Saturday, April 11, 2009

Emperor penguin Aptenodytes forsteri

The Emperor penguin Aptenodytes forsteri is one of only two species of penguin that inhabit the Antarctic continent: Adelie penguins breed there in summer, while Emperors breed in winter.

Emperors are the largest of all penguins, easily recognised by their black cap, blue-grey neck, orange ear-patches and bills and yellow breasts. There is a thick layer of blubber under the Emperor's skin. In their chicks, it is covered by a dense layer of woolly down. An overlapping coat of feathers grows over this layer. The outer feathers are covered in a greasy waterproof coating. It's mainly the layers of feathers that keep the water off the penguins' skin and help retain heat. The feathers are highly specialised and modified compared to the feathers of flying birds. For example, they are smaller, stiff, and lanceolate. There are far more feathers on a penguin than on a flying bird of comparative size. The feathers of an adult have tufts of downy underfeathers attached to the bottom of the rachis (feather shaft). So down and main feather are all part of the same structure. Feathers are very complex structures made of keratin (same as our hair and finger nails) that is naturally water-repellent. What birds - and particularly penguins - need to do is keep their feathers in good condition so they remain supple and properly aligned . Only when feathers are aligned properly will they keep out the water. Birds have a gland, the preen gland, at the base of their tails that produces a very complex and chemically complicated preen oil. It is more like a conditioner for feathers than grease.

The Emperor penguin grows to around 115 cm. It weighs 25-40 kilograms, but male weight can vary by up to half that amount depending on the stage of the breeding cycle and how much body reserves he has laid down before the breeding season started.

Males and females are indistinguishable during most of the year. However, when it becomes time for the male to switch responsibilities with the female, the male can have slimmed down to half his weight. Over forty colonies are known, ranging in size from less than 200 pairs in the Dion Islands to over 50,000 pairs on Coulman Island. Perhaps 200,000 stable breeding pairs can be found on the Antarctic ice shelves. Some like Dion Island are doing extremely badly. Others like Coulman Island are probably doing alright but we don't really know. The problem is that the only way to find out how many breeding pairs there are, it's necessary to count the incubating males in winter (one male = one breeding pair). The trouble is that most colonies are so remote that nobody can get there to do the job.

The Emperor penguin feeds primarily on shoaling fish, small crustaceans and squid. They can dive more than 300 metres deep, and remain under water for as long as 22 minutes; but these are extremes. Most of the time, emperors are feeding down to around, say, 150-200 metres, particularly in winter. The majority of their dives last only 3-6 minutes.

Most Emperor penguin colonies are located on the fast ice, i.e. frozen sea-ice. That is not the same as an ice-shelf. Ice-shelves occur at the end of glacier as they are flowing into the ocean. Ice-shelves are freshwater ice. Only two colonies are known on land.

The female lays only one egg; it is too energetically expensive to rear more than one chick; and they can only fit one egg (and later one chick) onto their feet. Also, if an egg is lost, it cannot be relaid because by the time the female returns it is far too late to try again. The egg an Emperor penguin female lays is actually rather small. If a 28-kg female lays, say, a 465 g egg, that is less than 2% of her body mass. Compare this to two 125 g eggs laid by an Adelie penguin. That's around 6.5% of a 3.8-kg female's body mass (still small compared to the eggs Brown Kiwis produce!).

Emperors assemble at the breeding colonies early in winter, shortly after the sea ice has formed. They breed during the perpetual darkness of the Antarctic winter, gathering at rookeries up to 90 kilometers inland during the months of April and May.

Each Emperor, on returning from the north, first looks faithfully for his mate of the previous year. Unless that male or female has died, each penguin returns to the same partner. The Emperors go through a stage of courting before mating. A male may try to befriend a female who has not yet found her partner. If and when his true mate does arrive, the intruder leaves to find a different mate. Emperor pairs gather together near a solid iceberg to each lay a single egg. There are no special preparations or nest. Laying typically occurs in May or June at the start of the bitter Antarctic winter. The Emperors are believed to have developed this winter breeding pattern to allow the chick to grow to independence at a time when food is most plentiful.

After the female lays her egg, she passes it over to the male - though not quite immediately. Sometimes females sit on a newly-laid egg for hours before their mates finally get them: eggs are very precious commodities, and the changeover is a very hazardous transition. If the male does not manage to scoop up the egg very quickly, it freezes and the breeding season is over for a pair before it has really begun. So the females are not very keen to risk loosing their valuable egg. The female travels across the ice to feed in the fish-filled waters far away in the north. She spends the winter at sea.

The male Emperor fasts through the winter during incubation of the egg. Incubation is solely his responsibility. He positions the egg on top of his feet and covers it with a warm fold of feathered abdominal skin. The incubation lasts nearly two months. During the Antarctic winter, the period of darkness can last more than 20 hours. Huddling emperor penguins may spend most of a 24-hour period sleeping while they incubate eggs. Sleeping conserves energy while they fast.

Between mid-July and the beginning of August, the young are hatched. A freshly hatched emperor penguin chick weighs somewhere between 120 and 160 g and they are approximately 15 cm (6 inches) long. When the chicks finally emerge, they are very hungry. The females return to the colonies seven to eight weeks after laying to relieve their mates and tend the newly-hatched chicks. If the female hasn't yet arrived, the father regurgitates a white secretion and feeds it to his chick. The chicks huddle together: the climate is extremely harsh. Winter temperatures may fall below -60C. Wind velocities can reach 180km per hour. But inside the huddles, the temperature can be as high as 20°C above ambient conditions. Adults recognize and feed only their own chicks. Parents are able to identify their young by their chick's distinctive call. The contact call of emperor penguins can be heard up to a kilometer away.

Chicks grow slowly at first, more rapidly in late spring. Once the young are about seven weeks old, they join other chicks in a crèche, which is protected by a few adults. By midsummer, the fledglings are independent. They will be ready to breed in 4-8 years. Giant petrels prey upon chicks, whereas at sea their predators are orcas and leopard seals. Emperor penguins can live up to twenty years or more; exceptional cases have been recorded of over forty years, though such extremes of longevity are rare. Mortality among the chicks and fledglings is high, especially after after fledging in their first year of life when the young Emperors must figure out how to live at sea. The learning curve is steep, and inevitably many youngsters perish; but the survivors typically enjoy a long life.

Information via the Emperor Penguin @

Images by Flickr

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