By Matt Walker
Editor, Earth News
The hunt is on
Orcas regularly feed on penguins in Antarctica, scientists have discovered.
Although orcas are known to predate penguins further north, around sub-Antarctic islands, this is the first time they have been seen hunting the birds in Antarctic continental waters.
What is more, the orcas seem to be particularly choosy about which bits of the penguins they eat; being inclined to take only the best cuts of penguin breast meat.
Details are published in Polar Biology.
They seemed to be mainly interested in eating just the breast muscles, rather like humans would do
Marine biologist Dr Robert Pitman
Marine biologists Dr Robert Pitman and Dr John Durban of the US National Marine Fisheries Service based in La Jolla, California, made the discovery while researching orca foraging behaviour around the Antarctic peninsula.
"We had expected to see killer whales taking Antarctic minke whales and seals, which we did," Dr Pitman told the BBC.
"But we were quite surprised to find them catching penguins also."
Three type of orca are known to live in Antarctic waters, each differing in size, colouration and the prey they hunt.
Type A orcas are the largest, are black and white and look most like orcas found elsewhere in the world. Type As hunt minke whales.
Unhappy feet: an orca rolls on its side with a gentoo penguin in its mouth
Type B orcas are smaller and have a yellow tinge. They also prey on minke and perhaps humpback whales but tend to prefer to hunt seals.
Type C orcas are also smaller, with different markings, and prefer to live within inshore waters and among the pack ice, and to date have been recorded only feeding on Antarctic toothfish.
Biologists have long questioned whether any of these orcas take penguins, which are also abundant on the continent, but until now there has been no evidence.
That was until Drs Pitman and Durban witnessed several instances of predation on two different species of penguins: chinstraps and gentoos.
The attacks occurred over three separate days, and were instigated by type B orcas, with some evidence that type A orcas may also have hunted and fed on a penguin.
During each attack the orcas chased a penguin, which porpoised out of the water and erratically changed direction under water in a bid to avoid capture.
Surprisingly, when the orcas did catch a penguin, they often refrained from eating the whole bird.
"We were surprised to find killer whales eating 4 to 6kg penguins, and even more surprised to find that they seemed to be mainly interested in eating just the breast muscles, rather like humans would do," says Dr Pitman.
Often the orcas handled their prey with meticulous care, removing skin and feathers to expose the breast muscle, sonetimes working cooperatively to do so.
The breast tissue may be particularly nutritious, but it is still unclear how much nutritional benefit a 3000kg orca would get eating selective cuts of a penguin weighing just a few kilograms.
Juvenile orcas catch and kill a gentoo penguin
"Penguins have enormous breast muscles to power winged flight through the water, but it is still a small part of an already small prey item. It would be like us chasing around after individual peanuts," says Dr Pitman.
"Ours is the first clear documentation of penguin predation in Antarctic waters but we don't know how commonly it occurs."
If it is common, then predation by orcas could have a significant impact on penguin populations in Antarctica.
In the past, biologists have suspected that orcas may have been responsible for a 50% decline in numbers of emperor penguins residing at Adelie Land, eastern Antarctica during the 1970s.
But no penguin remains were found inside the stomachs of orcas in the area.
The observations of Dr Pitman and Durban, however, suggest that these whales may have also been fussy eaters, removing breast tissue and not bones, which would leave little trace in an orca's stomach.