The sad, imploring expression held such power over humans during 33,000 years of canine domestication that the preference for dogs that could pull off the look steered the evolution of their facial muscles, researchers have said.The result is that dogs gradually acquired a new forehead muscle named the levator anguli oculi medialis, or LAOM, and have used it to deploy the doleful look to devastating effect ever since.
“They are very powerful animals in how they capture our hearts,” said Prof Bridget Waller, the director of the Centre for Comparative and Evolutionary Psychology at the University of Portsmouth. “We pay a lot of attention to faces , they are meaningful to us, and this expression makes dogs look juvenile and sad. It induces a nurturing response. It’s a cute factor.”
Puppy dog eyes are achieved by the LAOM raising the inner eyebrows, in some cases quite dramatically. The movement makes the eyes look larger and the face more babyish. Humans use different muscles to produce a similar expression when they are sad, which may explain why it brings out the caregiver in people.
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To investigate how the look developed in dogs, the UK-US research team acquired wolf and dog cadavers from taxidermists and US state organisations and dissected their heads to compare the facial muscles. No animals were killed for the research.Dissections of six dogs – a chihuahua, a labrador, a bloodhound, a German shepherd, a Siberian husky and a mongrel – found all had the LAOM muscle. But in the four grey wolves studied, the muscle was missing, save for a few scant muscle fibres. Since all dogs are derived from wolves, the comparison suggests the LAOM arose in the domestication process.
Only one other difference was noted in the head dissections. A muscle called the retractor anguli oculi lateralis (RAOL), which pulls the eyelids out towards the ears, was less prominent in wolves than dogs. The Siberian husky, one of the most ancient breeds, was the only dog found to lack the RAOL muscle, according to the report in the journal Proceedings of the National Academy of Sciences. After establishing that dogs and wolves have different muscles around the eyes, the researchers filmed the animals to see how their expressions varied. They filmed nine wolves in two different animal parks, and 27 dogs, mostly Staffordshire bull terriers, in shelters across the UK. The footage was reviewed by a trained specialist who was not told about the scientists’ hypothesis. The specialist recorded when the animals made the puppy dog eyes expression, and rated its intensity on a five-point scale.
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Dogs pulled the doleful face far more frequently than wolves, but the most striking finding was the intensity of the expressions. While dogs and wolves both produced “low intensity” expressions, only dogs appear to have weaponised the look and achieved what the scientists classified as “high intensity expressions”.The look has a real impact. In a previous study, Waller showed that the more dogs deployed the expression, the faster they were rehomed from shelters. In that regard, puppy dog eyes were more effective than tail wagging or the speed at which dogs bounded over to visiting humans.
Waller does not believe dogs originally produced the expression to win humans over. More likely, she said, is that animals that happened to deploy puppy dog eyes tapped into a response humans had evolved over millennia of living in large groups, where reading facial expressions was crucial.