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With their tongues out, tails wagging, and hearts full of seemingly unconditional love, it’s easy to assume man’s best friend is happy by nature. But a new study suggests that, just like their human counterparts, certain dogs may see their bowls half empty.

It seems some canines are born optimists, while others are pessimists, according to the research from Dr. Melissa Starling, a faculty member of Veterinary Science at the University of Sydney.

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To determine which dogs were happy-go-lucky and which were generally down in the dumps, the researchers set up a test: First, they taught the study animals to touch a target after hearing a tone associated with a more favorable lactose-free milk reward and refrain from touching the target after hearing a tone associated with plain water. Once the dogs learned the sound associations, researchers presented them with unfamiliar tones to see how they would respond.

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Animals that did not respond to the tones were considered pessimists, while dogs that touched the target after hearing the unknown sounds were categorized as optimists, because they expected a favorable reward. Since optimistic canines hoped for a positive outcome, they’re more likely to take risks and try again—even if the initial result isn’t favorable.

Pessimistic dogs, on the other hand, are more cautious because they’re hardwired to expect a negative outcome. A darker disposition doesn’t necessarily mean those dogs are unhappy though—they’re just less willing to try new things, as failure can be distressing.

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The findings are particularly useful for determining which dogs may be better suited for certain service roles: “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 said in a statement.

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“The remarkable power of this is the opportunity to essentially ask a dog ‘How are you feeling?’ and get an answer,” she said. “It could be used to monitor their welfare in any environment, to assess how effective enrichment activities might be in improving welfare, and pinpoint exactly what a dog finds emotionally distressing.”

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