XAVIER BELLÉS answers:
Blushing is a specifically human characteristic, since, as Charles Darwin showed in his work The expression of emotions in man and animals, our closest relatives, such as the chimpanzee, gorilla, or orangutan, do not blush.
The physiological mechanism by which we turn red with embarrassment is related to the dilation of blood vessels. The more blood flows through them, the more they paint our faces red. What is unclear is why only the face and adjacent areas experience embarrassment, and why situations of anxiety also trigger it. Darwin, in his book, rejects any adaptive value of blushing, citing particularly the fact that people with dark or black skin experience the phenomenon, even though it is not visible.
Xavier Bellés, Institute of Evolutionary Biology (CSIC-UPF, Spain).