Nature

04
Jun

When Looks Are More Than Skin Deep

On a grassy slope in New Zealand, Cody Dey and his research assistant wrangle a knee-high bird. Dey grips it carefully in his lap while the other man dips a brush into quick-drying black paint and traces around the fleshy red appendage on the bird’s forehead, shrinking it subtly. They release the bird with its fresh markings, almost like war paint—appropriate, since its station in life has just gone down, and its aggressive peers are going to let it know. How it responds will shed light on a kind of feedback biologists have only recently started to appreciate. They’re learning how deeply a bird’s appearance is tied to its status, and, in a twist, how its status drives its appearance.

pukeko
The pukeko is a New Zealand native that lives in wetlands.

The bird is a pukeko (rhymes with gecko), a mostly land-bound animal with glossy blue-and-black feathers. It has a thick beak, and the soft bit above which just got painted is what scientists call its “frontal shield.” Dey, a PhD student at McMaster University in Canada, discovered that this ornament is a status symbol in pukeko society. When the birds challenge each other for dominance, they often stretch their necks and tilt their faces down to display their shields. Dey found that birds with larger shields tend to be higher on the totem pole.

Ornaments like the pukeko’s shield are common in birds. Some have absurdly long tails, others sport brightly colored patches, UV-reflecting feathers, wattles, and more. These flourishes are not just decorations, but signals to an animal’s peers. As far as we can tell, says Ken Yasukawa, a professor of biology at Beloit College, “only those individuals that can afford the cost can possess a good ornament.” If you’re starving or fighting parasites, for example, you might not have the resources to grow a showy tail. A well-developed ornament may tell an animal’s potential mates that it has good genes or warn its rivals not to start a fight.

Much of what evolutionary biologists know about ornaments comes from manipulating them artificially and seeing how animals respond. Although many kinds of creatures have ornaments—sheep have horns, deer have antlers, and tigers have bright stripes—birds are particularly common research subjects, both because of their often elaborate ornaments and how easy it is for humans to tweak them, says Maren Vitousek, a senior research associate in the Department of Neurobiology and Behavior at Cornell University.

“A lot of the early manipulation studies focused on things like tail length,” Vitousek says; researchers clipped birds’ long tail feathers or glued extensions onto them. But more recently, scientists have realized they can manipulate birds’ colors in a subtler, more varied ways that look entirely natural to the animals. The cosmetic changes may seem minor to humans, but they are meaningful to other birds. Some of what researchers are finding provides a counterpoint to the classic view of what, exactly, an ornament does for the animal, and reveals some surprising similarities between birds and humans.

Preferential Treatment

Though we humans don’t grow elaborate tail feathers, it’s no secret that our looks matter. “Whether we like to admit it or not, our physical appearance impacts our daily lives,” says David Sarwer, a psychology professor at the University of Pennsylvania’s Center for Human Appearance. We perceive attractive people as warmer, more self-confident, and even smarter. “Studies have shown that more attractive individuals often receive preferential treatment” throughout their lives, Sarwer says. For example, in studies that simulate job searches, interviewers have again and again tended to prefer the more physically attractive applicants.

Like the boost a better-looking job applicant gets, cosmetic changes to birds’ ornaments can have far-reaching effects. All it takes is a few art supplies. In one of Vitousek’s recent studies, she used a brown marker to tint female barn swallows’ bellies. She already knew that swallows of both sexes with naturally darker feathers are “sexier,” she says, and have more offspring. But she discovered that females with naturally dark bellies also had lower blood levels of chemicals showing oxidative damage, a common sign of stress or illness.

A week after Vitousek darkened the bellies of naturally lighter birds, she noticed that their oxidative stress had dropped—their blood chemistry now resembled that of naturally darker-bellied birds. She thinks birds with darker feathers may have better social status, leading to less aggression from their peers and lower stress as a result. Darkened birds also had lower testosterone in their blood, perhaps another result of receiving less aggression.

Similar effects may happen in humans, Sarwer says, as we experience different social interactions based on our appearance. “Obviously, healthy and positive social interactions can enhance mood and self-esteem as well as reduce stress,” Sarwer says. Increasingly, research is showing that these effects are physiological, too: social interactions can cause changes in brain functioning, as well as in levels of the stress hormone cortisol.

Communicating Changes

Unlike a human who’s had a makeover, a bird won’t look in a mirror and see the difference. But that doesn’t mean it’s oblivious to its new status. In the 1980s, scientists discovered that female zebra finches are powerfully attracted to males wearing red bands on their legs. Green leg bands, though, are unattractive. These bands somehow tap into females’ innate preferences. Many studies have examined the effect on females, but in 2010, researchers at Macquarie University in Australia instead looked at how leg bands affect the males that are wearing them.

Wild male zebra finches were caught, banded in red or green, and kept in all-male groups for five months. Afterward, males with red bands were in better physical condition than green-banded ones. When they were allowed to court females, the males who’d worn red bands sang the most—a behavior that attracts females.

zebra-finches-on-wires
Male zebra finches who are given red bands on their legs tend to be healthier than those with green bands.

Remarkably, the birds kept up the same song rate even when researchers swapped their leg bands for the opposite color. This showed that a female’s reaction wasn’t what made a red-banded male sing more heartily or a green-banded one stay quiet. Instead, something they’d learned during their months of living with other males had taught them to act either attractively or unattractively. A handful of studies in other species have also shown that changing a bird’s visible ornament can lead to feedback that ultimately affects its song.

Humans aren’t so different, Sarwer says. “Absolutely, the verbal and nonverbal communication that we receive from someone else can go a long way to influencing how we feel about our appearance.” For example, he says, if we notice that someone isn’t engaging us in conversation, we may assume it’s because of how we look—concluding, like the green-banded finch, that we’re not very attractive.

Shrinking Shields

If a bird’s ornament elicits feedback from its peers, and that feedback can change the bird’s behavior and even its physiology, then maybe those physiological changes will manifest themselves visibly on the bird’s body, maybe even change the ornament itself. In fact, there’s one study that’s shown that, Vitousek says: Cody Dey’s painted pukeko.

Dey had already observed that pukeko with larger face shields are naturally dominant. When he painted the birds, he traced a narrow black line around the perimeter of each shield to make it look smaller. Then he returned to his hiding place and observed the pukeko again. He saw that painted birds, with their artificially shrunken shields, received more aggression than others. These “quite minor scuffles,” Dey says, establish dominance among the birds.

After a week, Dey recaptured his subjects and measured their face shields again. To his surprise, underneath the paint, their actual shields had shrunk.

It was a slight change—only about a millimeter. But this difference is meaningful to a pukeko, Dey says. “Basically it gave them the shield size you would expect of an individual who was two or three dominance ranks lower in the group.”

PurpleSwamphen
A pukeko stands along a riverbank.

However, it’s not clear that the painted birds were challenged only because of their smaller-looking shields. Other birds may have been “testing” their status, Dey says, because the painted pukeko behaved in a way that didn’t match their looks. “If something looks weird because some signals and behavior aren’t lining up, maybe you should just challenge [that bird] and try to figure it out directly.”

Scientists have seen this behavior in female paper wasps with artificially painted facial markings, as well as Harris sparrows whose black chest badges were darkened with hair dye. In both species, cosmetically altered animals often received more aggression—but when they were given hormones that matched their new status, their peers gave them a break.

Yasukawa, the Beloit biology professor, thinks the same kind of testing might have happened in one of his own studies of red-winged blackbirds. He brightened male birds’ shoulder patches, called epaulets, with a red art marker, which should have made them more attractive to females. But the birds didn’t get any extra mates—neighboring males wouldn’t let them. He speculates that the mismatch between these males’ looks and their behavior may have made them targets of aggression.

Reinforcing Appearances

All of these interactions with other individuals, Vitousek thinks, is similar to holding up a mirror for some species. “With these repeated interactions, individuals are beginning to sort of get a picture of themselves,” she says. Other studies have supported this idea, she adds, and that may be what happened with her barn swallows that were less stressed after she darkened their bellies. “In the beginning, the manipulated individuals are behaving like lower-quality individuals and are receiving a lot of aggression as a result. But over time, they begin to show the behavior—and in some cases the physiology—of naturally stronger signalers.” In other words, their actions start backing up what their looks are broadcasting to others.

Humans, too, may experience others’ expectations of them as a self-fulfilling prophecy. Mock-interview studies have suggested that interviewers’ early impressions of an applicant can affect how they conduct the interview. And an interviewer’s behavior, in turn, affects how an applicant performs. If the interviewer acts coldly, the applicant may respond to that feedback by doing worse in the interview, like a painted pukeko being put in its place.

Mutability, as in the pukeko’s face shield, isn’t common in the animal kingdom. Most animal ornaments are fixed features such as feathers or antlers. They develop early and don’t change easily. Vitousek points out that this may be a feature of the system: “These signals may be able to convey current information about the bearer by influencing it.” In other words, constant reinforcement of social status by peers may be what keeps an individual’s signals honest throughout its life. If other animals see you as low-status for your signals, they’ll treat you that way; the resulting stress or lack of mating opportunities will make it true.

To learn more about how feedback influences the signals that can change, Dey plans to return to New Zealand and his pukekos. This time, instead of artificially shrinking pukeko face shields, he’ll grow them. “We worked with a makeup artist to design artificial shields,” he says. “We’re hoping to attach them with some sort of adhesive.” Assuming the birds cooperate, of course.

What he finds could further upend our understanding of ornamentation. “The classic view is, something controls signal size, and that influences your social interactions,” Dey says. But what’s emerging is something more complicated: a system where appearance both creates status and is reinforced by it. “There’s this much more dynamic relationship than we might have thought of before.”