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Beak size points to sex of Galápagos penguins

Beak size is nearly a perfect indicator of whether a Galápagos penguin is male or female, new research shows.

The discovery means scientists can quickly and accurately determine a bird’s sex in the wild—no blood sample needed.

“For Galápagos penguins, we really wanted to understand if there was a simple ‘rule’ we could employ to determine sex—a sign that would be fast and reliable,” says lead author Caroline Cappello, a biology doctoral student at the University of Washington.

galápagos penguin on rock
(Credit: Caroline Cappello/U. Washington)

Galápagos penguins are the only penguins to live in an equatorial region. Like all penguin species and most birds, they lack external genitalia.

What’s more, males and females look similar to one another and both share parenting duties, incubating eggs and rearing the offspring. Males do tend to be slightly larger than females, but the difference is slight.

A DNA test is available to determine the sex of a Galápagos penguin, but that requires obtaining a blood sample from each individual—an invasive and time-consuming process.

“We were hoping to find a physical trait to determine sex in Galápagos penguins that would be simple to measure in the field,” Cappello says. “By finding such a sex-specific trait, we could use that in our field studies on whether shifting climactic conditions affect male and female Galápagos penguins differently.”

To search for a physical sign of sex, senior author P. Dee Boersma, a biology professor, collected body measurements from 61 adult Galápagos penguins in the wild—including head size, flipper length, foot length, and a half-dozen measurements of the beak and its surrounding feathers.

Boersma also obtained blood samples from each penguin and sent them to Patty Parker, a professor at the University of Missouri-St. Louis, to determine the sex of each adult using the DNA test.

Cappello combined the body-size measurements and DNA test results into a statistical analysis to search for physical characteristics that correlated with the sex of the bird.

The findings showed that males had slightly thicker beaks—measured from top to bottom—compared to females. Using beak size alone, Cappello could correctly determine the sex of more than 95 percent of the study penguins.

This has been reported for several other penguin species, but Cappello and Boersma are the first to test it by genetic analysis for the Galápagos penguin.

The findings will help researchers study Galápagos penguins in what Boersma calls a “predictably unpredictable” place. Straddling the equator, the Galápagos Islands are perfectly placed to take advantage of an upwelling of Pacific Ocean currents.

The upwelling typically brings nutrients that support small-schooling fish to the islands for the penguins and other marine animals to feast upon, Boersma says.

But El Niño events can disrupt these currents and collapse the Galápagos food web, leading to starvation. This puts a strain on the penguin population, which numbers between 1,500 and 4,700 individuals.

Some penguins mooch off parents after leaving the nest

According to Boersma’s research, in particularly lean El Niño years, Galápagos penguins can even stop breeding altogether. Climate change is expected to increase the occurrence of El Niño conditions and extreme weather events.

Other preliminary studies have suggested that male Galápagos penguins are more likely to survive extreme environmental conditions compared to females. Scientists don’t know why, and studies of sex-specific survival during El Niño years were hampered because researchers lacked easy ways to determine the sex of individual penguins—that is, until now.

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“Now we can start to look at whether climate change will impact male and female Galápagos penguins differently, and what kind of strain this might put on their ability to survive as a species,” Cappello says.

The research, which appears in Endangered Species Research, took place in partnership with the Galápagos National Park. The David and Lucile Packard Foundation, the Galápagos Conservancy, the Disney Conservation Fund, the National Geographic Society, the Leiden Conservation Foundation, the Detroit Zoological Society, the Sacramento Zoo, and the Wadsworth Endowed Chair in Conservation Science at the University of Washington funded the work.

Source: University of Washington