The "blonde" penguin, seen at the edge of one of the South Shetland Islands (map), "astonished" tourists on a National Geographic Journey to Antarctica cruise, naturalist David Stephens, of the Lindblad Expeditions cruise company, wrote on his blog.
(The National Geographic Society owns National Geographic News and National Geographic Expeditions, which operated the cruise in association with Lindblad Expeditions.)
Though the penguin looks like an albino, the bird actually appears to have isabellinism, said penguin expert P. Dee Boersma of the University of Washington in Seattle.
The condition is a genetic mutation that dilutes pigment in penguins' feathers, according to a 2009 study on isabellinism published in the journal Marine Ornithology.
This results in a "uniform lightening" of a bird's dark colors, turning the animal a grayish yellow or pale brown, the study said.
"Isabelline" Penguins Not Albino
Though they technically represent separate conditions, the terms "isabellinism" and "leucism" are sometimes used interchangeably. Leucism is a mutation that prevents any melanin at all from being produced in feathers. Albinism occurs when an animal produces no melanin at all throughout its entire body.
(See pictures of albino animals.)
"Many species of penguins have a few rare individuals with this color pattern," Boersma said via email.
For instance, scientists have observed the most cases of isabellinism in gentoo penguins, which are found throughout the Antarctic Peninsula. Magellanic penguins, which live on South American coasts, seem to have the lowest incidences of the condition. (See penguin pictures.)
In the ocean, penguins' black backs camouflage the birds from both predator and prey swimming above, so Boersma suspects isabellinism would affect the South Shetland bird's survival, although there are no studies on the subject, she said.
Indeed, aboard the National Geographic Explorer cruise ship, "many wondered about this unusual bird's chances of success," Lindblad's Stephens wrote.
On the bright side, "while odd coloration may make fishing a bit more difficult," he said, such "birds are regularly found breeding normally."