The mature mobile reproductive cell (see gamete) of male animals, which is produced by the testis (see spermatogenesis). It consists of a head section containing a haploid nucleus and an acrosome, which allows the sperm to penetrate the egg at fertilization; a middle section containing mitochondria to provide the energy for movement; and a tail section, which contains a single flagellum (sense 2) that lashes to drive the sperm forward.