Anyone who has tried diving will know the uncomfortable feeling of water pressing on their ears. For divers, the knack of holding their noses and exhaling hard to relieve the pressure is second nature. But other gas-filled chambers in the body are also at increasing risk of damage as air compresses at depth, and it may not be as easy as blowing your nose to equalise the pressure within. Paul Ponganis from the Scripps Institution of Oceanography, USA, explains that the lungs of birds are rigid, so as they dive and air in their lungs is squeezed, the delicate tissue is at risk of rupture. However, no one seems to have told penguins about the risk; they routinely plummet to depths of hundreds of metres. ‘The question, “How does the avian lung avoid damage at high pressure?” has never been addressed’, says Ponganis, so he teamed up with Judy St Leger and Miriam Scadeng to find out more about the champion divers’ survival strategy (p. 720).
Ponganis explains that in addition to their lungs, birds are equipped with compressible air sacs that function as bellows to drive air through the lungs. Could the air sacs provide a reservoir of air that can be injected into the lungs to equalise the pressure as penguins plumb the depths? The only way to find out was to CT scan emperor, king and Adélie penguins to measure the size of their lungs and fully inflated air sacs. However, before Ponganis and Scadeng could begin investigating the penguins’ bodies, they had to make sure that the polar birds were comfortable, so St Leger arranged a lift in SeaWorld's refrigerated truck to keep the penguins cool in transit to the local veterinary hospital and the University of California San Diego. Then, after carefully anaesthetising the birds, Ponganis and St Leger gently inflated their air sacs and scanned the sedated animals to find out how much air they could hold. ‘We even put little cooling packs on their [the emperor penguins’] wings and feet during anaesthesia to manage temperature’, adds Ponganis.
However, after analysing the images of the penguin's bodies, it was clear that the air sacs did not carry enough air to compress into the lungs and protect them from damage. ‘We were surprised by how large the air sac volumes were’, says Ponganis, but adds, ‘Despite the large maximal air volume, that volume is not enough to prevent barotrauma [pressure damage]’.
The team suspects that the animals use other strategies to keep them safe. Ponganis suggests that the lung and trachea may not be as rigid as previously thought, allowing the tissue to compress and avoid ruptures. He also suggests that the birds may be able to contract muscles in the walls of the lung airways to reduce the volume as the pressure rises, or inflate blood vessels to fill the volume as the air compresses. Ponganis also suspects that by fully inflating their air sacs the birds are able to carry down significantly larger oxygen reserves, allowing them to swim longer and go deeper before resorting to less effective anaerobic metabolism to fuel the dive.