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Barking dogs have something to say

The emotion in a dog's bark often seems obvious to us humans. But new research shows just how clear the message can be, at least to other dogs.

The study is the first concrete evidence that dogs can perceive the difference between barks arising from different situations.

While dog barking is hardly as complex as human language, experts now think it's clear that dogs are conveying their feelings to humans and other dogs.

"[Dogs] express basic emotions, and we have not yet found signs for more complex meanings, like 'this is the postman', 'this is the bill collector', 'this is the neighbour', etc," says Hungarian co-author Péter Pongrácz, a professor of animal behaviour at Eötvös Loránd University in Budapest.

"We might call this functionally referential communication, as we are able to tell what kind of situation could elicit a particular kind of barking," he says.

In earlier research, Pongrácz's team found that people could indeed distinguish between different types of barks.

For decades, however, dog experts were stumped about how to show dogs could do the same.

For the new study, published online in the journal Applied Animal Behaviour Science, Pongrácz and his team found a way to do that.

Listen and learn

The researchers first recruited pet dogs of various breeds from training schools, to serve as listeners.

Next, the scientists made recordings of Hungarian mudis (herding dogs) barking in two different situations.

One set of recordings was made when a stranger entered the property where a given dog lived. The second set was made when the dogs were tethered to a tree and left alone.

Two types of mechanical noise, an electric drill and a refrigerator, were control sounds.

The scientists next fitted each of the listening dogs with a heart rate monitor.

While the sound of all dog barks caused a listening dog's heart rate to jump, hearing a certain type of bark consistently over time stabilised the heart rate.

Even though they could get used to the distress barks, the listening dogs always showed a jump in heart rate when the researchers switched from one type of recorded bark to the other.

This evidence for a change in attentiveness shows that not all barks sound the same to other dogs.

The researchers also think it's likely that the dogs understand the different contexts producing the barks they hear.

Barking for each other or humans?

Previously, other researchers thought domesticated dogs barked primarily for our benefit, since neither adult wolves nor feral dogs bark.

"We think barking existed in the ancestor of the dogs, but the present form of variability and abundance of barking is the product of domestication in dogs," Pongrácz says.

He says that domesticated dogs must have learned how to bark to other domesticated dogs later, as a form of communication additional to visual and scent cues.

Anna Taylor, a UK researcher in the Department of Psychology at the University of Sussex, also studies dogs.

"[The new study] gives a convincing demonstration that dogs do indeed perceive acoustic differences between barks recorded in different contexts," she says.

She adds that the findings will inspire future studies to determine how this ability to decipher barks affects dog behaviour.