New research has found that dogs evolved the ability to put on "puppy dog eyes" over thousands of years because their human masters were suckers for it.

The University of Portsmouth study suggests that, since the first attempt to domesticate a wolf over 33,000 ago, the animals have slowly been evolving facial expressions to help them communicate with humans.

By comparing the anatomy of dogs and wolves, the researchers found that their facial muscles are nearly identical, except for one trait - almost all dogs have evolved a muscle that allows them to raise the inner eyebrow intensely, something that wolves can't do.

File photo of therapy dog Lexy, 18-02-2014. Image: AP Photo/Alex Sanz, File

An analysis of dog behaviour also found that dogs move their eyebrows far more often and more intensely than wolves.

The researchers said the ability to lift the levator anguli oculi medialis muscle makes the dogs' eyes "appear larger and more infant like" while also resembling a "movement humans produce when they are sad."

A three-week-old Chihuahua puppy named Tom Thumb in a tea cup, 08-04-2009. Image: Andrew Milligan/PA Archive/PA Images

They said that this may trigger a nurturing response in humans - giving those that put on their "puppy dog eyes" a selection advantage over others.

As a result, the study suggests that dogs' almost universal ability to produce the expression is the result of thousands of years of evolution by human selection.

Alfred Furtado. Image: Amanda Ximenes Furtado