With their wagging tails and tongues hanging from their mouths, dogs generally seem to have a sunny disposition, but a recent study finds man's best friend can actually be a pessimist.

Researchers at the University of Sydney taught dogs to associate two different sounds with either the preferred reward of milk or simply a drink of water.

The canines were then presented with "ambiguous tones" — sounds they hadn't been taught to associate with any action — to see how they'd respond.

Dogs that responded to the ambiguous tones demonstrated that they expected the sound to lead to a reward. In short, they proved to be optimistic.

"Of the dogs we tested we found more were optimistic than pessimistic but it is too early to say if that is true of the general dog population," Dr. Melissa Starling, who led the study as part of her PhD research, said in a news release.

According to Starling, dogs with optimistic personalities are more likely to take risks in hopes of receiving rewards, and setbacks often don't deter such dogs from taking risks in the future.

Pessimistic dogs, on the other hand, are more cautious and less likely to take risks. When these dogs do take a risk — such as responding to that ambiguous tone — not receiving a rewarding response will be more discouraging.

"Pessimistic dogs appeared to be much more stressed by failing a task than optimistic dogs," Starling said. "They would whine and pace and avoid repeating the task while the optimistic dogs would appear unfazed and continue."

But just because a dog is identified as a pessimistic pooch doesn't mean he's unhappy. He’s simply content with the status quo and may require more encouragement to try new things.

Starling has been working with Assistance Dogs Australia, a charity that trains dogs to help people with disabilities, to determine whether an optimism measure could aid in selecting dogs for training.

"If we knew how optimistic or pessimistic the best candidates for a working role are, we could test dogs' optimism early and identify good candidates for training for that role. A pessimistic dog that avoids risks would be better as a guide dog while an optimistic, persistent dog would be more suited to detecting drugs or explosives."

Starling describes her research in greater detail in the video below.

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