You know how it hurts to hold a smile for too long?
Well, that's because, like many human facial expressions, smiles rely on certain tiny facial muscles that can flex fast, but tire quickly.
It now appears that dogs also have lots of these kinds of muscles in their faces too.
And this could be what helps dogs communicate with humans, and why we find them so expressive and appealing, researchers say.
The team, led by Anne Burrows of Duquesne University in Pittsburgh, previously found that fast-twitching muscles above the eyebrows are what enables dogs to put on their famous puppy-dog eyes.
That's the slightly worried vulnerable look that dogs get that makes us want to go, "Awww … so cuute … come and let me look after you."
"Someone described it as a way of hijacking our emotions," Dr Burrows said.
And it certainly works in dog shelters.
In their new study, Dr Burrows and colleagues have looked further afield than the eyebrows and found there are a lot more fast-twitching muscles in dogs' faces.
These muscles also create other facial expressions that get us to pay attention to dogs.
"It's all about the dog-human bond," said Dr Burrows, who presented her preliminary findings at a scientific meeting held in Philadelphia this week.
Dogs evolved muscles for barking
Fast-twitching muscles around the mouth are also important for the short, sharp movements required in barking, which is another tool dogs use to communicate with humans.
Dogs bark to be playful, to protect their territory, and warn us of intruders, Dr Burrows said.
By contrast, wolves, which are essentially the undomesticated ancestors of dogs, tend to howl rather than bark — and it's to communicate with other wolves, not humans.
Dr Burrows found wolves have relatively few fast-twitching facial muscles and more slow-twitching ones.
These are more suitable for howling, which require facial muscles to contract for a long time, she said.
Dogs only tend to howl in response to something like a fire engine or a musical instrument — it's definitely not the norm.
There has been much speculation on when barking evolved, but it is thought to have been as a result of domestication, and may have evolved after the fast-twitching muscles that gave dogs flexibility for facial expressions.
"It was part of the domestication process somehow — whether humans chose dogs consciously that were barking, or whether it was a by-product of domestication," Dr Burrows said.
Interestingly, she added, most of the "ancient dog breeds" don't bark much.
While humans have more fast-twitching facial muscles than wolves — useful for quick smiles and frowns and the like — we have fewer of these muscles than dogs.
Dr Burrows thinks this is because humans use speech, which requires us to hold our mouth in certain shapes for longer periods of time, to form various sounds.
'Dogs' faces are full of expression'
Bradley Smith of Central Queensland University, who studies the cognition and behaviour of canines, said the study was "a great step" in our understanding of how dogs can communicate with us.
"They manipulate us into caring for them, taking them for walks, feeding them treats and all those things."
Dogs have more forward-looking eyes than wolves, which makes it easier for them to make eye contact with humans, Dr Smith said.
And the "mutual gaze" between dog and humans triggers the release of the love drug oxytocin in us, he added.
Australian animal behaviourist Melissa Starling, an honorary affiliate with the University of Sydney, knows all too well the mutual gaze.
"Dogs' faces are full of expression and when I'm working with dogs, I'm always looking at their faces," Dr Starling said.
"I think it's quite likely that even without meaning to, we would selectively breed dogs for communication styles that speak to us."
Dr Starling says the position of a dog's ears or the corners of their mouth can also be important in communication, as is tension that shows on their face.
In some cases, Dr Starling said, dogs try to reduce the intensity of eye contact by giving soft slow blinks to indicate they are not a threat, or showing their front teeth in a "submissive grin".
"It may be the kinds of facial expressions produced by these muscles causes us to engage with animals, because it's showing a mirror of ourselves," she said.
