Sleeping sperm whales hover several metres below the surface of the water lined up vertically like a row of toy soldiers.
Now, scientists have shown these napping whales have a nifty way to manage their buoyancy so they don’t rocket up to the surface: blowing bubbles. The findings are published in the Journal of Experimental Biology.
Sperm whales – the largest toothed whale on the planet – are mammals that breathe air consciously, meaning they must think about taking every single breath.
Sleeping like a human is impossible because they would be unable to breathe and would drown. Instead, it’s likely that they switch off one half of their brain at a time, like dolphins – known as unihemispheric sleep. This also allows them to keep an eye out for threats.
Researchers haven’t yet managed to measure the brain activity of slumbering sperm whales using an electroencephalogram (EEG) to confirm they are asleep, so they tend to describe the animals as ‘resting’ rather than sleeping.
During these bouts of rest, which usually last between 10 and 15 minutes, the whales hang several metres deep where they’re close enough to the surface to come up for air but far enough down to avoid getting knocked around by the waves.
As they kip, the whales sometimes release bubbles. Scientists think this might be to help control their buoyancy and prevent them from popping up to the surface.
“As sperm whales are breath-hold divers, if they slowly drift up while resting, the gas in their lung will slowly expand,” write the authors in the study. As scuba divers will know, this makes them more positively buoyant so they could float up to the surface. “Releasing bubbles would enable them to counteract the expanding diving gas and achieve neutral buoyancy.”
The study’s authors Noémie Freymond, a scientist at the Université de Neuchâtel, Switzerland and Alec Burslem and Patrick Miller, both from the University of St. Andrews’ Sea Mammal Research Unit tested this theory.
The team tagged 42 sperm whales with special recorders that could record depth and capture the sound of bubbles while the whales were snoozing and ran a simulation to find out “whether bubble releases can help sperm whales regulate their net buoyancy,” they write.
Sperm whales catching 40 winks closer to the surface blow more bubbles than those sleeping on the way back from deep dives of 200m or more: around 11 bubble releases compared to three or four, respectively.
In deeper waters, the air in the whales’ lungs is more compressed and so less buoyant.
“In cases where resting dives started at depths exceeding 100m, the number of bubble releases decreased,” they say in the paper. “This suggests that these individuals did not need to modulate buoyancy via bubble release to rest long enough because of the greater ascent distances.”
There may also be other reasons sperm whales blow bubbles during their naps. “While our results indicate that presence and timing of bubble releases suggest they are used to maintain neutral buoyancy, it does not necessarily follow that this represents the only function of bubble releases during resting,” they say in the paper. It could be related to the process of taking in oxygen and releasing excess carbon dioxide.
Blowing bubbles to stop themselves getting too close to the surface means the dozing whales must be aware of their position in the water column, according to the study, which says: “If the whales are indeed sleeping during these resting periods, our findings indicate that they are capable of fine positioning control even during sleep.”
Top image credit: Stéphane Granzotto, stephanegranzotto.com
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