Masters of manipulation: The evolution of 'puppy dog eyes'          

Masters of manipulation: The evolution of 'puppy dog eyes'          

Masters of manipulation: The evolution of puppy dog eyes

Ever wondered how dogs learned to use their "puppy eyes" to bend us to their will? It turns out our pet pooches have evolved human-like eyebrow muscles, which let them make the sad faces that melt our hearts, according to a new study published on Monday in the Proceedings of the National Academy of Sciences (PNAS).

It involved dissecting the cadavers of domestic dogs and comparing them to those of wild wolves, our best friends' ancestors, whom they branched off from around 33,000 years ago.

A separate part of the study saw scientists videotaping two-minute interactions between dogs and a human stranger, then repeating the experiment with wolves, to closely track how much they used a specific muscle around the eye that produced an inner eyebrow raise.

The researchers found two muscles around the eye were routinely present and well formed in the domestic dogs, but not the wolves, and only dogs produced high-intensity eyebrow movements as they gazed at the human.

"It makes the eye look larger, which is similar to human infants," Professor Anne Burrows of Duquesne University in Pittsburgh, who was one of the co-authors, said. "It triggers a nurturing response in people."

Since the muscles were robust in the dogs but not wolves, "that tells us that that muscle and its function are selected," she added.