> In the games, the Norns have their own simulated biochemistry, haploid genetics, and neural network to serve as a brain. From Creatures 2 onwards, Norns also possess organs determined by their genetics;[31] mutation and injury can render these defunct, with typically fatal consequences. Each of these has grown somewhat over the course of the series, allowing the creatures to become more complex and intelligent. Norns are omnivores and eat a variety of foods, including carrots, lemons, seeds, insects, fish, honey, and other things. The Norns that the player starts with require some degree of training and discipline from the player in order to make good decisions to maintain their health; however, later on they can evolve to be more independent and hardy, with better-developed instincts and a more robust biochemistry. The player can take advantage of this to breed certain colours or traits through artificial selection, and through many generations the changes can become quite pronounced.
Source: From wiki link above.