Dog owners can tell their animal companions are not the same as they were when they were puppies, but they cannot tell exactly what personality has changes as they grew older.

A recent study published on Wednesday in Scientific Reports looks into the dog’s personality and find some personality traits that changes or not over the course of their lives.

“Similar to humans, dog personality is both stable and malleable,” Borbala Turcsan, study’s lead author and research fellow at Eotvos Lorand University in Budapest, Hungary, told NBC News. “Dogs that are active and curious when young will remain active and curious when they get old, but only compared to other dogs. A dog’s personality changes over time, and, on average, every dog becomes less active and less curious as they age.”

The research team invited 217 border collies from six months to 15 years old. The dogs were evaluated using a series of tests or commonly known as the Vienna Dog Personality Test. Four years later, the researchers invited the dogs who were still alive, back to the lab for retesting. Out of the 217, 37 dogs and their owners showed up.

The tests include, exploration test (where the dogs were allowed to explore a room and the different objects for one minute, while the owner stood in the middle of the room ignoring the dog); frustration test (a large piece of sausage was swung on a string in front of the dog’s nose, just out of reach, for a minute); novel object test (the dog encountered a self-moving toy that made a sound for a minute); ball playing test (the owner threw a tennis ball thrice and encourage the dog to retrieve it); obedience test (owner gave the dog four basic commands – sit, lie down, stay, and come, while experimenter tries to distract the dog with rustling noises); and the problem-solving test (owner showed the dog how to remove the lid of bin to get a piece of sausage from it, and the dog had to to din in a minute).

As a result, the researchers found that the most active and curious ones in the first test were still the most active and curious ones four years later, but they were less active and curious.

The researchers also figured that the dog’s attentiveness and ability to solve problems improved up until they were six years old, and remained stable. The novelty-seeking trait did not change much in the dogs’ early life, but when they turned three, their curiosity about novel objects started to decline.

“Something that was surprising to me is that dogs don’t seem to get particularly more tolerant of frustration as they get older,” Dr. Katherine Houpt, professor emeritus at the Cornell University College of Veterinary Medicine, told NBC News.

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