A Castrato is a male soprano, mezzo-soprano, or alto voice produced either by castration of the singer before puberty or one who, because of an endocrinological condition, never reaches sexual maturity. This prevents a boy’s larynx from being transformed by the normal physiological events of puberty. As a result, the vocal range of pre-pubescence is largely retained, and the voice develops into adulthood in a unique way. As the castrato’s body grew, his lack of testosterone meant that his epiphyses (bone-joints) did not harden in the normal manner. Thus the limbs of the castrati often grew unusually long, as did the bones of their ribs. This, combined with intensive training, gave them unrivalled lung-power and breath capacity. Operating through small, child-sized vocal cords, their voices were also extraordinarily flexible, and quite different from the equivalent adult female voice, as well as higher vocal ranges of the un-castrated adult male.
Boys are no longer castrated for this purpose. Very few recordings survive of castrati.