Life-forms transmit information vertically (from generation to generation) via replication of genes. Memes can replicate vertically or horizontally within a single biological generation. They may also lie dormant for long periods of time. Memes spread by the behaviours that they generate in their hosts. Imitation counts as an important characteristic in the propagation of memes. Researchers have observed memetic copying in just a few species on Earth, including hominids, dolphins and birds (which learn how to sing by imitating their parents or neighbours).
Some commentators have likened the transmission of memes to the spread of contagions. Social contagions such as fads, hysterias and copycat suicides exemplify memes seen as the contagious imitation of ideas. Observers distinguish the contagious imitation of memes from instinctively contagious phenomenon such as yawning and laughing, which they consider innate (rather than socially learned) behaviours.