Editor, Earth News
A king penguin adopts and defends a skua chick. (C. Oosthuizen)
A king penguin has been observed trying to adopt the baby of its mortal enemy.
The adult penguin kidnapped a skua chick on Marion Island, in the sub-Antarctic, then attempted to raise it.
During the incident, reported in Polar Biology, the penguin vigorously defended the chick and tried to brood it upon its feet.
Penguins often try to raise chicks that aren't their own, but it's surprising for one to try raise the young of its natural predator, say scientists.
The king penguin tried to brood the skua chick on its feet. (C. Oosthuizen)
Chris Oosthuizen and Nico de Bruyn of the Mammal Research Institute at the University of Pretoria, South Africa, spotted the king penguin while en route to Goodhope Bay on Marion Island, where they research southern elephant seals, fur seals, killer whales and penguins.
"We initially thought the penguin was brooding a king penguin chick," says Oosthuizen.
"However, it was too early in the breeding season, and not at a regular breeding site. Closer inspection revealed it was actually brooding a skua chick."
Mature skuas eat penguins, preying on their chicks and occasionally taking adults.
An hour later, the researchers returned to find the chick's real parents, two skua birds, attempting to win back their offspring. One skua repeatedly harassed the penguin, spreading its wings, and calling in a bid to have its chick returned.
Twice, it succeeded in driving the penguin from the chick, only for the penguin to retaliate by beating its flippers and pecking at the skuas. After each attack, the penguin won the chick back, placing it back on its feet as it would if it was brooding a chick of its own.
The tug of love only ended when a human observer stepped in and returned the chick to its real parents.
King and Emperor penguins often adopt chicks that aren't their own, either taking on abandoned baby birds or kidnapping those of other penguins.
A parenting hormone bonds adults and offspring. (C. Oosthuizen)
They may be driven to do so by an increased level of the hormone prolactin, known as the "parenting hormone" because it's thought to help maintain the bond between chicks and adults when they're away foraging.
It's not unheard of for a bird to try to raise the young of another species.
Although it is rare, one scientific review collated 140 separate records of it happening, involving 65 species.
Usually it occurs when a parent fails to correctly identify their offspring, because their original nest has been lost and they've flown to another nearby, or because they can't resist the begging cues of chicks that are not their own. But, usually, the behaviour and diet of adopting and adopted species are analogous.
"It's surprising that the penguin adopted a chick of a species that preys on penguin chicks," says Oosthuizen.
Why it would do so remains a mystery, but Oosthuizen has his suspicions.
"It's likely the penguin simply happened to stumble across this chick, and adopted it, perhaps due to the similar brown colour of the down in both species' chicks," he says.
Skuas like to prey on penguin chicks. (C. Oosthuizen)
Thinking it to be a penguin, the adult then thought it should defend the chick against the skuas, which are natural predators, rather than "knowingly kidnapping a skua chick and defending its claim from the rightful parents".
"Seeing an 'abandoned penguin chick' at the mercy of the predatory skuas might have been enough to stimulate the parental care instinct," he says.