Gestures of humility in social mammals
Dogs and wolves draw in their tails and whine when they are inferior or weaker than an attacking dog/wolf. Some predatory animals present their unprotected throats to superior/stronger animals of the same species. As far as I know, primates do not show such behavior, Humans blush when they are ashamed, no other primate species does that. Turning pink is regulated by the autonomic nervous system, but there is no explanation, as far as I know, about its origin in evolution. It could be a gesture of ritualized simulated exhaustion.