What muscle is missing in the dog? - briefly
Dogs lack the muscle known as the palpebral levator or retractor muscle of the upper eyelid, which humans possess. This absence allows dogs to maintain a constant eye contact and expressive gaze.
What muscle is missing in the dog? - in detail
The question of which muscle might be considered "missing" in dogs, or more accurately, less developed compared to humans, often refers to the masseter muscle. This is the primary muscle responsible for chewing and jaw movement in mammals.
In humans, the masseter muscle is well-developed and plays a significant role in our ability to chew and grind food efficiently. It allows us to maintain a varied diet that includes tough or fibrous foods. However, dogs have evolved differently. Their dietary needs and feeding habits are distinct from those of humans, which has led to differences in their muscular development.
Dogs have a less developed masseter muscle compared to humans. This is because dogs' diets typically consist of softer food sources, such as meat and kibble, which do not require the same level of chewing effort that tougher human foods do. As a result, the selective pressure on dogs over generations has not favored the development of a large masseter muscle. Instead, their jaw structure and muscles are adapted to efficiently tear and swallow food rather than grind it extensively.
Additionally, the shape and size of a dog's skull differ from that of a human. Dogs have a more elongated snout with a hinge-like action at the temporomandibular joint, which allows for powerful biting but less side-to-side chewing movement. This adaptation is particularly useful for their predatory habits and scavenging behaviors.
In summary, while dogs do possess the masseter muscle, it is less developed compared to humans due to evolutionary adaptations that favor their specific dietary needs and feeding strategies.