Researchers from Duquesne University in Pittsburgh found that dogs have similar muscles in their faces to humans, allowing them to form facial expressions close to our own.
'Throughout the domestication process, humans may have bred dogs selectively based on facial expressions that were similar to their own, and over time dog muscles could have evolved to become "faster," further benefiting communication between dogs and humans,' said Professor Anne Burrows, senior author of the study.
Researchers set out to understand why dogs' facial expressions appeal to humans through their study.
'Dogs are unique from other mammals in their reciprocated bond with humans, which can be demonstrated through mutual gaze, something we do not observe between humans and other domesticated mammals such as horses or cats,' Professor Burrows explained.
The team focused on the anatomy of mimetic muscles – tiny muscles in the face that are used to form facial expressions – in both dogs and wolves.
In humans, mimetic muscles are dominated by 'fast-twitch' myosin fibres.
As the name suggests, these fibres contract quickly. However, they also fatigue quickly, which explains why we find it hard to maintain facial expressions for a long time.
In contrast, 'slow-twitch' myosin fibres are slower to contract and don't tire as quickly.
Their analysis revealed that dogs and wolves have facial muscles that are dominated by fast-twitch' myosin fibres like humans.
During a previous study, the team found that dogs have an additional mimetic muscle that contributes to the puppy-dog eye expression, which wolves lack.
These findings suggest that dogs' ability to have 'expressive eyebrows', similar to humans, resulted in an unconscious preference during the domestication of dogs by human breeders.
The study found that when dogs make movements with their eyebrows, it seems to elicit a strong desire from humans to look after them.
This offers a selection advantage over others and ensures the 'puppy dog eyes' trait was bred into future generations of the animal.