London - When dogs look up at you with those big, cute eyes, it’s hard to resist giving them a stroke or treat.
And that’s no accident as the canines have evolved to look more appealing with sad "puppy dog eyes".
Scientific research has revealed that they have developed muscles allowing them to raise an eyebrow and make their eyes seem larger and more child-like. The effect is to give them facial expressions similar to our own, triggering a nurturing response in humans.
Experts said the evolutionary step took place around 33 000 years when our ancestors began to domesticate wolves.
This is considered a rapid change in evolutionary terms and is the only example of an animal whose facial expression has changed through domestication.
It means dogs are able to show more ‘human-like’ eye expressions than even our closest relative in nature, the chimpanzee.
Scientists made the discovery by comparing the anatomy of dogs and wolves, finding that their facial muscles are similar apart from the eyebrow muscles.
Over time, those dogs with cuter expressions would have had more puppies and the feature would have become reinforced, reported the journal Proceedings of the National Academy of Sciences.
Project leader Dr Juliane Kaminski, of the University of Portsmouth, said: "The evidence is compelling that dogs developed a muscle to raise the inner eyebrow after they were domesticated from wolves. It seems to elicit a strong desire in humans to look after them."
Co-author Professor Anne Burrows, at Duquesne University, Pittsburgh, USA, said: "The muscle that allows the eyebrow raise in dogs was in wolves a scant, irregular cluster of fibres. This is a striking difference for species separated only 33 000 years ago."