It's a complete mystery how penguins avoid getting the bends. The birds, which take a lung-full of air before leaving the surface, not only face an increased risk of decompression sickness but have to battle against increased buoyancy too. Katsufumi Sato wanted to know how they manage to plumb the depths against these odds. After recording over 1000 penguin dives, he has collected evidence that the animals `brake' during the final stages of ascent, which could protect them from decompression sickness (p. 1189). They also appear to adjust the size of their last breath, so they're carrying just enough air to last until their return to the surface.

Measuring a diving bird's acceleration pattern as it swims through the water can tell you a lot about how hard the bird is working. The bird accelerates and decelerates with every wing beat. Sato realised that if he measured the acceleration profile he would have a record of the lift, drag and buoyancy forces acting on the penguin throughout the dive, as well as recording when the penguin flapped its wings to swim. Combining this information with knowledge about the bird's depth and speed, the Japanese team realised that they could estimate how much air a diving bird was carrying as it returned to the surface.

These weren't experiments that Sato could do in the lab, he had to head towards the Antarctic to work with King penguins that routinely dive to 300 metres to get the diving data. At first he attached the acceleration logger and a depth/speed logger to five penguins, and waited for them to return from their month long foraging trips. Unfortunately, only two of the loggers produced enough data to show that Sato was on to something, and by then the breeding season was coming to an end. Sato had to wait another year before he could return to the Antarctic to collect more data.

This time round Sato attached the modified data loggers to smaller Adélie penguins who's foraging trips only lasted for a day. This time all of the modified data recorders worked well, and by the end of the season, he had recorded over 650 dives from ten birds.

Back in Japan, Sato noticed that the penguins flapped continually as they descended, but after the first half of the return, they stopped swimming, and took advantage of their natural buoyancy to glide back to the surface. Sato was also surprised to see that instead of bobbing to the surface like a cork, the penguins delayed their return by applying a `break' and changing the angle they approached the surface. Sato thinks that this combination of tactics might help the birds to avoid decompression sickness.

When Sato compared the volume of the bird's final breath with the depth the birds dived, he was startled by the strength of the correlation. Birds that went to greater depths took a much deeper breath than birds that stayed in the shallows. The only conclusion Sato could draw is that penguins plan every dive before they leave the surface. Buoyancy is the major problem the birds face until they have dived deep enough for their cargo of air to become compressed. Penguins that intend to stay in the shallows only take a small breath to keep their buoyancy to the minimum, while birds that have planned a deeper dive can take a much larger breath, because they only have to fight their increased buoyancy until the pressure has increased enough to compress the air. Then they are free to forage to their heart's content!