Dogs were played the sound of humans crying. Their reaction shows a unique emotional bond with us that other animals lack.

• Dogs have developed a special kind of emotional bond with humans over millennia.
• They connect with us in ways that other domestic animals haven't, per a new study.
• That's why dogs bring us so much joy, one expert said.

Pet owners worldwide came together for a scientific experiment proving how emotionally bonded dogs are to us.

For the experiment, scientists asked the owners to play separate recordings: one of a human crying and another of a human humming. The owners then watched and video-recorded their dog's reactions.

When the 30 dogs (both purebreds and mixed breeds) in the study heard sad human cries, they would start to scratch, shake, lift a paw, lick their mouths, and some even whined — behavior that suggested the animals were stressed or agitated, the scientists reported in a paper published in the peer-reviewed journal Animal Behavior .

However, when the dogs heard the neutral humming, they were less responsive and didn't seem so concerned.

Dogs of all shapes , sizes, and ages, from golden retrievers and boxers to beagles, labs, and Border Collies — all between 2 to 11 years old — participated in the study. And their different reactions prove an important point about our canine companions.

Dogs understand us on a deeper level

The dogs in the study exhibited what scientists call emotional contagion , which is a person's (or animal's) unconscious tendency to mimic another's emotions. Study co-author Paula Pérez Fraga, a researcher for Eötvös Loránd University's Department of Ethology, said it's related to empathy.

Typically, emotional contagion exists within the same species because it's how animals bond and communicate . Dogs, however, have developed this social tactic with an entirely different species: humans.

Vanessa Woods, who wasn't involved with the study, has worked with hundreds of dogs as director of Duke University's Puppy Kindergarten, which studies puppy traits that lead to the best companions as adults.

She's witnessed dogs' emotional contagion firsthand many times.

"They just know when I need a hug, or they know when I'm stressed out," Woods told Business Insider, adding, "The emotional connection that we have with our dogs is real , and that's why they bring so much joy."

Of course, the level of emotional contagion in dogs varies. Some have more than others, Woods pointed out. But, in general, there's a deep connection there .

The researchers of this new study were trying to determine whether this connection existed among other domesticated animals or if dogs are — indeed — special.

"Our main idea was that we asked ourselves if this was something special from dogs or if this kind of emotional contagion can be shared among other animals," Fraga said.

Fraga and her colleagues played the same crying and humming sounds to 22 house pigs to find out . The pigs responded very differently.

How pigs responded differently to human crying than dogs

The pigs didn't appear to mind the crying sounds and instead seemed more agitated by the humming.

However, this doesn't mean pigs are dumb or indifferent, Fraga said. Fraga added that humming is an unusual sound that the pigs likely weren't familiar with and didn't understand.

"What we suggest is that for the pigs, this humming was a highly novel vocalization. So this is why they might be more stressed after hearing this humming," Fraga said.

Fraga added that the findings clearly indicate dogs' strong connection with humans. They "have been with us for more time than any other domestic species, and they have been selected for cooperation and dependency on us," Fraga said.

She added that more research is needed to determine whether pigs or other domesticated animals share that same bond.

In the meantime, it's probably safe to say that your pooch is pretty special .

"I love pigs, but I think it just shows how specialized dogs are," Woods said of the study, adding, "They've really integrated into our world and into our lives in a way that other domesticated animals haven't."