Thickset, blunt-snouted toothed whale (Physeter catodon, family Physeteridae) with small, paddlelike flippers and rounded humps on the back. Sperm whales have an enormous head, squarish in profile, and a narrow, underslung lower jaw with large conical teeth that fit into sockets in the toothless upper jaw when the mouth is closed. They are dark blue-gray or brownish. (Herman Melville's Moby-Dick was presumably an albino.) The male grows to 60 ft (18 m). Herds of 15–20 live in temperate and tropical waters worldwide. They commonly dive to 1,200 ft (350 m), feeding primarily on cephalopods. The whales have been hunted for their spermaceti (a waxy substance in the snout, used in ointments and cosmetics) and for ambergris. The pygmy sperm whale (genus Kogia) is a black dolphinlike whale, about 13 ft (4 m) long, of the Northern Hemisphere that lacks commercial value.
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Male reproductive cell. In mammals, sperm are produced in the testes and travel through the reproductive system. At fertilization, one sperm of the roughly 300 million in an average ejaculation (see semen) fertilizes an egg (see ovary) to produce an offspring. At puberty, immature cells (spermatogonia) begin a maturation process (spermatogenesis). A mature human sperm has a flat, almond-shaped head, with a cap (acrosome) containing chemicals that help it penetrate an ovum. It is essentially a cell nucleus, with 23 chromosomes (including either the X or Y that determines the child's sex). A flagellum propels the sperm, which may live in a woman's reproductive tract for two to three days after sexual intercourse, to the egg. Sperm may be frozen and stored for artificial insemination.
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