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Beaked whales can dive 2,000 metres below the ocean's surface. Why and how do they do it?
Around 2,000 metres below the surface, the water is freezing, black and seemingly impenetrable.
No light reaches the murky depths, and no human could survive the crushing pressure from the water above. It is easier for a person to exist in space than it is to explore the ocean floor at such depths.
But one group of mammals can swim through the heavy darkness: beaked whales.
Despite the extreme conditions, these whales manage to hunt two kilometres deep, surviving on a single breath for near to an hour, many times each day.
Marine mammals as a group are accomplished divers. Elephant seals have been recorded diving for hours at depths of more than 1,500 metres.
So far, the deep diving record holder is the Cuvier's beaked whale.
A 2014 study, published in PLOS ONE, used satellite-linked tags to follow the dives of eight beaked whales off the southern California coast. The deepest recorded dive was 2,992 metres and the longest lasted 137 minutes, breaking the record for diving mammals.
Experts have suggested that this was an unusually deep dive for this species, and a more normal depth is 2,000 metres.
A more recent 5-year study of nearly 3,700 deep dives by 23 beaked whales found that half of all foraging dives lasted an hour or more and 5% exceeded 77 minutes. One individual made two extremely long dives of 173 minutes and 222 minutes, which the researchers thought might be reaching the limits of the species' diving capacity.
All whales rely on coming to the surface of the water to breathe oxygen - so they must hold their breath for remarkably long periods of time.
Dr Sascha Hooker of the University of St Andrews is studying marine mammal diving behaviour.
'Beaked whales are amazing animals,' she says. 'It is not just the occasional deep dive that we see, but repeated deep dives many times a day. Yet these animals are small compared to something like a sperm whale - so how do they do this?'
Beaked whales are difficult to study, mainly because they spend their lives in deep offshore waters and shy away from boats.
However, with perseverance and patience, depth-recording tags can be attached to the flank below the dorsal fin to monitor their underwater behaviour. These studies show that the whales dive far underwater to find the best hunting spots, which are full of deep-sea fish and squid.
When diving to such great depths, whales face two challenges: storing enough oxygen to hunt successfully and withstanding the enormous pressure.
High pressures change the uptake of gas in the body. Increasing pressure shrinks the air in the lungs and by 200 metres deep, both human and whale lungs will have collapsed.
This is the danger zone for whales. Once the lungs have collapsed, no more gas from the lungs will enter the blood. However, at depths shallower than this point, the pressure will compress gasses like nitrogen, increasing the amount dissolved inside blood and tissues.
Human divers face the same problem with uptake of compressed gas. If we then surface too quickly, dissolved gas will form bubbles in the body, which expand as pressure drops and can be fatal.
It is not fully understood how whales deal with these problems, as they too are at risk of their body tissues becoming heavily saturated with harmful levels of nitrogen.
One theory is that marine mammals collapse their lungs in a way that forces air away from the alveoli, the tiny air sacs in the lungs that transfer gasses like oxygen and nitrogen into the blood.
This would solve the problem of nitrogen bubble formation leading to decompression sickness, but it doesn't explain how the whales supply their bodies with oxygen on those long underwater hunts.
Whales' bodies are specially adapted to store oxygen in their blood and muscles, instead of keeping it in their lungs like humans do.
Whales have extraordinarily high levels of proteins called haemoglobin and myoglobin, which store oxygen in the blood and muscles. This also makes their muscles and blood a very dark red, almost black colour.
Diving mammals reduce their heart rate and stop the blood flow to certain parts of the body, temporarily shutting down organs such as their kidneys and liver while they hunt.
Dr Hooker says, 'These animals have an extraordinary physiology to help them dive for so long and at such depths.
'Alongside these adaptations in terms of their dive response, beaked whales have indentations, or pockets for their flippers, which enable them to assume a torpedo-like shape.
'Their streamlined body shape helps them to swim, and often to glide, with minimal effort and extend their oxygen stores for as long as possible.'