How does a Harbour Porpoise sleep under water?
Breathing effectively is one of the difficulties we humans face while swimming in the sea; it is a constant challenge to keep the splashing seawater from penetrating our nostrils and mouths.
The Harbour Porpoise has similar problems. It is the only representative that inhabits our territorial waters of the six species of porpoise found worldwide. Like us, it breathes air but unlike us it does so in a very different way.
We breathe unconsciously. We don't have to think about drawing a breath; air just flows in and out of us automatically. It continues to happen subconsciously while we are in deep sleep.
The Harbour Porpoise is a conscious breather; for it breathing doesn't happen subconsciously, it must consciously decide when to breathe and when not to breathe. It has evolved in that way to prevent it drowning in its watery world. Our inability to overcome unconscious breathing by holding our breath leads, tragically, to fatal incidents in the water each year.
The porpoise has other great adaptations: it cannot breathe through its mouth and its nostrils open in a single blowhole on top of its head. When it is travelling at speed it breaks the surface of the sea, breathes out violently to blast any water off its blowhole and gulps in a lungful of fresh air while continuing the slow, fluid flow of its undulating forward movement.
Porpoises stay down for only a few minutes, exceptionally up to six minutes and when they surface their exhaling blow sounds like a violent human sneeze.
Seeing that it is a conscious breather, how does the Harbour Porpoise breathe when it is sound asleep? Nobody knew the answer to that question for a long time. It was a mystery. Studies of captive animals now suggest a remarkable solution to that particular problem.
All mammals have brains that require sleep. Brains come in two halves joined together along the centre line. When the Harbour Porpoise it at rest, it appears that each side of the brain takes a turn at sleeping separately. The left side sleeps while the right side continues to control breathing and allow the animal to function normally. When the left side has had a sleep, it takes control of things allowing the right side get a well-earned nap.
Both sides of the brain become fully alert when prey, predators like dolphins and Killer Whales, other marine mammals or other porpoises are about.