Humpback Whales’ Bubble Nets Should Be Classified as Tools

The bubbles that humpback whales blow to bewilder and trap their minute prey should be regarded as tools.

That is the conclusion of scientists studying these remarkable marine mammals in Alaskan waters, where they alter their surroundings to improve their prospects of survival.

A detailed investigation, using cameras that provide a whale’s-eye view of events, has revealed that humpback whales (Megaptera novaeangliae) do not simply release bubbles at random. Instead, they adjust and direct them to ensure they secure as much food as possible.

Humpback whales and their bubble-net tools

"We discovered that solitary humpback whales in southeast Alaska craft complex bubble nets to catch krill, which are tiny shrimp-like creatures," explains marine biologist Lars Bejder of the University of Hawaiʻi at Mānoa.

"These whales skillfully blow bubbles in patterns that form nets with internal rings, actively controlling details like the number of rings, the size and depth of the net, and the spacing between bubbles. This method lets them capture up to seven times more prey in a single feeding dive without using extra energy."

This striking behaviour puts humpback whales in the small group of animals that both create and use their own hunting tools.

Krill hunts in Alaskan waters during summer and autumn are especially important. Winter is the whales’ calving period, when many North Pacific humpbacks travel to warmer waters near Hawaii to give birth and rear their calves.

Adults do not feed during that period, so they must consume enough throughout the feeding season to sustain them.

How bubble nets trap krill

Their approach to gathering krill (Euphausiacea) is remarkably inventive. A whale descends, swimming in a circle around a cluster of prey while releasing bubbles. As the bubbles rise, their ring becomes a cylindrical barrier that stops the prey from dispersing. The whale then rises from underneath with its mouth open wide. The krill have no route of escape except into that vast mouth.

The technique also seems to be learned. Not every humpback whale feeds in this manner, and several approaches appear to exist. It can also be a cooperative activity, with whales working together so that each animal gets to feed.

To examine the foraging behaviour of solitary humpback whales off Alaska in greater detail, scientists used drones to record the animals from above while they created bubble nets. To observe what occurred beneath the surface, they fitted the whales with cameras and sensors using harmless, non-invasive suction cups.

They recorded 83 bubble nets made by solitary whales. This information enabled the researchers to assess the bubbles’ shape, size and separation within a bubble net, and to determine how these features affected the amount of prey consumed.

Sensor data also allowed the team to examine breathing rates, lunge kinematics and diving behaviour, in order to calculate the energy required to construct a bubble net. The findings indicate that the nets increase the quantity of prey a whale can capture and eat without adding to its energy costs.

Evidence that bubble nets are whale-made tools

According to the researchers, the nets fulfil the requirements for tool creation and use.

"Bubble-nets are unattached and employed externally by the whales. The large number of individual whales we observed using bubble-nets and the tight temporal coupling of net deployment with lunging strongly support the argument that bubble-nets confer a benefit to foraging whales," they write in their paper.

"Furthermore, that there is consistency between individuals in several key, yet modifiable structural components of the nets they produce, suggests that whales exert control over the nets' three-dimensional form to optimize that benefit."

The results indicate that humpback whales may be included among tool-making and tool-using animals, alongside crows, parrots, orangutans and chimpanzees.

"Many animals use tools to help them find food, but very few actually create or modify these tools themselves," Bejder says.

"There is also data coming in from humpback whales performing other feeding behaviors, such as cooperative bubble-netting, surface feeding, and deep lunge feeding, allowing for further exploration of this population's energetic landscape and fitness."

Understanding how whales hunt and forage is vital to their continued survival. Numbers of humpback whales increased after commercial hunting of the species was prohibited in 1985, and the cetacean is no longer classed as threatened or endangered. However, a study released earlier this year identified a worrying fall in their numbers from 2012 to 2021.

Research into their foraging methods will support efforts to protect their feeding grounds.

"What I find exciting is that humpbacks have come up with complex tools allowing them to exploit prey aggregations that otherwise would be unavailable to them," says marine biologist Andy Szabo of the Alaska Whale Foundation. "It is this behavioral flexibility and ingenuity that I hope will serve these whales well as our oceans continue to change."

The research has been published in Royal Society Open Science.