Some like it hot, but emperor penguins definitely like it cold. New research has found that these Antarctic predators died off in large numbers during a particularly warm period in the 1970s.
This does not augur well for these magnificent birds, given the uncertainties of the world's climate. "Our results indicate that emperor penguins may be very susceptible to environmental variability," writes Dr Christophe Barbraud and Dr Henri Wirmerskirch of the Centre d'Etudes Biologiques de Chize, CNRS, Villiers en Bois, France, in the journal Nature.
They used the longest series of data available on penguin populations near the Terre Adelie weather station, stretching back to 1952. The figures showed that warmer water and the consequential reduction of the sea-ice shelf was bad news for the emperors.
Met station data indicated that average annual temperatures were minus 17.3 degrees until the 1970s. That decade saw average temperatures reach minus 14.7 degrees, an enormous jump over such a short period. The average has cooled somewhat to minus 16.6 degrees, but the warming caused a decrease in adult survival during the 1970s.
The net result is that the emperor penguin population is now 50 per cent lower than it was 50 years ago. The impact was greater on males than females, and the factors of greatest importance were the sea surface temperature and the extent of shelf ice.
The researchers found a clear correlation, with higher water temperatures and reduced ice cover both contributing to population decline.
"To our knowledge this is the first time that the consequences of changes in major oceanic parameters on the dynamics of an Antarctic large predator have been identified," they write.
Krill, fish and squid are the main elements of a penguin's diet, but krill in particular doesn't do as well when there is reduced ice cover. "In years with high sea surface temperatures, emperor penguins probably have difficulties in finding food, which could increase mortality," the researchers say.
They could not say, however, whether the pressures caused by warmer water took effect during winter when males and females undertake long incubation and chick-brooding fasts or during the summer when the birds disperse to moult.
The researchers also discovered that when there is less ice cover, the adult emperor penguin population declines, but the number of chicks that hatch is higher. Conversely, when there is more ice cover, the adults do better but the number of hatchlings falls. "Therefore there exists in emperor penguins a trade-off between the advantages and disadvantages of extensive pack ice."
Higher adult survival, however, means more mating adults, so more ice favours the total numbers of the birds.
The penguin population has stabilised, but things could become difficult for the birds if climate change progresses as expected.