Nuchal Hump: It may look like this fish swam right into a ledge and got the nastiest bump ever, but that big bump on its head is actually a nuchal hump, also known as a kok. The formation of this forehead bulge is hormonally induced and swells up on male cichlid fish right around mating time. They do rarely appear on females of certain species but are always smaller than the male bumps. Two explanations have been postulated about the purpose of a nuchal hump. (1) They are used for sexual recognition or (2) as fat stores. Either way, now you know if you see a cichlid with a swollen forehead, he is probably feeling the urge to get down.