Abstract
Domestication shaped wolves into dogs and transformed both their behavior and their anatomy. Here we show that, in only 33,000 y, domestication transformed the facial muscle anatomy of dogs specifically for facial communication with humans. Based on dissections of dog and wolf heads, we show that the levator anguli oculi medialis, a muscle responsible for raising the inner eyebrow intensely, is uniformly present in dogs but not in wolves. Behavioral data, collected from dogs and wolves, show that dogs produce the eyebrow movement significantly more often and with higher intensity than wolves do, with highest-intensity movements produced exclusively by dogs. Interestingly, this movement increases paedomorphism and resembles an expression that humans produce when sad, so its production in dogs may trigger a nurturing response in humans. We hypothesize that dogs with expressive eyebrows had a selection advantage and that "puppy dog eyes" are the result of selection based on humans' preferences.
Highlights
Domestication shaped wolves into dogs and transformed both their behavior and their anatomy
Nagasawa et al [13] showed that, between dogs and humans, mutual gaze seems to lead to an oxytocin feedback loop analogous to the one that exists between human mothers and infants
While the levator anguli oculi medialis muscle (LAOM) was routinely present in dogs, in the gray wolves, it was typically represented only by scant muscle fibers surrounded by a high quantity of connective tissue
Summary
Domestication shaped wolves into dogs and transformed both their behavior and their anatomy. Mutual gaze between dogs and humans seems to trigger an increase of oxytocin in both species, which increases the motivation to establish eye contact [13] As this cross-species oxytocin loop can be found in dogs and humans, but not between dogs’ closest living relative (the wolf) and humans, selection processes during domestication must have played an important role whereby dogs hijacked the human caregiving response [15]. The movement makes the eyes appear bigger, more infant-like and potentially more appealing to humans This inner brow raise resembles a facial movement humans produce when they are sad, potentially eliciting a nurturing response from humans [17, 18]. The study showed that dogs that produce this facial movement more were rehomed
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