Vocal learning, in which animals modify their vocalizations to imitate those of others, has evolved independently in scattered lineages of birds and mammals. Comparative evidence supports two hypotheses for the selective advantages leading to the origin of vocal learning. The sexual selection hypothesis proposes that vocal learning evolves to allow expansion of vocal repertoires in response to mating preferences for more complex vocalizations. The information-sharing hypothesis also proposes that vocal learning evolves to allow expansion of vocal repertoires, but in this case in response to kin selection favoring sharing of information among relatives.