Laryngeal sac buoyancy aid makes dives effortless for humpback whale mums Free

Novice human divers will tell you that stabilising your buoyancy is a fine art. A delicate balance is required to get the right amount of air in your buoyancy control device to hang perfectly suspended at depth. Most other mammals have to swim hard against the buoyancy of their lungs until they reach the depth at which they are neutrally buoyant, descending with ease when deeper. And returning from great depths requires effort to reach their neutral buoyancy point, after which they almost float to the surface. But, in addition to their lungs, baleen whales – such as humpback whales (Megaptera novaeangliae) – are equipped with an additional air-filled sac, known as the laryngeal sac, which is surrounded by muscles that could compress and expand the chamber. Might baleen whales use their laryngeal sac to adjust their buoyancy, allowing them to descend and ascend freely? But implanting sensors inside a humpback whale to find out isn't possible. Fortunately for Maevatiana Ratsimbazafindranahaka, from Université Paris-Saclay, France, colleagues at Association Cétamada, Madagascar, had been attaching motion sensors to humpback whale mums and their calves since 2013 to find out more about their lifestyles. Could Ratsimbazafindranahaka and his colleagues reanalyse the recordings to determine whether the whales might be using their laryngeal sacs for buoyancy control?

‘Approaching humpback whales is always intense and memorable’, says Ratsimbazafindranahaka, who joined the whale-tagging team in 2018 and describes using a small boat to get close to the colossal animals, temporarily attaching the motion sensor to the whale's back with a 4 m long pole. He then painstakingly reconstructed the recorded descents and ascents of the mothers and their offspring – keeping track of how hard the whales were swimming and the speed of their descents and ascents – and it was clear that the mothers did not swim harder as they descended initially, when other mammals work hard against their buoyancy. And as he scrutinised the recordings, it was evident that the mothers could hang out at pretty much any depth down to 60 m with minimal effort; they could fine-tune their buoyancy to achieve neutral buoyancy at any shallow depth. It was also apparent that the whales were not exhaling air to manage their descent, as they were able to initiate an ascent immediately after reaching the deepest point of a dive without exertion.

In contrast, their offspring – ranging in age from newborns to 3-month-old calves – had to swim hard to descend from the surface, in addition to investing more effort when ascending from depth. And each youngster could only achieve neutral buoyancy at one depth, ∼38 m for the youngest newborns and ∼18 m for calves up to 3 months old. They were not able to regulate their buoyancy and had to swim harder than their mothers to remain at shallow depths.

So, adult female humpback whales are able to regulate their buoyancy actively – more like fish than other mammals – down to depths of 60 m and they have to develop or learn the skill, much like novice human divers, when young. But Ratsimbazafindranahaka warns that the baleen whale's laryngeal sac buoyancy aid may not be effective at extreme depths if the sac collapses as a result of the high pressure, although the laryngeal sac may remain inflated at depths at which the animals’ lungs are known to collapse. But even if the laryngeal sac ceases to function as a buoyancy aid at extreme depths, the amount of effort it saves humpbacks during shallower dives could help the lactating mothers to conserve energy at one of the most energy intensive times of their lives.