Many dog owners have found themselves unable to resist treating a pet that makes "puppy dog eyes" and now scientists have discovered the cute expression taps into our subconscious.
Research by the University of Portsmouth found dogs that raise their inner brows, to make their eyes appear larger, are chosen by prospective owners more quickly than other dogs.
The study suggests that dogs have evolved in response to the human preference towards childlike faces.
In conjunction with the Waltham Centre for Pet Nutrition, researchers used a newly developed tool called DogFACS to analyse dogs' facial expressions.
They studied every muscle movement made by 27 dogs in re-homing shelters when a person came and stood in front of their pen.
The tool counted the number of times the dogs raised their inner brows and widened their eyes as a prospective owner approached.
Dogs that produced more of these movements attracted an owner quicker with their "child-like" expression than those that did not.
Previous studies have repeatedly shown that humans find large eyes appealing - not just in human infants but also in animals - but the research claims to be the first to show that childlike facial expressions play a key role in the current selection of pet dogs.
The study focused on Staffordshore Bull Terriers and Mastiff dogs aged seven months to eight years old, all needing a new home.
"The results of this research suggest that wolves which produced childlike expressions may have been more tolerated by humans, and so modern dogs have inherited these features," said lead author, Dr Bridget Waller, an expert in the evolution of social communication at the University of Portsmouth.
"Our study suggests that dogs' facial movements have evolved in response to a human preference for childlike characteristics.
"In other words, we might have automatically opted for dogs which produced facial movements that enhanced their baby-like faces.
"Raised inner brows are also closely associated with sadness in humans and so another possibility is that humans are responding to a perceived sadness in the dog."
Co-author Dr Juliane Kaminski, an expert in dog cognition who is also at the university, said: "Little is known about the early domestication of wolves and it is likely to have been a complex evolutionary process.
"It is clear that specific physical features were actively selected for as wolves were domesticated to become dogs, but other features may have also been selected for unconsciously.
"The results suggest dogs have evolved childlike facial features which make them more attractive to humans. It is highly likely that these facial expressions do not make a dog a better pet than one that doesn't widen its eyes, but this superficial trait is still preferred over other traits, such as tail wagging."
Dr Sandra McCune, scientific leader of human-animal interaction at Waltham, said: "The research contributes to a growing body of work that is building a clearer picture of the evolution of the bond between humans and dogs."
Dogs have departed from wolves in both their behaviour and physical traits, and in many ways, dogs resemble wolf puppies more than adult wolves.
Previous research assumed this was an accidental by-product of humans actively selecting against aggression, but the new study supports the idea that childlike facial expressions in domestic dogs have arisen as a result of indirect selection by humans.
- DAILY MAIL