Dolphins live entirely in water, leading many to wonder about their classification and assume they are fish. Despite their streamlined bodies and fins, which superficially resemble fish, dolphins possess distinct biological characteristics.
Dolphins: Clearly Mammals
Dolphins are classified as marine mammals, belonging to the order Cetacea, which includes whales and porpoises. Despite their aquatic life, dolphins share fundamental biological traits with land mammals. Their lifestyle results from millions of years of evolution, adapting from land-dwelling ancestors to life in the sea.
Distinctive Mammalian Characteristics
Dolphins breathe air using lungs, not gills, necessitating regular trips to the water’s surface to inhale. They breathe through a blowhole located on top of their heads, which is a specialized nostril that seals tightly underwater. This adaptation allows for efficient air exchange, with dolphins capable of exchanging up to 80% of the air in their lungs with each breath, significantly more than humans.
Dolphins are warm-blooded, maintaining a consistent internal body temperature regardless of the surrounding water. They achieve this through metabolic processes and a thick layer of blubber, which insulates against the cold ocean.
Dolphins give birth to live young, typically a single calf, rather than laying eggs. Calves are usually born tail-first to prevent drowning. Female dolphins nurse their young with milk from mammary glands, which is rich in fat and protein, aiding rapid growth of the calf’s blubber. Calves may nurse for several years, learning survival skills from their mothers.
Dolphins also possess hair, a characteristic shared by all mammals at some point. Newborn dolphins often have a few whiskers around their rostrum (beak area) at birth, which typically fall out shortly after. The presence of these hairs, even if sparse or temporary, is an evolutionary remnant.
How Dolphins Differ from Fish
A primary distinction between dolphins and fish lies in their respiratory systems. Fish breathe underwater using gills, extracting dissolved oxygen. Dolphins, in contrast, use lungs to breathe air and must surface regularly, unable to extract oxygen from water.
Most fish are cold-blooded, or ectothermic, meaning their body temperature fluctuates with their environment. While some fish species exhibit warm-blooded traits, the vast majority do not maintain a constant internal temperature. Dolphins, conversely, are warm-blooded and actively regulate their body temperature.
Reproduction also differs significantly; most fish lay eggs, fertilized externally and developing outside the mother’s body. Dolphins, however, give live birth to fully formed young and provide parental care through nursing.
The external coverings of dolphins and fish are distinct. Most fish are covered in scales, providing protection and hydrodynamic advantages. Dolphins have smooth skin without scales, relying on blubber for insulation and protection. While both have fins and tails, dolphins possess horizontal tail flukes for propulsion, whereas most fish have vertical caudal fins.