Cetaceans, an infraorder of aquatic mammals, encompass all whales, dolphins, and porpoises. While most mammals have hair, cetaceans appear largely hairless. This raises questions about their hair presence and purpose.
Presence of Hair in Cetaceans
Cetaceans do possess hair at some point in their lives. Many species are born with limited hair, typically around the snout or head, which is often shed shortly after birth. This temporary hair presence links them to their mammalian ancestry.
Baleen whales (Mysticetes) tend to exhibit more prominent hairs, even into adulthood. Humpback whales, for instance, have visible bumps called tubercles on their heads, each containing a single hair follicle. Other baleen whales, such as fin, sei, right, and bowhead whales, can have 30 to 100 hair follicles along their jawline, chin, and head. In contrast, most toothed whales (Odontocetes) generally lose their hair quickly after birth. The Amazon river dolphin, also known as the boto, is a notable exception, retaining stiff hairs on its beak throughout its life.
Adaptations for Aquatic Environments
The near-complete absence of body hair in most adult cetaceans results from evolutionary adaptations to their fully aquatic lifestyle. One primary reason relates to thermoregulation. A thick layer of blubber beneath their skin provides insulation to maintain body temperature in cold ocean waters, making a fur coat unnecessary.
Fur functions as an insulator by trapping air, but this mechanism becomes ineffective underwater where hydrostatic pressure compresses the air. A hairless body also allows cetaceans to dissipate excess heat efficiently during high activity. The smooth, streamlined body surface plays a significant role in hydrodynamics. The absence of hair reduces drag, allowing these animals to move through water with minimal resistance, crucial for efficient swimming, hunting, and escaping predators. Their skin is also highly specialized for aquatic life, being thick and smooth with a rapid cell turnover rate.
Remaining Hairs and Their Functions
Some cetaceans retain specialized hairs or hair follicles that serve specific functions. These hairs, particularly around the head and mouth, are often sensory, akin to vibrissae or whiskers in land mammals. They are highly innervated, meaning they are surrounded by many nerve endings, enabling them to detect subtle changes in water pressure, currents, or prey.
For example, the stiff hairs on the beak of the Amazon river dolphin assist in foraging by helping them sense their surroundings in murky river environments. The hairs located within the tubercles on the heads of humpback whales are also sensory, potentially aiding in detecting water flow or prey density. Bowhead whales possess sensory hairs on their chin, rostrum, and near their blowhole, which may help them detect air and ice interfaces in their Arctic habitat. For many cetacean species, however, any hairs present are largely vestigial, representing remnants from their land-dwelling ancestors that serve little to no current purpose and typically disappear shortly after birth.