The group of marine mammals known as cetaceans—which includes all whales, dolphins, and porpoises—presents a biological contradiction. As warm-blooded, air-breathing mammals, they should possess a defining trait of their class: a covering of hair. However, these sleek aquatic animals appear largely hairless, an adaptation that seems to defy their mammalian heritage. The near-absence of a coat raises two primary questions for these descendants of land-dwelling ancestors: whether any hair remains at all, and what evolutionary forces led to the dramatic loss of this fundamental mammalian feature required for a fully aquatic life.
The Presence of Hair in Cetaceans
All cetaceans develop hair at some point during their lives, typically while they are still in the womb. This early presence of hair is a clear indication of their lineage from terrestrial mammals. For the majority of species, especially toothed whales (Odontocetes) like dolphins and porpoises, this prenatal hair is shed before or very shortly after birth.
A few species, however, retain a limited number of hairs into adulthood. Among baleen whales (Mysticetes), species such as the Bowhead, Right, Humpback, and Fin whales possess coarse, scattered hairs. These hairs are concentrated around the head, particularly on the snout, chin, and jawline. Humpback whales, for instance, have prominent, golf ball-sized bumps called tubercles on their heads, each housing a single hair follicle. The Amazon River dolphin, a toothed whale, is one of the rare exceptions that retains stiff hairs on its beak throughout its life.
The Sensory Function of Remaining Hairs
The few hairs or specialized hair follicles remaining in adult cetaceans are not remnants of an insulating coat. Instead, they function as tactile sensors, known as vibrissae, similar in purpose to a cat’s whiskers. These structures are densely innervated, surrounded by a network of nerves and blood sinuses that amplify minute physical signals. The follicles are converted into mechanoreceptors, specialized in detecting pressure changes and movement in the surrounding water.
For filter-feeding baleen whales, these sensory hairs help locate the densest patches of plankton or krill. The hairs sense subtle hydrodynamic cues created by prey movement, allowing the whale to determine the optimal moment to begin feeding. In the Amazon River dolphin, which inhabits murky water, the stiff hairs on its snout are used to feel for prey along the riverbed, complementing their primary use of echolocation.
Evolutionary Drivers for Near Hairlessness
The primary pressure leading to hair loss in cetaceans was the need for hydrodynamic efficiency in an aquatic environment. A dense coat of hair or fur, which traps air for insulation on land, creates significant drag in water. This drag increases the energy expenditure required for swimming, making hair a liability for a fast-moving marine predator. The evolution of smooth, nearly hairless skin allows for laminar flow, where water glides over the surface with minimal turbulence, enabling efficient movement through the water column.
The second major driver was a shift in thermoregulation strategy. While terrestrial mammals rely on fur for insulation, cetaceans evolved to depend on blubber, a thick, dense layer of subcutaneous fat, to maintain their body temperature. Blubber provides superior insulation that does not compress under the immense pressures experienced during deep dives, which would render an air-trapping fur coat ineffective.
The genetic basis for this transformation involved changes to genes that regulate hair growth and cycling. Specifically, the Hr (Hairless) gene, which helps boost hair growth, experienced a functional loss, effectively halting the early development of hair follicles. Concurrently, the FGF5 (Fibroblast Growth Factor 5) gene, which controls the cessation of hair growth, was subject to positive selection. The accelerated function of the FGF5 gene promoted the early termination of the hair growth cycle, ensuring that any hair that did sprout was quickly shed. This combination of genetic changes provided the mechanism for the near-complete loss of the mammalian hair coat.