These Birds Make Their Own Citrus-scented Cologne

29th May 2018

On Alaska's rugged Shumagin Islands, no fruit tree dares take root, yet the smell of fresh lemons and tangerines hangs heavy in the air.

A nearby colony of lovesick crested auklets are the culprits: Males produce a citrus odor to attract females. And as new research in Behavioral Ecology reveals, it's the strength of their citrusy scent along with the size of their crests that really matters.

Researchers have long known that male auklets with larger crests - a tuft of head feathers that curl forward—have more sexual appeal. But unlike an elaborate display or vocalization, these ornaments cost little energy to produce, making them an unreliable signal of fitness to females looking for a robust male.

Scientists assumed there must be something else that makes these longer-crested individuals so attractive. 

Perhaps it’s their smell, thought study leader Hector Douglas, an independent marine biologist. “The citrus odor is incredibly intense,” he says. “They’re the strongest smelling birds I know of.”

But in 2002, when he began his research, even the idea that birds had a sense of smell was a radical one, let alone the notion that perfume could function as a signal of fitness. So he put it to the test.

That summer, Douglas captured dozens of crested auklets on the Shumagin Islands and inserted them, one by one, into a specialized chamber to quantify their scent. 

A stream of purified air passed around the bird’s body, picking up volatile molecules that produce the aroma, which was then filtered and analyzed. These organic compounds, called aldehydes, are found in citrus fruit rinds, as well as in off-the-shelf perfumes, including Chanel No. 5.

Before releasing them back into the wild, Douglas measured the birds' fitness through a stress response hormone called corticosterone—the bird equivalent of cortisol. The faster the birds produce the hormone, the stronger their physical condition. 

After sifting through results back on the mainland, an exciting pattern emerged, he says: Birds with larger crests produced a higher quantity of the citrusy perfume, suggesting that odor is attractive to females. “Something happened in evolution to favor the production of these chemicals,” Douglas says.

And not only that, more citrusy auklets released corticosterone at a faster rate, suggesting that auklets’ capacity to produce perfume is tied to their physical ability - and thus fitness.

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