After a female lays her single egg, she leaves her male partner at the nesting ground for up to two months to incubate the egg.
The female returns to care for the chick after it has hatched, allowing the hungry male a chance to return to the sea to hunt.
"The females are out foraging at sea for long periods after laying the egg and have no visual stimuli" of the chick, Chastel said. That is, emporer penguin mothers don't need to see their chicks in order to continue prolactin production.
"The [persistence of the] hormone is a possible evolutionary adaptation that provides [female penguins] with an incentive to return to their nests."
But some females return to an empty nest. Because they are still pumped up with the parental hormone, the researchers surmised, the penguins might be more likely to kidnap a chick if their own is lost.
"She is still full of prolactin, and when she doesn't find her baby, she resorts to kidnapping," Chastel suggested.
Chastel's team tested their theory on a group of emperor penguins living in the Pointe Géologie archipelago in Antarctica.
The researchers injected about 25 females that had lost their eggs with a substance that lowers prolactin secretion. An additional 25 birds were injected with a control substance.
Observation of the birds' behavior after a week suggested that penguins with decreased levels of prolactin were four times less likely to kidnap chicks.
Other experts are cautious about attributing the penguins' behavior directly to the birds' hormone levels.
John Wingfield, a zoologist at the University of Washington in Seattle, said, "It is entirely possible" that the hormones are responsible.
"However, it is important to remember that hormones do not cause behavior, neuronal circuits do that," he said.
"But [hormones] do increase the likelihood that [certain] behavioral traits will be expressed."
"I suspect [the kidnapping] is a conflict between hormone levels promoting expression of parental care and individual recognition signals indicating that the chick is not theirs [resulting in eventual abandonment]," he added.
"Thus, when having lost a chick, but still having high levels of parental hormones, they are likely to express parental care and acquire a chick.
"What do [the females] get out of it? Probably nothing, which may explain why the penguins abandon the chick so quickly."
According to Chastel, the French biologist, it is still not clear which behavioral traits might motivate the penguins to kidnap chicks.
"Perhaps it is a way for females to manipulate males and make themselves more attractive to males for the next season by suggesting they are able to bear a chick and care for it," he said.
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