The next time a dog looks up at you with those big, soppy eyes and your heart begins to melt, be aware: you are being manipulated by 33,000 years of evolution.
That is the conclusion of a new scientific study which argues that the raised eyebrows so beloved of owners are more of a survival strategy than an expression of friendship.
The clues, its authors argue, lie in a comparison with wolves, which dogs began evolving from after they were first domesticated by Stone Age man.
Anatomical analysis reveals that while modern dogs have developed small muscles around the eyes which allows them to raise their inner eyebrows, wolves possess hardly any comparable muscles.
Such a stark diversion in what in evolutionary terms is a short period of time has led the researchers to believe that humans naturally favour canines able to look at them with big “puppy dog” eyes, giving them a selection advantage.
Their findings complement previous studies which suggest that big eyes trigger a caregiving response in humans because it reminds them of babies.
Scientists already know that dogs’ ability to read human behaviour and emotion is almost unique within the animal kingdom, and there is growing evidence that eye contact is crucial to this.
Dr Juliane Kaminski, who led the research at the University of Portsmouth, said: “The findings suggest that expressive eyebrows in dogs may be a result of humans' unconscious preferences that influenced selection during domestication.
“When dogs make the movement, it seems to elicit a strong desire in humans to look after them.
“This would give dogs, that move their eyebrows more, a selection advantage over others and reinforce the ‘puppy dog eyes’ trait for future generations.”
The researchers dissected the facial anatomy of four wild grey wolves and six domestic dogs, focusing on a region of muscle they called Action Unit 101.
They found stark differences in muscle fibre concentration between the species.
The team also compared the behaviour of live wolves and dogs when exposed to humans for two minutes, finding that the dogs raised their inner eyebrows more and at a higher intensity than wolves.
“This is a striking difference for species separated only 33,000 years ago and we think that the remarkably fast facial muscular changes can be directly linked to dogs’ enhanced social interaction with humans,” said co-author Professor Anne Burrows, who led the anatomical research at Duquesne University, Pittsburgh.
A study in 2015 showed that a shared gaze between humans and dogs results in a hormonal response in both parties similar to that between a mother and baby.
Two years earlier, scientists proved rescue dogs were more likely to find an owner if they raised their inner eyebrows.
Surveys have also shown that humans tend to favour dogs with infant-like characteristics such as a large forehead, as well as eyes.
“The AU101 movement is significant in the human-dog bond because it might elicit a
caring response from humans but also might create the illusion of human-like communication,” said Dr Kaminski.
The new research is published in the Proceedings of the National Academy of Sciences.