Penguins' extraordinary ability to survive in the Antarctic is down to an accident of evolution - the heating system in their wings. Roger Dobson explains
Wednesday, 23 February 2011
Penguins may owe their survival in the coldest and most inhospitable place on earth to evolutionary chance during a period of global warming millions of years ago. Far from adapting to the cold in Antarctica, where temperatures can plunge below minus 60C and wind speeds reach in excess of 200mph, they have been able to thrive because of a form of central heating of the wings they evolved when the climate on Earth was hot.
New research shows that when the earth warmed up nearly 50 millions years ago, penguins evolved a wing heating system, a highly efficient heat exchanger.
The mechanism, which is so effective that the birds have to cool down after vigorous swimming in sub-zero waters, evolved to help the birds keep warm while foraging in ever deeper, ever cooler waters. "Penguins had a lucky break with the evolution of heat retention 49 million years ago, as it allowed them to survive the coming cold," says Dr Daniel Thomas of the University of Otago, New Zealand, who led the study. "The fossil evidence reveals that it evolved during a Greenhouse Earth interval.
"Its evolution is therefore unrelated to global cooling or development of polar ice sheets, but probably represents an adaptation to foraging beneath the surface in waters at temperate latitudes. As global climate cooled, the heat exchanger was key to the invasion of the much more demanding environments associated with Antarctic ice sheets. The climate has never been so hot as it was back then, nor has it been so cold as in recent millennia, and penguins have weathered it all. We are seeing a dramatic shift in climate, however, and it will be a true test for the thermal tolerances of penguins."
Penguins probably evolved from flying birds between 50 and 60 million years ago. Exactly why they lost the ability to fly is not known, but the dominant theory is that it was the survival trade-off for perfecting flight beneath the surface of the sea. It became more important to be able to dive and swim for food than to fly, so over time the birds lost the ability to fly, and wings became flippers.
The first penguins came some time after the Cretaceous-Tertiary or KT mass extinction event, around 65 million years ago when almost all the large vertebrates on Earth – dinosaurs, plesiosaurs, mosasaurs, and pterosaurs – suddenly became extinct for reasons that are still hotly debated.
After KT, climates warmed with temperatures peaking in the Eocene epoch, around 49 million years ago. Geochemical signals, telltale signs of how the earth once was, suggest that it was much warmer. Sea temperatures were around 25C, and the area around what is now Seymour Island in Antarctica, a major penguin habitat, was a balmy 15C, while parts of the now ice-covered region were subtropical. It has long been believed that penguins gradually adapted to increasingly cold conditions after the area became glaciated about 34 million years ago.
But the new research, by scientists from New Zealand, America and South Africa, based on analysis of modern penguins and of fossils dating back more than 60 million years, shows that the key anatomical change that was to become pivotal to survival in the extreme cold many years later occurred when global warming reached its peak, 49 millions years ago. In the study, the researchers examined the remains of several types of modern penguins, including the little penguin, yellow-eyed penguin, king penguin, and Humboldt penguin, all of which had died naturally.
Dissection of the birds showed that each had a major adaptation that allows penguins to forage in cold water, the humeral arterial plexus, a counter current vascular heat exchanger, or CCHE, that limits heat loss through the wing. The core body temperature of a penguin is around 38C. Although blubber and feathers offer some protection to the body, the large wings, with tightly attached skin and little insulation, have a huge surface area. Without protection, heat in the wing or flipper would be rapidly lost to the surrounding colder water or air with the threat of hypothermia and death. Heat is lost 20-30 times faster in water than in air.
The CCHE is an ingenious web of arteries and veins that stops this happening. Blood is pumped to the wings of birds through a single major artery, but in penguins there are up to five arteries, each of which runs alongside two or more veins. Blood in the arteries being pumped into the wing from the heart is much warmer than that in the veins which is returning from the wing extremities exposed to the cold. Heat from the arterial blood is given to the venous blood, and redirected back to the body instead of being lost to the ocean. It is so effective that penguins emerging from the sea often stick their wings out to the sides in order to cool them down.
But when did it evolve? The results of the dissections show that the presence of this heat exchanger mechanism requires grooves in the bones of the wings to carry the arteries, which should be detectable in fossils. The researchers looked at penguin fossils dating from 62 million years ago for signs of these grooves. In the early period birds there were none, but from around 49 million years ago they are present. At around the same time, other changes and adaptations were taking place that improved buoyancy in the water and reduced drag. Body size increased too, and a hydrofoil wing evolved.
But while all of these changes improved the penguins' abilities for long distance swimming and deep diving, there was a snag. While much of the land was now tropical or subtropical, temperatures in the sea had not increased as much and it was still very cold, and significantly cooler than penguin body temperature.
According to the researchers, it was these longer feeding excursions far from the shore, and spending long periods of time in cold waters, that led to the evolution of the heat exchanger.
Once equipped with the CCHE, the now flightless penguins were able to travel vast distances, and colonise new areas. And millions of years later, when the earth began to cool, the onboard heat exchanger meant the penguin was uniquely equipped for a successful invasion of icy Antarctic, an environment where it has walked, or waddled tall, ever since.
Ironically, there have been warnings that the penguin is at risk from the effects of global warming, including rising temperatures and a loss of sea ice, with a consequent reduction in nesting and breeding grounds, as well as a drop in food availability. "This is very interesting research and it suggests the heat exchanger is an adaptation to allow longer to be spent in the water," says Dr Jonathan Green, lecturer in marine biology at the University of Liverpool. "It is an adaptation which developed to promote foraging in warmer water and by coincidence proved to be helpful in cold waters and air temperatures. The same mechanism is employed in other parts of the penguin body, including the feet, which stops them freezing when they are in contact with ice.
"Our own research has shown that global warming is a threat. Only two of the 17 species of penguin breed on the Antarctic continent. The others live in the sub-Antarctic and temperature regions in South America, South Africa and Australasia. Our research in Australia has shown that temperature rises can cause problems for the penguins. A danger from global warming is that this overheating may lead them to abandoning breeding attempts, resulting in declining population numbers."
Huddling: The emperor's other way of warming up
Penguins keep warm on land by huddling.
Scientists have found that it is highly effective for keeping warm on land and can generate a tropical environment in one of the coldest environments on earth.
The emperor penguin breeds during the severe Antarctic winter, and the males have the job of incubation, which involves them being deprived of food for around 65 days.
But successful breeding requires a temperature of around 35C, and so to keep warm, the males huddle.
Researchers from the Scott Institute of Polar Research, who investigated what exactly happens inside huddles for the first time, showed that the birds spend an average of 38 per cent of breeding time huddling, with huddles lasting around 90 minutes. The birds moved around in the huddle so they all had access to the inner warmth.
Temperatures during tight huddling increased from 20C to 37.5C in less than two hours. "This complex social behaviour enables all breeders to get a regular and equal access to an environment which allows them to save energy and successfully incubate their eggs. Huddling behaviour of emperor penguins is a far more complex behaviour than previously described," say the researchers.