Making a More Perfect Penguin

A long-term study shows the subtle hand of natural selection on Argentina’s Magellanic penguins.

Published November 29, 2016

If you ask the internet, penguins are pretty much perfect as they are: cute, curious, and clumsy. But the truth is, the “perfect” penguin might not always take the same form.

Since 1983, biologist P. Dee Boersma has been studying the world’s largest colony of Magellanic penguins at Punta Tombo, Argentina, and has found that as environmental conditions shift, the penguins show a resilience to those changes. Boersma and her colleagues have measured a lot of penguins. And they’ve been tracking whether and how everything—from their bills to their flippers to their feet—has been changing over time.

“It’s kind of like a big general census,” says Laura Koehn, a doctoral candidate in fisheries science who worked with Boersma’s team as an undergraduate.

By marking penguins with ID bands, the researchers tracked how successful each was at raising its young, which traits were passed down through the generations, and whether certain parental traits, such as a longer bill or shorter feet, led to a boost in survival for a penguin’s offspring.

In general, Koehn says, they weren’t able to detect any evidence of modern penguin evolution—yet. The penguins aren’t all getting bigger, for instance. But the team did find that, sometimes, being a certain shape or size has perks, and that these traits are passed down, which paves the way for evolution. Since different body shapes provide different benefits, what could be considered the “ideal” penguin for the conditions in any given year is fickle and variable, and often reverses itself over time. But overall, the takeaway is clear: even when conditions in their highly dynamic habitat aren’t the best, at least some Magellanic penguins will have what it takes to succeed.

Boersma’s team found evidence of natural selection—that is, when penguins of a particular size or shape raised chicks more successfully than their peers—in seven of the 28 years examined. In those years, they found that the trends for males and females seemed to be headed in different directions. Furthermore, the timing didn’t always overlap.

In years when food seemed scarce and more than half the chicks died of starvation, successful male dads tended to have larger bodies and bills, perhaps because they could catch larger prey. In years when food appeared abundant, males of all sizes raised chicks equally well.

In some years, female penguins with more petite bodies, or just with shorter feet or smaller bills, had chicks which survived better. In other years, the opposite was true.

Detecting even this small signal of ongoing natural selection is impressive, says biologist Mary Bomberger Brown, especially in a bird that splits its time between land and sea. It requires precisely tracking and measuring large numbers of individuals over long periods of time. Even then, she says, “the differences aren’t going to be gigantic. A penguin flipper isn’t going to be four inches [10 centimeters] longer one year than it was the previous year.”

“Overall, nobody’s shrinking or getting bigger,” emphasizes Koehn. Instead, these year-to-year fluctuations tend to wash out over time. That suggests, says Koehn, that whatever factors are driving these changes—whether climate, food, or even behavior—are relatively stable over time.

But if that stability is upset, the penguins have shown they have the capacity within their population to survive. “When they are pushed, they do respond,” says Brown.

Just how far and how fast that flexibility can be pushed, however, is uncertain.