The question of whether a shark is a mammal is common, often because some species give birth to live young, leading people to group them mentally with marine mammals like whales and dolphins. However, the definitive biological answer is straightforward: sharks are fish. This classification is based entirely on specific anatomical and physiological characteristics.
Biological Requirements for Mammals
Membership in the Class Mammalia is determined by a unique combination of traits that sharks do not possess. All true mammals maintain a consistently high internal body temperature through metabolic activity, a characteristic known as endothermy, or being “warm-blooded.” The circulatory system supports this high metabolism with a four-chambered heart that completely separates oxygenated and deoxygenated blood.
Mammals are also defined by integumentary features, specifically the presence of hair or fur covering the body at some stage of development. The class name itself points to a defining reproductive trait: mammary glands. These specialized glands produce milk to nourish the young after birth, a nursing process that is a universal requirement for classification. A defining skeletal characteristic is the presence of three small bones in the middle ear, which are derived from jawbones found in their reptile ancestors.
Shark Classification: The Cartilaginous Fish
Sharks belong to the Class Chondrichthyes, a group of jawed vertebrates known as the cartilaginous fishes. The most immediate and defining difference from mammals is the skeletal structure, as a sharkâs entire endoskeleton is composed of cartilage rather than true bone tissue. This lighter, more flexible material is the same type of tissue that shapes a human nose and ears.
Their respiration system is fundamentally different from a mammal’s air-breathing lungs. Sharks extract dissolved oxygen from water using five to seven pairs of exposed gill slits, lacking the bony gill cover (operculum) seen in most other fish. The skin is covered in tiny, tooth-like structures called placoid scales, or dermal denticles, which reduce drag and provide protective armor.
The internal physiology also aligns them with fish, not mammals. Sharks possess a two-chambered heart, which is typical for fish, circulating blood in a single loop throughout the body. Unlike most bony fish, sharks also lack a swim bladder, which is an internal gas-filled organ used for buoyancy control. To compensate, sharks must rely on a large, oil-filled liver and constant movement to avoid sinking through the water column.
Sources of Biological Confusion (Reproduction and Warmth)
Two biological features of sharks sometimes cause public confusion by superficially resembling mammalian traits: their reproductive strategies and their ability to generate heat. While most fish are oviparous, meaning they lay eggs that develop outside the mother’s body, many shark species exhibit either ovoviviparity or viviparity. Ovoviviparous species retain the eggs internally until they hatch, with the pups nourished by a yolk sac before being born alive.
Viviparous sharks develop a placental connection that transfers nutrients from the mother to the embryo, resulting in a live birth that mirrors the process in mammals. However, the young are never nursed with milk after birth, which immediately disqualifies them from the Mammalia class. Reproduction method alone cannot override the fundamental skeletal or respiratory characteristics.
Another point of confusion is the thermal regulation observed in certain species. These sharks exhibit regional endothermy, a physiological adaptation where they use a specialized vascular network, called the rete mirabile, to retain metabolic heat in specific areas like the swimming muscles and brain. This allows them to maintain a body temperature several degrees warmer than the surrounding water, enhancing their speed and predatory success. This is not true endothermy, which involves maintaining a consistently high, whole-body temperature independently of the environment, but rather a localized warming unique to a few active fish species.