Any of us who wish to take to the water have no choice but to don scuba diving gear before plumbing the depths. Yet many ocean going mammals and birds regularly dive on a single breath. One of the avian world's champion divers is the emperor penguin, which routinely dives for several minutes while searching for food. Intrigued by the bird's dive tolerance, Paul Ponganis from the Scripps Institution of Oceanography at the University of California San Diego decided to investigate how the birds handle perilously low oxygen levels. From previous studies, Ponganis and his colleagues knew that towards the end of a dive, oxygen levels in the penguin's air sac, which provides additional ventilation for the lungs, fell even lower than those experienced by birds at extreme high altitudes. Wondering how the birds manage their oxygen stores during a dive, the team decided to investigate penguins' blood oxygen levels as they foraged beneath the sea ice(p. 4279).

In previous field seasons, Ponganis and his team had developed a system where they fitted a tiny oxygen probe inside the penguin's air sac and collected oxygen level data in a microprocessor carried on the bird's back. This time, Ponganis wanted to measure the penguins' vascular oxygen levels. Adapting techniques that he practices routinely as a cardiac anaesthesiologist, Ponganis inserted the probe into a vein or artery before releasing the penguins from an isolated ice hole, allowing the birds to come and go freely for several days before retrieving the probe and backpack crammed with oxygen data.

Scrutinising the data from 130 dives, the team was amazed to see that a few birds' venous blood oxygen levels had fallen almost to zero by the end of a dive and almost a third of the returning divers had blood oxygen levels where humans black out. And when the team compared lung, arterial and venous blood oxygen levels at the end of long dives, they were virtually indistinguishable.`Emperor penguins clearly push the limits of hypoxemia' says Ponganis `and are capable of “returning on empty'” he adds.

But how do penguins pull off this remarkable feat? Ponganis believes that the answer lies partly in the penguin's haemoglobin. He suspects it must have very different oxygen carrying characteristics from most ducks and flying birds. According to Ponganis, his idea builds on Bill Milsom's work in the 1970s and David Jones's from the 1980s. He explains that Milsom suggested that haemoglobins with a high oxygen affinity (that are able to bind oxygen at very low concentrations) may allow divers to use their respiratory oxygen stores more completely. And in 1986, David Jones found that the Pekin duck was unable to use all of its oxygen stores; the duck was at the point of `imminent cardiovascular collapse' despite having used only 75% of its respiratory oxygen. Based on these ideas, Ponganis suspects that the emperor penguin's haemoglobin may have a high oxygen affinity to allow the bird to fully exploit its oxygen reserves during a dive and is keen to investigate this idea during his current stay in Antarctica.

Ponganis, P. J., Stockard, T. K., Meir, J. U., Williams, C. L.,Ponganis, K. V., van Dam, R. P. and Howard, R. (
). Returning on empty: extreme blood O2 depletion underlies dive capacity of emperor penguins.
J. Exp. Biol.