Sharks are not mammals. While some marine mammals, like dolphins and whales, share the aquatic environment with sharks, these two groups of animals belong to entirely different biological classes. Sharks are fish, specifically a type of cartilaginous fish, possessing distinct characteristics that set them apart from mammals.
Key Traits of Mammals
Mammals are a diverse group of vertebrates characterized by several specific traits. A primary characteristic is their ability to regulate internal body temperature, classifying them as warm-blooded or endothermic. Mammals also possess hair or fur at some point in their lives, which provides insulation.
Another distinguishing feature of mammals is the presence of mammary glands, which produce milk to nourish their young. Most mammals give birth to live young, though there are exceptions like monotremes which lay eggs. Regardless of their habitat, all mammals breathe air using lungs. Their internal support structure, known as an endoskeleton, is primarily composed of bone.
Key Traits of Sharks
Sharks, on the other hand, exhibit a different set of biological characteristics. Most shark species are cold-blooded, or ectothermic. While a few species, such as the great white and mako sharks, can maintain higher internal temperatures in certain body parts, the majority rely on their environment for temperature regulation.
Sharks breathe by extracting oxygen from water as it passes over their gills. Their skeletal structure is another defining trait, as it is made entirely of cartilage, a flexible tissue, rather than bone. This cartilaginous skeleton is lighter than bone, which aids in their movement through water. Sharks also have a unique skin covering composed of tiny, tooth-like structures called dermal denticles or placoid scales, which give their skin a sandpaper-like texture. Regarding reproduction, sharks display a variety of methods, including laying eggs in protective cases, live birth, and ovoviviparity, where eggs hatch inside the mother before live birth.
Fundamental Differences in Classification
Mammals are warm-blooded, actively regulating their body temperature, while most sharks are cold-blooded and depend on external conditions. Respiration also varies significantly; mammals use lungs to breathe air, even aquatic species like whales and dolphins, which must surface regularly. Sharks, conversely, use gills to extract dissolved oxygen from water.
The skeletal composition provides another clear division. Mammals possess bony skeletons, whereas sharks have flexible, cartilaginous skeletons. Their skin coverings are also markedly different, with mammals having hair or fur for insulation, while sharks are covered in specialized dermal denticles that reduce drag in water. Furthermore, mammalian reproduction involves mammary glands producing milk to nourish young, a characteristic absent in sharks. Although some sharks give live birth, they do not produce milk.