What classification do dolphins fall under, despite living underwater?

Dolphins are classified as mammals, which is the correct answer. Despite their aquatic environment, they possess several key characteristics that define mammals. Dolphins breathe air through lungs, coming to the surface regularly to inhale oxygen. They are warm-blooded, maintain a constant body temperature regardless of their environment, and give birth to live young, which they nourish with milk produced by mammary glands, a distinct feature of mammals.

Their smooth, streamlined bodies are adapted for swimming, allowing them to thrive in the ocean, yet their biological and anatomical structures align with those of land mammals. This classification distinguishes them from fish, which have gills for breathing underwater, are cold-blooded, and lay eggs in water. Amphibians typically have a dual life, spending part of their life in water and part on land, and although reptiles may have some marine species, they differ significantly in their reproductive and respiratory systems that do not align with the mammalian classification.