Whales and dolphins belong to the cetacean group (a term derived from the ancient Greek meaning “large marine animal”). These animals have several adaptations for living in aquatic environments, such as a hydrodynamic body shape, appendages modified for swimming, and organ systems that allow prolonged immersion in saltwater and freshwater systems.
Cetaceans are divided into two main groups: mysticetes (cetaceans that capture food using a baleen) and odontocetes (cetaceans that capture food with their teeth).
This cetacean group captures food using baleen plates, which are structures that serve as filters capable of retaining small mollusks, crustaceans and schooling fish. Scientists usually refer to these cetaceans as “true whales,” and they are medium to large animals that measure between 6 and 30 m in length when fully grown and may weigh over 150 tons.
Nearly all mysticete species migrate, reproduce in tropical waters during winter and return to temperate or polar oceans where they encounter a large concentration of food during the summer and early fall. After accumulating energy in the form of fat, mysticetes migrate again, restarting the reproductive cycle.
Currently, 13 mysticete species are known, and they are divided among 4 families:
- Family Balaenidae – This family is known as right whales, which were the first whales hunted by man because of their coastal habit, approachability and large amount of fat. The name right whale is derived from the fact that these whales were ideal for hunting because they float when dead, facilitating towing after harpooning.
- Family Balaenopteridae – This family is known as rorquals, and they have several ventral folds in the neck region that expand for the ingestion of large amounts of food. The most well-known members of this family are the humpback whale and blue whale.
- Family Eschrichtiidae – This family includes the gray whales, which only inhabit the Northern Hemisphere. Members of this family are only found in the Pacific Ocean because the Atlantic Ocean population became extinct as a result of hunting in the 18th
- Family Neobalaenidae – This family only has one species, the pygmy right whale, which inhabits temperate Southern Hemisphere seas. These whales are rare and the smallest mysticetes in the world, measuring up to 6 meters in length.
The Odontocetes are toothed cetaceans and include freshwater and saltwater dolphins, orcas and sperm whales. With the exception of the sperm whale, which can measure 18 m and weigh over 60 tons, the odontocetes are small- or medium-sized animals. Unlike mysticetes, most odontocetes do not migrate long distances between reproductive and feeding areas.
This group is composed of over 80 species and subdivided into at least 6 different groups.
- Family Delphinidae – This family includes true dolphins. They are are found in all oceans and in certain estuarine and freshwater habitats. The bottlenose dolphin is one of the most well-known representatives of this family and achieved notoriety through the television series Flipper. They are the best-adapted dolphins to captivity and are found in aquariums and water parks around the world. The orca is also a well-known Delphinidae. The orcas are the largest ocean predators and feed on a wide range of prey (fish, squid, sea lions, fur seals, true seals and even other whales and dolphins).
- Family Monodontidae – This family includes narwhals and belugas, which are cetacean species that only live in regions close to the Arctic. Belugas are white whales known as sea canaries because of their large vocal repertoire. Narwhals are known as the unicorns of the sea because males have a long modified tooth that projects forward, resembling a horn, and is used to seek prey on the ocean floor.
- Family Phocoenidae – This family includes porpoises, which are small cetaceans similar to dolphins, although they do not have a beak. The main characteristic of these animals is the rounded and flat-ended tooth shape. Porpoises are more common in the Northern Hemisphere, and the vaquita is the most popular member of this family (a small dolphin that lives in the northern Gulf of California and is considered the most endangered cetacean in the world).
- Family Physeteridae – This family includes the sperm whales, and it only has three species: the sperm whale and pygmy and dwarf sperm whales. These animals have oceanic habits and dive deep to find prey. The sperm whale is the largest of the toothed whales and became known worldwide as the protagonist of the book Moby Dick (an albino sperm whale), which was written by Herman Melville.
- Superfamily Platanistoidea – This family includes five mostly freshwater species living in rivers in Asia and South America. They are among the most endangered cetaceans in the world, and a recent study in China found that the baiji dolphin, which lived in the Yangtze River, is practically extinct. Only captive animals have survived. A famous and well-known platanistid in Brazil is the pink river dolphin that inhabits the Amazon.
- Family Ziphiidae – This family includes the “beaked whales,” which are rare animals that inhabit deep waters where they perform great dives to capture prey. Some of these animals are so rare that they have never been seen alive.
All cetaceans are mammals; therefore, their calves grow within a placenta inside the mother’s body and are fed milk during the first months of life. The gestation period varies among species and may last from 11 to 12 months for mysticetes or 10 to 15 months for odontocetes.
Cetaceans are generally top-of-the-food-chain animals; thus, they have few natural predators. Therefore, the growth of whale populations in the oceans may be used as an indicator of the health of marine environments and availability of biodiversity for the future.
As with other mammals, the mother teaches their young how to survive, and this behavior is especially true in odontocetes. In most dolphins, an offspring stays with the mother for approximately two years. Orca calves stay with their mothers throughout their entire lives, with their mothers teaching them how to feed and communicate
It is rare for whales or dolphins to have twins. The offspring require a large amount of energy from their mothers, and females are generally incapable of feeding two or more offspring at the same time. In rare cases where twins are born, one usually does not survive, which is a reason why cetaceans deserve special care. Because females produce one offspring per pregnancy and require periods of between two and six years before their next pregnancy, cetacean populations do not grow quickly, and these animals are more vulnerable to environmental impacts.