Dogs evolved ‘sad eyes’ to trigger nurturing response in their owners

Dogs evolved muscles that give them ‘sad eyes’ and make them look more childlike to trigger a nurturing response in their owners, experts say

  • Scientists say the evolutionary step took around 33,000 years to happen
  • This dates back to when our ancestors first started to domesticate wolves
  • It’s the only example of an animal whose facial expression has changed as a result of domestication

Dogs have evolved muscles around their eyes to look cute to humans, scientific research has shown for the first time.

The muscles allow dogs to raise a quizzical eyebrow and to look sad – giving them facial expressions similar to our own.

The authors say that the eyebrow raising movement triggers a nurturing response in humans. 

They makes the dogs’ eyes appear larger and more child-like.

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The authors say that the eyebrow raising movement triggers a nurturing response in humans. They makes the dogs’ eyes appear larger and more child-like  (stock image) 

Scientists say the evolutionary step took around 33,000 years – dating back from when our ancestors first started 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 as a result of domestication.

It means that dogs are able to show more ‘human-like’ eye expressions than even our closest relative in nature, the chimpanzee.

Scientists made the discovery in comparing the anatomy of dogs and wolves.

Apart from the eyebrow muscles, the facial muscles of dogs and wolves are similar.

Over time, dogs with more appealing expressions would have had more puppies, leading to the feature becoming more pronounced.

Dr Juliane Kaminski, at the University of Portsmouth, who led the research said: ‘The evidence is compelling that dogs developed a muscle to raise the inner eyebrow after they were domesticated from wolves.

‘We also studied dogs’ and wolves’ behaviour, and when exposed to a human for two minutes, dogs raised their inner eyebrows more and at higher intensities than wolves.

‘The findings suggest that expressive eyebrows in dogs may be a result of humans unconscious preferences that influenced selection during domestication.

WHY CAN WOLVES AND DOGS INTERBREED?

Wolves and dogs are interfertile, meaning they can breed and produce viable offspring and these offspring are capable of reproducing themselves.

They are members of a wider group, the genus Canis, containing multiple species such as wolves, coyotes, jackals, dingoes, and dogs

The members of Canus can potentially all potentially interbreed.

Dogs and wolves share an evolutionary past and thus share many physical and behavioural traits.

Dogs evolved from wolves through a centuries-long process of domestication which led to an alteration of the dog’s life cycle and behaviour. 

People who own wolf-dog hybrids often find that their pet’s behaviour makes it a challenge for them to care for. 

The diversity of genetic composition leads to inconsistent behaviour and makes them more difficult to predict.

Wolves and dogs mature at different rates, which makes the physical and mental development of a hybrid animal unpredictable. 

Sexual maturity of wolves signals a shift in hormone quantity and balance. 

This hormonal change is often coupled with these changes in the animal.

‘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.’

Dr Kaminski’s previous research showed dogs moved their eyebrows significantly more when humans were looking at them compared to when they were not looking at them.

She said the eyebrow raising 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.’

The muscle is called the levator anguli oculi medialis. The authors say that dogs are more skilled at using human communication ‘cues’ such as pointing gestures, or gaze direction than our closest living relatives, chimpanzees, as well as wolves or any other domesticated species.

Lead anatomist Professor Anne Burrows, at Duquesne University, Pittsburgh, USA, co-author of the paper, said: ‘To determine whether this eyebrow movement is a result of evolution, we compared the facial anatomy and behaviour of these two species and found the muscle that allows for the eyebrow raise in dogs was, in wolves, a scant, irregular cluster of fibres.

‘The raised inner eyebrow movement in dogs is driven by a muscle which doesn’t consistently exist in their closest living relative, the wolf.

‘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 inter-action with humans.’

Bridget Waller, professor of evolutionary psychology at the University of Portsmouth, said: ‘This movement makes a dogs’ eyes appear larger, giving them a child-like appearance. It could also mimic the facial movement humans make when they’re sad.

‘Our findings show how important faces can be in capturing our attention, and how powerful facial expression can be in social interaction.’

Co-author and anatomist Adam Hartstone-Rose, at North Carolina State University, USA, said: ‘These muscles are so thin that you can literally see through them – and yet the movement that they allow seems to have such a powerful effect that it appears to have been under substantial evolutionary pressure. It is really remarkable that these simple differences in facial expression may have helped define the relationship between early dogs and humans..’

Soft tissue, including muscle, doesn’t tend to survive in the fossil record, making the study of this type of evolution harder.

The only dog species in the study that did not have the muscle was the Siberian husky, which is among more ancient dog breeds, and historically has been bred to pull sleds rather than as a pet.

The full findings of the study were published in the journal Proceedings of the National Academy of Sciences.

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