One reason I love (working) spaniels and can’t imagine having any other kind of dog is how expressive their eyes and face are.
Based on what I’ve read recently, I think it’s due to more finely developed facial muscles (e.g. eyebrows) – which in itself could be because they are bred to be a close working companion to hunter, both reading human expressions and expressing their own.
My hypothesis is based on research that suggests humans unknowingly selected for paedomorphic (childlike) features when domesticating dogs, a selection that was also based on behavioural characteristics . Unlike wolves, their closest relatives, dogs have developed a small facial muscle allowing them to raise their inner eyebrow which is believed to have developed to ease communication with humans – and even to heighten their bond with us .
It is hypothesized that dogs evolved this particular musculature trait as a way to selectively enhance their bond with humans who, either consciously or unconsciously, exhibited a preference for specific animals with these facial movements. 
Additionally, the researchers hypothesised that “this movement increases paedomorphism and resembles an expression humans produce when sad, so its production in dogs may trigger a nurturing response” . This muscle is also responsible for creating the classic “puppy eyes look” that spaniels in particular have perfected. Picture below shows the muscle in action:
Although there is no breed-specific research on this, it would make sense that, as a side effect of selecting the best hunting dogs for breeding, we may have also unconsciously selected for other features such very expressive faces if they are an advantage in the work.
Another feature is that most spaniels show a lot more sclera (the white part) which is important in signalling where someone is looking. Dogs use this information in humans to follow their gaze, and it would make sense that we also prefer dogs where we can see their gaze, especially if it helps to work with them. Again, this might not have been deliberate but just an unconscious byproduct of the selection process.
While I can read my own dogs really easily, I find it harder to read dogs with dark eyes and little sclera. The other bird gundogs like pointers and setters – they’re originally from the same spaniel family – also tend to have quite expressive faces, but I have a feeling that the close range gundogs have more of this vs. the independent hunters. Of course, spaniels aren’t alone in this and many other breeds also have highly expressive faces as well – but this is a spaniel blog and I’m spaniel enthusiast so that’s what I focus on 🙂
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