When we raise our eyebrows, our hairline also tends to lower. As you can see in the video above, this hairline drag does not just occur in humans!

The primary brow elevator for humans and chimpanzees is the frontalis. In both species, frontalis originates near the eyebrows and inserts upwards onto a tendinous sheet (galea aponeurotica) along the scalp. Because frontalis has no bony attachments, it has no stable anchor points when it exerts force onto the eyebrows. This lack of tethering puts strain on the scalp during frontalis contractions, causing the skin above the frontalis to drag downward as a secondary effect. 

The degree to which the hairline is dragged downward varies from person to person and chimpanzee to chimpanzee. Humans and chimpanzees have individual differences in muscle strength and facial structure. There is also an important interplay between frontalis and occipitalis that influences the degree or hairline drag. Learn more about frontalis and occipitalis relationships and interactions in “Forehead Dynamics.”