It turns out that dogs have more "fast-twitch" muscle fibres - which give them greater facial mobility and faster muscle movement than wolves.

It's yet another example of the effects domestication has had on dogs. The US researchers' findings suggest humans have contributed to dogs' ability to form facial expressions through thousands of years of selective breeding.

The research has been presented at the American Association for Anatomy annual meeting during the Experimental Biology (EB) 2022 meeting, which was held from April 2-5 in the US.

"Dogs are unique from other mammals in their reciprocated bond with humans which can be demonstrated though mutual gaze, something we do not observe between humans and other domesticated mammals such as horses or cats," says senior author Anne Burrows, professor in the Department of Physical Therapy at Rangos School of Health Sciences, Duquesne University, US.

"Our preliminary findings provide a deeper understanding of the role facial expressions play in dog-human interactions and communication."

It's estimated that domestic dogs (Canis familiaris) and wolves (Canis lupus) diverged from each other sometime between 27,000 and 40,000 years ago, when humans began domesticating wolves by selectively breeding them for desirable traits.

Mimetic muscles are used to form facial expressions, and in humans these are dominated by fast-twitch myosin muscle fibres that contract and fatigue quickly. This is why we can form facial expressions rapidly, but struggle to hold them for a long time.

On the other hand, muscle cells with more "slow-twitch" fibres are more efficient for long, controlled movements and don't tire as quickly.

The researchers compared the myosin fibres in facial muscle samples from the orbicularis oris muscle and the zygomaticus major muscle (muscles involved in lip movement) from a range of domestic dog breeds and grey wolves.

They did this by staining the samples with antibodies that bind to myosin proteins, to assess the percentages of the different myosin fibre types present in the tissue. They found that domesticated dogs had more fast-twitch fibres than wolves.

Their fast-twitch fibres ranged from 66 per cent to 95 per cent, while wolves averaged about 25 per cent. Slow-twitch fibre percentages in dogs averaged only 10 per cent and wolves averaged 29 per cent.

Having more of these fast-twitch fibres enable small movements - such as a raised eyebrow - and the short, powerful muscle contractions involved in barking. Slow-twitch fibres are important for extended muscle movements, such as when wolves howl.

"These differences suggest that having faster muscle fibres contributes to a dog's ability to communicate effectively with people," Burrows said.

"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."

The researchers believe further research is needed using antibody stains suitable for differentiating even more myosin fiber types, and this could shed new light on the anatomical differences between dogs and wolves.