it possible to demonstrate this rigorously. Following a population of humpback whales in the Gulf of Maine over thirty years, they reconstructed the spread of a new feeding technique: lobtail feeding, a variant of the bubble net that begins with a slap of the tail at the surface, from one individual to the next, and showed that it followed the patterns of social transmission by observation.

The invention of lobtail feeding

The history of lobtail feeding is particularly telling. Before 1980, no whale in the Gulf of Maine had been seen striking the surface with its tail fluke before diving to produce a bubble net. That year, a single individual was observed inventing the sequence. In the years that followed, a few whales geographically and socially close to the original inventor took up the same technique. By 2007, nearly thirty years later, about thirty-seven per cent of the Gulf of Maine population was lobtail feeding. The spread follows the observed social ties: whales learn by watching those they keep company with.

This is one of the rare cases in which a complete temporal record of a cultural innovation is available for a wild population of a large mammal. Compare it with the appearance of potato washing among Imo’s macaques in Japan, or the making of “sponge tools” by the dolphins of Shark Bay: all of them examples that force a reconsideration of the idea that culture is an exclusively human affair.

A precise energy investment

Why this complicated technique rather than hunting alone? For reasons of energy economy. An adult humpback whale has to consume about a tonne of food a day during the feeding season. It has to build up enough blubber over a few summer months to cross thousands of kilometres to the tropical breeding waters, where it will stop feeding. Every mouthful has to count.

Bubble net feeding multiplies the return per mouthful. By trapping an already dense school, concentrating it further with the call, and engulfing it several at once, the whales take in more prey per unit of effort than each would hunting alone. The technique also shares out the energy cost: blowing bubbles is less tiring when you are circling in a spiral and not passing through the school yourself. Social ties play a stabilising role as well: whales that hunt together regularly recognise one another, coordinate better, and improve their collective return over time.

Sources

• Allen, J., Weinrich, M., Hoppitt, W. & Rendell, L. (2013). “Network-based diffusion analysis reveals cultural transmission of lobtail feeding in humpback whales”. Science, 340(6131), 485-488.
• Wiley, D., Ware, C., Bocconcelli, A. et al. (2011). “Underwater components of humpback whale bubble-net feeding behaviour”. Behaviour, 148(5-6), 575-602.
• D’Vincent, C.G., Nilson, R.M. & Hanna, R.E. (1985). “Underwater observations and recording of feeding behavior and underwater vocalizations of southeastern Alaska humpback whales”. Scientific Reports of the Whales Research Institute, 36.
• Whitehead, H. & Rendell, L. (2015). The Cultural Lives of Whales and Dolphins. University of Chicago Press.
• Hain, J.H.W. et al. (1982). “Feeding behavior of the humpback whale (Megaptera novaeangliae) in the western North Atlantic”. Fishery Bulletin, 80.

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