Whales, Dolphins, and Porpoises: How to Tell Them Apart
Wednesday, October 14th, 2020 | Blog
Usually the most obvious difference between a whale and a dolphin or porpoise is size. Other key differentiators include the presence of baleen or teeth, the shape and number of teeth in the latter case, the height of the spout or ‘blow’ and the shape of the rostrum. Whales have either baleen or teeth, while most toothed whale species found in Sri Lankan waters lack visible teeth in the upper jaw. All dolphins have a large number of teeth, usually conical in shape, located in both the upper and lower jaws. One exception is Risso’s dolphin, which does not have visible teeth in the upper jaw.
Baleen Whales & Toothed Whales
All cetaceans are divided into two scientific groups, baleen whales and toothed whales, depending on their oral anatomy and manner of feeding. Baleen whales, known to zoologists as belonging to the sub-order Mysticetes, comprise 15 species of whales, all of which have ‘plates’ made of a keratinous substance called baleen that hang down from their upper jaws. They also lack teeth. Mysticetes are filter feeders, living on organisms such as plankton, krill and small fish, which they strain out of the water using their baleen plates.
Toothed whales or Odontocetes belonging to the suborder comprise the remaining species of whales, dolphins and porpoises. They are predatory in their feeding habits.
Form & Function
This section covers the basic features of cetacean external anatomy, particularly those associated with aspects of behaviour most frequently observed by whalewatchers and seafarers.
Flukes and Peduncle
The tail of a cetacean consists of two lobes. Each individual lobe is called a fluke. The flukes are attached to the animal’s tailstock or caudal peduncle at right angles to the vertical plane of the body. They are constructed entirely of dense and fibrous tendons, which house a network of blood vessels that aid in body temperature regulation. Fluke size and shape varies with species.
The peduncle is a large muscular extension of the body containing longitudinal muscles that extend along the animals back and are used to raise and lower the flukes to propel it through the water. Curiously, thrust is provided by the upstroke of the flukes and peduncle, not the downstroke. The flukes also play a part in turning and balancing the animal.
The flippers, also called pectoral fins, vary greatly in length and shape. They can be short and round or long and narrow, with either rounded or pointed tips. They are essential to both steering and speed control but are also used for other purposes, such as communication – cetaceans often stroke one another with their flippers while engaged in social or sexual bonding.
Flippers are the equivalent of forelimbs in terrestrial mammals and possess the same basic bone structure. The bones are supported and held rigidly in place by dense connective tissue and cartilage. As with the flukes, a network of blood vessels in the flippers helps to regulate the animal’s body temperature.
The size and shape of the dorsal fin differs among cetacean species. Indeed, in some species it is vestigial or altogether absent. Where it is present, the fin lies along the dorsal centre line, nearer the posterior than the anterior end. While it can act as a keel, helping to stabilize the animal while swimming, this function appears to be relatively unimportant, as is evident from species that lack the fin altogether. In male orcas, which have the tallest dorsal fins of any cetacean, a welldeveloped fin is thought to be an advertisement of reproductive fitness, like a peacock’s tail. Traditional names used by Sri Lankan fishermen for the orca, such as talgas mulla, compare the animal to the palmyrah palm, a comment on the height and straightness of its dorsal fin.
The dorsal fin is a cartilaginous structure with no bones to support it. Bending dorsal fins are rarely observed in the wild. Tall dorsal fins may weaken and lean to one side or the other, particularly in adult male orcas. On occasion, an adult cetacean’s dorsal fin is severly bent (see Spotted Dolphin, p.177). Bent dorsal fins are thought to be caused by natural and/or anthropogenic sources. Like the flukes and pectoral flippers, the dorsal fin, too, is a site of thermoregulation.
The blowhole or blowholes of a cetacean are situated at or near the top of its head. Their number, shape and location vary according to species, as does the size and shape of the waterspouts they produce. Baleen whales have two blowholes side by side atop their heads, while toothed whales make do with a single one, as do dolphins.
Blowholes are protected by muscular flaps, which provide a watertight seal and are opened or closed voluntarily by strong muscles. The flap is closed in its relaxed position; thus cetaceans, unlike humans and most other terrestrial mammals, must make a conscious muscular movement in order to breathe. Baleen whales, having the largest blowholes, also posses a splashguard round them to keep water out when they are open.
While most cetaceans’ blowholes are centrally located on top of the head, sperm whale blowholes are located off to the left side, at an angle from the vertical. This causes the spout to slant off to the left (when the animal is viewed from behind).
As seen in the photos below, a whale’s blowhole looks like a pair of lips and, in fact, works in a somewhat similar fashion. It is kept tightly shut during dives to keep water out of the animal’s lungs.
Odontocetes, or toothed whales, possess an unique feature called the melon. This is a large organ in the forehead region filled with a lipid called ‘acoustic fat’ that has the same density as seawater and which serves as an acoustic lens, focusing the echolocation clicks and other sounds made by the animal. In some species, such as dolphins, the melon can be voluntarily reshaped to assist in sound production and focusing. Clicks bouncing off objects in the dolphin’s environment are received by an organ in the jaw which is filled with the same acoustic fat as the melon.
A sperm whale’s head contains the spermaceti organ, a large reservoir of clear liquid oil or ‘spermaceti’ that hardens to a wax-like consistency when cold. The organ is thought to serve as a buoyancy-control mechanism for these deep-diving whales, as well as having an echolocation-related function like the melon found in other toothed cetaceans. The clarity and other physical properties of spermaceti or ‘whale oil made it valuable for a variety of industrial and commercial purposes, such as cosmetics manufacture and for use as a lubricating fluid in vehicle automatic transmissions. As a result, sperm whales were driven almost to extinction until a complete, worldwide ban on hunting them was agreed in 1972.
The snout-like projection at the end of a cetacean’s head is known as the rostrum. In some species, such as the humpback dolphin and spinner dolphin, it is very pronounced; in others, such as the orca and pilot whale, the rostrum is subtly defined and hardly noticeable except on close examination. A cetacean rostrum is often very hard and can be used as an offensive weapon against predators. Orcas, for example, are capable of severely injuring or killing small whales by ramming them with their rostrums.
The rostrum also houses the animals baleen or teeth.
Odontocetes teeth take many different forms. Dolphins typically have conical teeth, while porpoise teeth are spade-shaped. Orcas have extremely large, curved teeth. All odontocete teeth are meant for grabbing and holding prey, not for ripping, tearing, or chewing. Food is swallowed whole, head first in the case of fish.
The number of teeth also varies greatly by species.
Most beaked whales have only two teeth at the tip of the lower jaw, whereas the spinner dolphin has 180-260.
The teeth of cetaceans are often an indicator of age, having annual growth rings like those of a tree trunk.
Dugongs have a pair of tusks and molar teeth.
Content by Howard Martenstyn