Have you noticed the phenomenon of puppy dog eyes, the inner eyebrow raising movement of dogs? Do you believe it a result of ＿＿（41）＿＿ for dogs to appeal to humans? Researchers dissected dog and wolf heads and found that the facial muscle anatomy of both animals was almost identical -- ＿＿（42）＿＿ an eyebrow muscle, found only in dogs. The inner eyebrow raising movement makes the dogs' eyes appear larger and more infant like, ＿＿（43）＿＿ prompts a "nurturing" response in humans. When dogs make the movement, it seems to elicit a strong desire in humans to ＿＿（44）＿＿ them. This would give the dogs that moved their eyebrows more a selection advantage over others, reinforcing the trait in ＿＿（45）＿＿ generations. The findings show how important faces can be in capturing our attention, and how powerful facial expression can be in social interaction.