Vestigial muscles try to pivot your ears like a dog or cat does.

Your Vestigial Muscles Try to Pivot Your Ears Just Like a Dog’s

Your Vestigial Muscles Try to Pivot Your Ears Just Like a Dog’s

The state of the universe.
Oct. 16 2015 1:01 PM

Your Vestigial Muscles Try to Pivot Your Ears Like a Cat’s

We are a glorious mish-mash of evolutionary history.

humans have vestigial ears.

Photo illustration by Juliana Jiménez. Photo by Hiroaki Shibata/Thinkstock.

Pet owners, you know the look: Your cat hears you pouring the Purina, and suddenly she’s all ears. As she stops in her tracks and turns to face you, her ears swivel straight toward the sound of all those salmon-flavored bits cascading into the bowl. This is your cat’s listening face, all senses trained on the all-important event happening before her. Feed me. Right. Meow.

It turns out that humans may do the very same thing! Well, kind of. Remarkably, our brains still retain the ancient neural circuitry that let our mammalian ancestors control the movement of their ears. Steve Hackley, a cognitive neuroscientist at the University of Missouri, reviews the evidence in a paper in the journal Psychophysiology. The existence of this “evolutionary fossil” is a reminder that humans evolved from species that utilized their ears more fully to assess important, loud, or startling noises, and to express emotions like fear and rage.

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While people have studied the vestigial muscles of the pinnae—a fancy word for ears—since the early 1900s, no one had yet pulled the information together to come up with a cohesive explanation, says Rickye Heffner, a psychologist at the University of Toledo who specializes in the evolution of hearing. The analysis was “clever and convincing,” says Heffner, who was not involved in the study.

To understand our ear-orienting abilities, Hackley combed through more than 60 published studies on vestigial ear muscles. When humans were startled with an unexpected sound behind them, the studies revealed, muscles behind the corresponding ear twitched to attention. In other studies, shifting the gaze to the right or left triggered a subtle curling of the ears. Finally, when people were asked to complete a reading task but were distracted with the sound of typing, sawing, or bird song, researchers found bursts of ear muscle activity.

Of course, modern human ears are firm and their muscles are weak, so no amount of muscle activity was likely to move them. But that didn’t stop our ears from trying! The subjects’ ear muscles rose to attention, as if they were “unconsciously trying to orient their ears toward the relevant sounds,” Hackley wrote.

Aside from a few modern people who have the uncanny ability to wiggle their ears, “such attempts are in vain,” he added. So the question is: Why do we still have brain space devoted to it?

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To answer that question, we have to take a quick detour into evolution. In short: You are a living fossil. (I don’t mean that to be rude.) While evolution selects for traits that are useful and allows the ones that are not to drift away, it’s hardly efficient about the process. As it tries out various features only to discard them, it leaves countless traces behind. And so you contain the evolutionary remnants of millennia, from your useless tailbone to your unnecessary appendix to your disease-prone tonsils to wisdom teeth to the way your skin goose bumps in chilly weather (it still thinks it has fur to keep you warm).

We call these remnants “vestigial structures.” And the system for controlling our ears is doubtless among them. Recently, many supposedly useless vestigial structures—like the appendix and the spleen—have been shown to actually not be so useless. But the ear-orientation circuit appears to be one of the clearest examples we have of a completely unnecessary feature left behind by evolution, say Hackley and Heffner. “The brain is astonishingly expensive,” Hackley says. “The fact that it would keep doing something like this, which is useless—that’s pretty surprising.”

The existence of ear-orienting neural circuits adds to the story of how we evolved from species whose ears could pivot and curl. The story goes something like this: Early mammals probably developed ear-orienting abilities more than 150 million years ago. When our primate ancestors became diurnal, we began to rely more on vision than sound to pinpoint important stimuli, says Heffner. So while cats and the amazing fennec foxes and more distantly related mammals like marsupials kept their ability to orient their ears—think of big-eared cuties like koalas, lemurs, bush-babies—apes and humans lost the skill. Yet the circuits for ear-orientation remained intact, buried in your nervous system just waiting for scientists to find.

Part of the study’s strength stemmed from the fact that Hackley compared our ears to that of an array of other species, including bats, foxes, cats, and dogs. “So many scientists tend to focus on a single species,” says Heffner. “You can gain a lot of perspective on humans by comparing what we have and what we do with what other animals do. If you don’t know what’s out there, you don’t know what’s so special about us.”

Rachel E. Gross is the science web editor at Smithsonian.