If you are a fan of sci-fi cult movies, chances are you have seen ‘2001: A Space Odyssey’. The opening of the iconic film about the future, made in 1968, shows a chimpanzee using a bone as a tool. This is often considered a unique trait of humans, yet other animals create and use tools to their advantage. For example, humpback whales (Megaptera novaeangliae) create bubble-nets as a tool for capturing prey by exhaling air through their blowhole while swimming in circles. Although some studies have shown how humpback whales bubble-nets as tools for feeding, no one had studied how they adapt their bubble-nets to increase how many fish or krill they capture and how much energy they use while creating the fishing tool. So, scientists from the USA, Denmark and New Zealand recorded how solitary whales created their incredible bubble-nets to learn how the animals adapt these to help them increase the amount of fish they can gulp down and how this tool impacts the amount of energy they use while foraging.
Andy Szabo (Alaska Whale Foundation, USA) and eight more scientists monitored the maneuvers of whales feeding alone using bubble-nets in northern Southeast Alaska between 14 and 19 July 2019, filming 23 of the massive mammals producing 83 bubble-nets, with drone cameras from above. In addition, they attached tags that recorded the movements of five whales, while also filming beneath the water, to get a closer view of what was going on. Szabo and the team then used this information to quantify the number – ranging from one to six – and area of the concentric bubble rings, as well as the distance between each ring. The team also used the information from the tagged whales to reconstruct 268 bubble-nets, showing that the whales often began producing the nets at ∼22 m depth, descending a further 3.4 m initially, while exhaling in pulses to produce the trains of bubbles, and exhaling 7.3 times more bubbles per minute while generating the inner ring of bubbles than the outer ring.
Analyzing the structure of the bubble-nets, Szabo and colleagues realized that each individual bubble, rising at 0.6 m s−1, is followed by a trail of smaller ‘capillary’ bubbles, traveling at just 0.1 m s−1 in its wake. The team also noticed that the bubbles forming the inner rings were smaller in size and were closer to each other than those in the outer rings. Szabo and the team found that the more rings within a net, the more concentrated the prey (krill) trapped within. According to the researchers, the whales lunge in to engulf the krill in a single mouthful shortly (∼10 s) after they stop making bubbles, reaching speeds that are typically 60% of those used by whales that have not produced a bubble-net when feeding. And when the team monitored the breathing rates of whales that were hunting conventionally and those that were producing bubble nets, they were similar, suggesting that building a bubble-net requires no more energy than feeding without.
In conclusion, humpback whales that use bubble-nets as tools do not use more energy when foraging than whales that simply lunge to engulf their prey. As krill do not like bubbles, the whales use bubble-nets to corral them, driving them inwards from the outer ring into the innermost ring, where they are easily consumed. Szabo and colleagues propose that the whales use bubble-nets for hunting when krill densities are low and krill are spread out in shallower waters. Humpback whales are a beautiful example of how some animals can think, plan and create a tool to hunt successfully, and they do this in locations where krill numbers would otherwise be too low to provide sufficient energy to justify the exertion.
