A hippo's mouth may be 2 feet wide at its lip. The mouth of an adult hippo is roomy enough for 36 teeth, including tusks that can grow up to 2 feet long. Incisors and bottom incisors thrust vertically to the front of the mouth to stab foes. More commonly, male hippos size one another up by opening their mouth up to 150 degrees, stretching it to its full 4 to 5 feet length.