Imagine diving into the deep ocean water only to notice several huge pillars hovering ~ 25 m under the surface. These aren't abandoned torpedoes from a sunken warship; they're a pod of sleeping sperm whales (Physeter macrocephalus). While little is known about how sperm whales sleep, scientists think that these giants nap underwater for 10-15 minutes at a time with their heads facing upwards, keeping away from the choppy waters of the surface while still remaining close enough to get a breath of fresh air when the need arises. However, one issue puzzles scientists. A sperm whale's head is massive – taking up nearly a third of its body – and filled with oil, wax and air. Given that all of these materials float, how do these whales stay underwater without popping to the surface like a cork as they sleep? Noémie Freymond of the Université de Neuchâtel, Switzerland, along with Alec Burslem and Patrick Miller of the University of St. Andrews, UK, shed some light on this conundrum and publish their discovery suggesting that the sperm whale's solution is to blow bubbles in their sleep in Journal of Experimental Biology .
However, watching the whales blow bubbles can be rather difficult. So, the team made use of some state-of-the-art devices that would allow them to tell whether the whales were asleep and also record the bubbles of the whales. These devices, suctioned to the whales, recorded the sound of the bubbles as they were released from the blowhole while also telling the researchers how deep the whales were and whether their heads were facing up or down. After affixing these recorders on 42 sperm whales in the Loften Islands off the coast of northern Norway, the scientists recovered their recorders after the popped of and floated to the surface and headed to the lab to see what they could hear.
Freymond and colleagues already knew that sperm whales can rest in three different ways: they can put their head down and dive a bit before their buoyant heads turn up towards the surface, they can sink gently, tail-first about 8 m down, or they can rest as they ascend from deep dives (over 200 m). The researchers found that those whales dozing closer to the surface blew more bubbles in their sleep, releasing bubbles ~11 times per power nap. However, those drifting up from the depths (over 200 m) only blew bubbles three or four times during their restful ascent. The scientists explain that different resting depths require the whales to carry different amounts of oxygen and last different lengths of time. Deep diving means the whales need a lot of oxygen, but the pressure of the water compresses the air in their lungs causing the whales to be less buoyant. This means they need to blow fewer bubbles to stop themselves from floating upwards. However, when the whales rest closer to the surface, the air in their lungs will slowly cause them to rise to the surface. Blowing bubbles releases some of the air, making them less buoyant in the process.
However, one question remains unanswered; is this a conscious decision by the whales to release some air as they sleep? No one knows whether sperm whales sleep as we do, or if they sleep like dolphins and keep one half of their brain awake while the other snoozes. Blowing bubbles to make sure they don't come too near the surface requires the whales to sense their environment and realise that they are coming too close to the surface. Whether they turn out to be partially awake or fully asleep, sperm whales have definitely come up with a creative solution to ensure a restful slumber beneath the waves.
