Hot Times For A Cool Bird
Snow stings the exposed flesh of my face. I am wearing pure wool long-johns, a woollen shirt, over trousers made from a metallic space-age material, a dacron vest, and a jacket - a huge jacket - filled entirely with goose down. On my feet I have a pair of thick-soled mukluks which would not look out of place on the moon and, inside them, a pair of fleece-lined booties. My hands are covered by polypropylene gloves and bear mitts: thick outer mittens made from the reversed hide of some furry animal long since dead. A woollen balaclava, polarised ski goggles and the down-filled hood of the jacket, complete with wolverine trim, shroud my head. But I am cold. Despite the combined warmth of the best natural and high-tech fibres available, I am cold. After three hours sitting still, the Antarctic wind, the snow and, perhaps, just the view of the ice bergs has chilled me to the bone. During that time I have been watching Adélie penguins and they have hardly moved. Lying on their nests, incubating eggs, they have stood only to shake the snow crystals from their backs before resuming their vigil, seemingly untroubled by the freezing temperatures.
Penguins are usually portrayed as I see them now: eking out an existence in a land of ice and snow. Consequently, when I speak of penguins and thermoregulation in the same frosted breath, it is natural to suppose that penguins need to find ways to keep warm rather than cool. That, however, could not be further from the truth: when on land, the problem most penguins face is how to keep cool.
It all stems from their life in the sea. Penguins are aquatic creatures and water has about 25 times the thermal conductance of air. To survive long in water, warm-blooded animals must be insulated. Whales have blubber to buffer their body heat from the body of water that would draw it away. Seals have, as well as fat, a thick pelt. But birds - birds have no fur and can have little fat: in their evolutionary past they eschewed features that would encumber flight too much. For penguins, their barrier against heat loss is provided by a survival suit of feathers. Their feathers are short and stiff and interlock to trap air beneath, forming the ornithological equivalent of a diver's dry suit. However, the same feathers that keep heat in when they are in water, also prevent heat from getting out when they are on land. It is their insulation demanded for a life in water that has allowed penguins to rush onto the land and ice of higher latitudes where other birds had feared to tread: keeping warm is not too difficult - even a snow storm is of little consequence for an Adélie. Keeping cool, though, can be a problem. Penguins ashore risk overheating. This is especially so for those penguins found in temperate and tropical zones: contrary to their cartoon image, most penguins do not live in Antarctica.
For a penguin of the temperate zone, such as the Yellow-eyed penguin breeding on New Zealand's South Island, walking around in a feather wetsuit in summer when temperatures may approach 30°C can be downright uncomfortable, and in the case of their young chicks, life threatening. Work by Phil Seddon, my student, and I showed that Yellow-eyed penguins avoid direct sunlight by choosing a sheltered place to nest. They nest in forest and the most important feature of their nest sites is the vegetative cover provided overhead between 0.5 and 1.0 metre from the ground. Birds with most of their nest shaded by such cover rarely display signs of heat stress, while those with less than 50% cover spend more time standing upright and panting.
Penguins breeding even nearer to the tropics seek shelter from the sun by nesting in burrows or caves. Some, like the Little Blue penguin breeding around the coasts of New Zealand and Australia, go one step further and are active on land mainly at night, usually coming ashore only after sundown. Humboldt penguins, found on the coast of Peru and Chile, dig burrows in their own guano: their food eventually recycled as shelter. A near relative breeding at the equator, the Galapagos penguin, avoids the sun by nesting in cracks in the lava rock derived from their islands' volcanic origins.
Even at the equator where the water temperature can be as high as 28°C, for a bird that must keep its internal temperature at 39°C the sea still acts as a huge heat sink and penguins must maintain the integrity of the survival shield provided by their feathers. This encasement of the penguin within an armour of feathers limits the potential for physiological management of excess body heat. Prevention from overheating is the best medicine, but shade provided by bushes and burrows can do only so much to ameliorate the heat. There must be windows for superfluous heat to escape the feathers.
Spheniscus penguins, like the Humboldt and Galapagos penguins, do have patches of unfeathered flesh on their face. However, it is the flippers and the bare feet of penguins that act chiefly as heat radiators. The feathers on the flippers of all penguins are extremely short and provide, at best, only modest insulation. By controlling the position of the flippers and feet, and the blood flow to them, the animal can minimise or maximise heat loss through those areas. When penguins are hot, like those Yellow-eyed penguins using poorly shaded nests, they hold their flippers out at an angle of about 45o to the body. The blood vessels in the feet and underside of the flippers dilate, turning those areas pink. This is easiest to see in a bird that has just come ashore and needs to rid itself of the heat generated by swimming. When even hotter, they pant. Hyperventilating evaporates body water using up body heat as it does so. My Adélie penguins and Phil's Yellow-eyed penguins have a circadian rhythm to their panting: it occurs just after mid-day during the hottest part of the day.
Chicks can also be in danger of overheating as on calm days their down is even more efficient at retaining heat than the feathers of their fathers or mothers. Parents of those penguins that breed in the open, shade their young chicks by standing over them. As the chicks get older, they are able to regulate their own temperatures and they may move away from the nest. The down on the chicks' flippers is not as transparent to heat as the flippers of an adult, and they rely more on their disproportionately large feet to act as radiators. On hot days they lie, flippers spreadeagled at their sides, feet exposed. Yellow-eyed penguin chicks may even seek out marshy ground and stand with their feet in water.
Water. Penguins are birds that belong in water. They come onto land mainly because they are compelled to by their phylogenetic past: as birds they must lay eggs and those eggs must be kept warm. The risk of hyperthermia from insolation when on land is the price they pay for the insulation they need to stay at sea. Ultimately, if the heat should become intolerable, then they can always go back to the sea from whence they came. As I pull my hood even tighter to ward off the cold, this seems an unlikely possibility in the Antarctic; but at the equator, Galapagos penguins have indeed been observed to abandon their nest and retreat to the water, leaving their eggs or chicks to literally fry in the sun. A not so hot ending for an ever so cool bird.
– Lloyd Spencer Davis
– From: Natural History 93/8: 48-51