The Hunger Bar displays how much food one of the player's creatures requires in the Creature Stage and the Tribal Stage. The Bar is orange, and appears beneath the health bar. When the Hunger Bar is low, the creature must eat something before it loses health and dies of starvation.
Depending on the creature's diet, (herbivore,carnivore, or omnivore) your creature can eat fruit, meat, or both. Herbivores can eat Fruit, which grows on large trees and small bushes. Carnivores can eat only meat. Any creature can be killed for meat, although dead corpses can also rarely be found in Creature Stage (there is one next to the player's first nest). Omnivores can eat just about anything. Eggs can be eaten by creatures of any diet type. However, this must be a last resort, because if you eat another creature's egg, they will get angry at you and attack, although eggs can also give a reasonable amount of bonus DNA Points.
In the Tribal Stage, the Hunger Bar appears in the squares where the player's tribe members are pictured. Rather than directly eating food, though, tribal creatures must collect the food that fits their diet (now including fish for carnivores and omnivores, and seaweed for herbivores from in any ocean/river) and place it on a food pile at the front of their village. When the creatures are hungry, they will go to the food pile themselves, but the player can also direct them to the food pile by selecting them and right-clicking on the food pile.