New Study Says Humans Are , Responsible for Puppy Dog Eyes.br Are you head over heels for the br sweet and soulful "puppy dog" br eyes looking up at you?.br Naturally, many of us are suckers br for our begging pups, but dogs weren't always so irresistibly sweet.br A recent study has found that "puppy dog" eyes are a result of breeding that started nearly 33,000 years ago.br Dogs are unique from other mammals in their reciprocated br bond with humans which br can be demonstrated br through mutual gaze ... , Anne Burrows, professor Rangos School of br Health Sciences at Duquesne University, via CNN.br ... something we do not observe between humans and other domesticated mammals such as horses or cats. , Anne Burrows, professor Rangos School of br Health Sciences at Duquesne University, via CNN.br Throughout the domestication process, humans may have br bred dogs selectively based br on facial expressions that br were similar to their own. , Anne Burrows, professor Rangos School of br Health Sciences at Duquesne University, via CNN.br Throughout the domestication process, humans may have br bred dogs selectively based br on facial expressions that br were similar to their own. , Anne Burrows, professor Rangos School of br Health Sciences at Duquesne University, via CNN.br Dogs have more "fast-twitch" facial muscles than their genetic cousin, the wolf.br Dogs have more "fast-twitch" facial muscles than their genetic cousin, the wolf.br Researchers say these muscles allow them to mimic the facial expressions of humans.br Over time dog muscles could have evolved to become 'faster,' further benefiting communication between dogs and humans. , Anne Burrows, professor Rangos School of br Health Sciences at Duquesne University, via CNN.br Presented at the American Association for Anatomy's annual meeting, the study analyzed the facial muscles of domesticated dogs and wolves.br Researchers say they've found br wolves have lower percentages of br fast-versus slow-twitch fibers.br Slow-twitch fibers in the face of br wolves are beneficial to their howl.br Fast-twitch fibers in the face of domesticated dogs benefit communication with humans via short, fast barks and varied facial expressions.br These differences suggest br that having faster muscle br fibers contributes to a dog's br ability to communicate br effectively with people.