1. Warm-bloodedness: Whales and dolphins are endothermic, meaning they can generate their own body heat to maintain a constant internal body temperature, regardless of the external environment. This is a characteristic feature of mammals.
2. Mammary glands and milk production: Female whales and dolphins possess mammary glands that produce milk to nourish their young. They nurse their offspring with milk, just like other mammalian species.
3. Hair: Although whales and dolphins are mostly hairless, they still have a few hairs during their embryonic development. Some species, such as the bowhead whale, have a few bristles around their mouths. Hair is a defining characteristic of mammals.
4. Three-bone middle ear: Whales and dolphins have a middle ear structure consisting of three small bones: the malleus, incus, and stapes. This configuration aids in sound transmission and is found in all mammals.
5. Four-chambered heart: Whales and dolphins, like other mammals, possess a heart with four chambers: two atria and two ventricles. This arrangement allows for efficient circulation and oxygenation of blood.
6. Breathing through lungs: Whales and dolphins breathe air using lungs, requiring them to surface periodically for respiration. This dependence on air for breathing distinguishes them from fish and other aquatic organisms.
7. Neocortex: Whales and dolphins have a well-developed neocortex, which is a region of the brain associated with higher cognitive functions such as learning, memory, and problem-solving. This advanced brain structure is found in mammals.
8. Viviparity and parental care: Whales and dolphins give birth to live young (viviparity) and provide extensive parental care and nurturing to their offspring. This characteristic is typical of mammals.
Given these shared characteristics, despite their aquatic lifestyle, whales and dolphins are rightfully classified as mammals. They belong to the mammalian order Cetacea, which includes whales, dolphins, and porpoises.