Seals are fin-footed, carnivorous, semi-aquatic animals belonging to the infraorder Pinnipedia, which includes 34 living species such as true seals, eared seals, and the walrus. While they spend the majority of their lives at sea, seals must return to land or ice to give birth, rest, and molt. This reliance on both aquatic and solid environments makes them unique among marine life, but their classification as mammals is firmly established by a set of universal biological traits.
The Four Pillars of Mammalian Status
The classification of seals as mammals is based on four biological characteristics shared with all other animals in the class Mammalia. Seals are endothermic, meaning they are “warm-blooded” and can maintain a consistently high internal body temperature independent of the surrounding cold water. This is achieved partly through a high metabolic rate, which can be 1.5 to 3 times higher than that of comparable land mammals.
A second characteristic is the presence of hair or fur, which all seal species possess, though it varies in density and function. This fur, along with a thick, insulating layer of subcutaneous fat called blubber, helps prevent heat loss in the frigid waters. Furthermore, seals exhibit viviparity, giving birth to live young rather than laying eggs, a trait common to nearly all mammals.
The fourth mammalian trait is the ability to nurse their young with milk produced by mammary glands. Female seals produce milk that is exceptionally rich in fat, enabling the pups to grow rapidly and build up their own insulating blubber layer. The mothers rely on their own fat stores and typically fast completely during the nursing period, which lasts anywhere from a few days to several weeks, depending on the species.
Seals Terrestrial Ancestry and Aquatic Adaptation
Seals are not fully aquatic mammals like whales, but are instead descendants of land-dwelling carnivores that returned to the water. Their evolutionary path began approximately 50 million years ago, diverging from ancestors closely related to modern bears and musteloids, such as weasels and raccoons. Early transitional fossils, like Puijila, show an otter-like animal that lived a more terrestrial life but possessed early aquatic adaptations like a streamlined body and webbed feet.
This terrestrial history explains why seals must still haul out onto solid ground or ice to perform specific life functions, notably breeding and giving birth. Over millions of years, their limbs evolved into specialized flippers, which are essential for propulsion and steering in the water. The streamlined, torpedo-shaped body also minimizes drag, and the thick blubber layer allows them to thermoregulate in the marine environment.
Seals in the Taxonomic Hierarchy
The formal classification of seals places them within the biological framework of land mammals. They belong to the order Carnivora, a group that includes dogs, cats, and bears, reflecting their ancestry. Within this order, seals, sea lions, and walruses are grouped into the infraorder Pinnipedia, a name that translates to “fin-footed”.
Pinnipedia is further divided into three families: the Phocidae, or “true seals,” the Otariidae, or “eared seals” (sea lions and fur seals), and the Odobenidae, which contains only the walrus. This grouping distinguishes them from other marine life, such as fish, which are non-mammalian vertebrates, or marine reptiles like sea turtles.