Do hippos sweat their blood and what causes this?

Hippos don’t actually sweat blood, but this erroneous belief is based on an oddity that bears mentioning.

A hippopotamus has no oil or sweat glands on its entire body; it does, however, sport a gland that only kicks into action when it gets excited or nervous.

This gland excretes a reddish, oozing liquid that is often mistaken for blood and coincidentally may function, in part, to scare off attackers.