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Dogs’ eyes appear wider and more infant-like than those of wolves
Dogs have evolved new muscles around their eyes in order to better communicate with people, researchers have found.
Led by comparative psychologist Juliane Kaminski at the University of Portsmouth, researchers found the AU101 movement – a term for ‘inner eyebrow raise’ – was present in dogs but not in wolves, suggesting that dogs with more expressive eyebrows might have had a selection advantage at breeding.
The study suggests the change in dogs’ facial anatomy, specifically the muscles involved in raising their eyebrows, has allowed them to communicate better with people and has played a role in the human-animal bond between the two species.
Researchers examined differences in both species through facial dissections and through recording the movements of live animals (nine wolves and 27 domestic dogs) individually for two minutes.
They found that the levator anguli oculi medialis (LAOM) muscle, responsible for high intensity eyebrow movement, was present in all of the dogs, compared to only a few muscle fibres being found in the group of wolves. This suggests that wolves are less able to raise their eyebrows to the intensity of domestic dogs as a result of this anatomical difference.
The team hypothesised that the movements observed resembled a sad human expression, typical of ‘puppy dog eyes’. The high intensity eyebrow movement observed in dogs made them wider and more infant-like, potentially invoking a care-giving response from people and suggesting a selection advantage for animals demonstrating those movements.
These findings are supported by previous research showing that dogs demonstrating more facial movements are likely to be rehomed from rescue shelters more quickly.
Co-author and anatomist Adam Hartstone-Rose 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.’
Currently, there is no evidence to suggest that dogs raising their eyebrows is done intentionally in attempts to manipulate people; however, it is determined that it is a voluntary movement demonstrated particularly when being under the observation of people. Only dogs and horses have been found to produce AU101 movements; all other non-human animals, including cats and chimpanzees, do not demonstrate such movements.
Kaminski, whose focus of research is in dog cognition, explained: ‘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 research is published in the journal PNAS and can be found at: www.nature.com/articles/s41598-017-12781-x •
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