Skeleton

A drawing on a sperm whale with the skeleton inside

Sperm whale skeleton © Lydekker, 1894 Royal Natural History. Vol 3.

Head and Teeth

In the course of evolution, the two nasal passages moved to the top of the skull. However, at the skin’s surface, while we find two blowholes in baleen whales, toothed whales have but one. As soon as they break the surface, cetaceans inhale without having to move their head. Likewise, the snout grew longer to become the rostrum. Most toothed whales have identical, cone-shaped teeth which they use to capture and tear apart their prey, while their stomach does all the digesting. The harbour porpoise sports spade-shaped teeth.

On the harbour porpoise skull, the two holes are the two blowholes. © GREMM

Vertebral Column

The vertebral column is long and rather stiff. The necks of most cetaceans have grown shorter and lost most of their mobility, the seven cervical vertebrae are compressed and sometimes fused to stabilize the head in the water and reduce energy expenditure. Rorquals’ vertebrae are free and separated, except those of the fin whale, whose 2^nd and 3^rd vertebrae are partially fused. Belugas, narwhals and freshwater dolphins have retained a certain flexibility and can turn their heads slightly. Thoracic and lumbar movement is rather limited. One vertebra serves as a pivot point in the caudal region and allows enough flexibility for complex and powerful movements of this horizontal fin used for propulsion.

A beluga skull © GREMM

Limbs

The front limbs of whales’ land-roaming ancestors grew much shorter and transformed into pectoral fins. The bones are no longer articulated and do not allow for any movement. The only point of articulation is the shoulder. These fins serve both as a stabilizer and a rudder. Five digits for toothed whales and right whales, four for rorquals: some are very long, with many more phalanges than in land mammals. The bones of the hand are embedded in a fibrous, rigid and resistant tissue and do not appear at the surface of the skin. The hind limbs of whales have disappeared. The pelvis is represented by two small free-floating, vestigial bones that serve as a framework for the male’s genital organs and as a point of attachment for the retractor penis muscles.