The tail fin, formally known as the fluke, is a powerful fin that works primarily to propel the animal forward. Unlike fish, which have tails that move from side to side (vertically), the whale’s fluke moves up and down, or horizontally, in powerful strokes. The fluke’s muscles are arranged in two masses. These two masses are the epaxial mass (also known as the back muscle because it lies along the upper length of the back bone) and the hypaxial mass (or under fillet), which is situated on the underside of the whale’s body.
The epaxial muscle mass pulls the tail upwards, while the hypaxial mass draws it back down to facilitate its swimming movement. Whalers favour these muscles as they provide plenty of boneless meat with no inedible trimmings. The top muscle, the epaxial, is larger and stronger than its lower counterpart. This is an essential feature of its structure because the epaxial muscle is responsible for raising the massive tail fin against the resistance and weight of the water around it.
The tendons of the fluke elevator muscle are attached to the last few vertebrae of the spine, giving them strength and stability. This muscle supports the fluke against the downward pressure of the water every time the epaxial mass pulls the tail upwards. There is no corresponding muscle for the downward movement of the tail. The belly muscles are attached to the hypaxial mass and tail, which means that even downward movements of the fluke aid in forward propulsion.
When the whale needs to accelerate, or when it is maintaining a slow, steady speed, it will move its tail in large up and down strokes. However, when it is maintaining a fast, constant speed, it only needs to move its fluke in regular, but relatively small, oscillations.
The bigger the whale is the less energy it needs to propel itself through the water in terms of the proportional amount of calories that it requires to cover certain distances.
Interestingly, the tail can also move (albeit to a very limited degree) to the left and right. These movements may be largely imperceptible to onlookers, but are highly effective means of steering and facilitating sharp corners.
Most whales have one dorsal fin (situated on their upper side, or back) and two pectoral fins on their undersides. The dorsal fins contribute to the stability of the whale, while the other two fins are used for steering. The size and positioning of these fins differ from one whale species to the next, and their role will need to be considered in a detailed discussion about each member of this fascinating species.