When you want to use a smile GIF, at least one in 10 are of dogs that grin or appear to smile, with their mouths wide open. But do dogs really smile? And if they do, does that mean they’re happy? Maybe, but a lot depends on context, say canine experts.

The dogs in those GIFs really are doing something with their faces. The lips are pulled back. The mouth is open. But what we see, i.e. a smile, and what the dog feels don’t always line up.

Dog faces are wired for expression

Anne Burrows, an anatomist at Duquesne University who studies the comparative anatomy of facial expression in dogs, has studied enough dog and wolf faces to know that the muscles underneath work very differently. Dog facial muscles are mostly fast-twitch—meaning they contract quickly and can produce the fleeting movements we associate with spontaneous expression.

“The faster your muscles are, the more genuine the facial expression is,” Burrows tells Popular Science. “Dogs are our best friends. They tell the truth. We can count on them to let us know how they’re feeling.”

Wolves have more slow-twitch face muscles, Burrows says. Those muscles are better suited to holding the lips in a steady funnel to howl than to flicker through expressions. Dogs aren’t just friendlier wolves. Their faces evolved differently. 

Burrows has noted in a study that dogs’ shift to fast-twitch facial muscles wasn’t accidental. Dogs whose faces moved quickly were easier to read, and over tens of thousands of years, those were the dogs that humans kept around. Wolves signal to wolves. Dogs signal to us. 

But a dog “smile” isn’t always a smile

That doesn’t mean every dogs’ grin-shaped expression means what we think. Karen Jesch, a PhD student at Boston College’s Canine Cognition Center, points out that human smiling is a strange behavior to begin with, and not always a happy one.

“If you look at our closest relative, the chimpanzee, when they pull their cheeks back and part their lips and expose their teeth, that’s usually a fear grimace. It signals that they’re anxious, displaying submission, or ready to fight,” Jesch says. 

Dogs do something similar. A dog might pull its lips back into something that looks like a smile, but that expression might actually mean the dog is uneasy, not happy. A relaxed mouth hanging loosely open is more likely to mean a dog is at ease. But humans tend to read both as smiling.

We’re worse at reading dog faces than we think

Research has shown that humans are not as good at reading dog facial expressions as they assume, Jesch says. Part of that is anthropomorphism, the urge to map our own feelings onto animals.

“We want to assume that dogs love us and are happy. So, we’ll look at a dog running around with other dogs and think, ‘If I were them, I’d be happy. So, they must be smiling.’” Sometimes, she says, that does the dog a disservice when they aren’t actually having a good time.

The classic example is the “guilty look.” A dog cowers when its owner discovers a chewed shoe, and we read shame on its face. But experiments suggest the look isn’t really about guilt. It’s what Jesch calls an “appeasement signal” triggered by the owner’s body language—the dog’s way of trying to defuse the situation before it gets worse. 

“They’re more likely just doing an appeasement signal to say, please don’t be mad at me,” Jesch says. 

Context matters. A “smiling” dog curled up on the couch next to you is probably content, Burrows says. The same expression in a loud, chaotic environment, paired with a tucked tail, might mean the dog wants to leave.

Why dogs have such expressive faces

Burrows favors a version of the domestication story in which dogs domesticated themselves. Ancestral wolves that were less afraid of humans began following hunting parties and scavenging leftovers. The ones better at showing that they meant no harm got closer to the camps.

“There’s some back and forth about how dogs and humans decided to be together,” Burrows says, “but it’s almost like dogs figured out a way to domesticate themselves.” Easier-to-read faces likely gave some dogs an edge—humans grew attached to them and kept them close. 

Dogs don’t communicate just using their faces, however. Burrows points out that dogs rely heavily on their ears, too. Relaxed ears usually mean a dog is approachable; ears pinned flat against the head usually signal fear or stress. (Her lab is now studying the muscles that move dog ears, and how they differ from those of cats, who barely move theirs.) 

Breeds like pugs, with their flattened faces, can be especially hard to read, Jesch adds. Their squashed-in features make subtle expressions harder to interpret than they would be on a Labrador.

So is a smiling dog actually happy?

Sometimes, yes. A relaxed open mouth, soft eyes, a loose body, and a wagging tail in a calm setting probably mean a dog is enjoying itself. The same teeth-baring grin in a tense moment, with a stiff body and pinned ears, is something different, and worth paying attention to.

The good news, Jesch says, is that humans can get better at this. “If everybody who loves dogs were to spend a little bit of time learning how to interpret their signals, I think we could help them live much happier lives.”

So, the next time you fire off the smiling-dog GIF, know that might not be an expression of joy. Certainly, the dog in the image is doing something with its face—what it means depends on what else is happening around it.

In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.

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If your dog stops mid-walk to chew on a patch of lawn, you’ve probably wondered whether something is wrong. Of the delicious food options available to them, why would they choose leafy, bitter grass? Many owners assume the worst: that the dog has an upset stomach and is eating grass to make itself throw up.

Dr. Melissa Bain doesn’t see it that way. “My dog enjoys it every day,” says Bain, a professor of clinical animal behavior at the University of California, Davis. “If we ever mow the grass, [he’ll] go out there and just start chomping on it.” To her, it reads as a snack, not a symptom.

The idea that dogs graze to purge a sick stomach is one of the explanations owners reach for most. But it’s not what the research shows.

Eating grass is normal dog behavior

Grass eating is extremely common. In a 2008 UC Davis study, 79 percent of owners whose dogs had daily access to plants said their dog ate them. A follow-up internet survey of more than 1,500 owners found that 68 percent of dogs grazed daily or weekly, and grass was by far the plant they ate most.

If a behavior turns up in roughly three out of four dogs, it’s hard to call it a sign of illness.

Most dogs don’t get sick from grass

If dogs really ate grass to purge, you’d expect them to look ill first and vomit afterward. Most don’t. 

The same 2008 study found that only about 9 percent of dogs seemed sick before grazing, and only around 22 percent regularly vomited after. 

Diet made no difference either. Whether dogs were fed raw food, kibble, or a vegetarian diet had no bearing on whether they ate grass. 

“There is no nutritional basis for that that we know of,” Bain says of the theory that grazing makes up for something missing in a dog’s food. It’s a normal behavior, she adds, and one she sees mostly in healthy animals.

Her interviews with owners point in the same direction. When Bain asked what a dog was doing right before it ate grass, the dogs that already seemed unwell were the ones more likely to throw up afterward. The dogs that seemed fine usually didn’t. So, when sickness does show up, it tends to come before the grass, not because of it. The vomiting looks like a side effect, not the goal.

A popular version of that idea is that dogs graze to flush intestinal worms out of their gut. But many of the dogs in the survey were on monthly heartworm medication, which also clears intestinal worms—so those dogs had nothing to flush out. They grazed anyway.

They probably just like it

Once you set illness and diet aside, the explanation that’s left is appetite. “Most dogs eat grass because it is a food they enjoy,” says Carlo Siracusa, professor of clinical small animal behavior and welfare at the University of Pennsylvania.

Bain has noticed the same thing. Dogs tend to go for moist, long-stemmed grass, the tender kind that comes up early in the morning. They’re choosing what tastes good to them.

The behavior may be inherited from wild ancestors

Why dogs like grass in the first place is harder to answer. The 2008 study proposed that grazing is a normal behavior, possibly an instinct carried over from wild canid ancestors. 

Bain finds that idea convincing. One ecological version of that idea holds that grass once helped wild canids clear intestinal worms—the fibrous strands wrap around the worms and carry them out in the droppings. Bain points to wild-canid droppings to support this idea: They often hold long strands of plant material, sometimes with parasites tangled in it. But it isn’t proof, she says.

A 2021 study of domestic cats had similar results: Very few cats looked ill before eating plants, and the behavior appeared normal and likely innate rather than a reaction to feeling sick. (Cats did vomit more often than dogs—up to a third of the time—which the authors say may reflect some gastric upset.) Why the instinct exists at all is still an open question.

When it’s worth a second look

Only rarely does grass-eating become a problem, Bain says—when it becomes compulsive. Siracusa says it can turn excessive enough to cause an intestinal obstruction. 

“I have seen this in anxious dogs, but it does not represent the norm,” he says. In nearly three decades of practice, Bain can remember only one dog whose grazing was truly compulsive, and that dog obsessively ate everything, not just plants.

What matters is the pattern. A dog that grazes constantly, looks sick before eating, or vomits regularly afterward is worth a trip to the veterinarian, since the underlying cause may be nausea or another gut problem. It’s also a good idea to keep grass-eating dogs off chemically treated lawns and away from plants that are toxic to dogs.

For most dogs, though, none of that applies. “Most owners should not be concerned if their dog eats grass,” Siracusa says. For a lot of dogs, grass is just the first snack of the day.

In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.

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