Dolphins are marine mammals, which are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They vary quite remarkably in size ranging from 1.2m to about 9.5m of the Killer whale. They are present worldwide, mostly in the shallower seas of the continental shelves. Dolphins are among the most intelligent animals and are often characterized by a friendly appearance and playful attitude that have made them popular in human culture.
The name “dolphin” is of Greek origin, interpreted to mean “a fish in a womb”. The word “dolphin” encompasses any member of oceanic dolphins, river dolphins and toothed whales.
Dolphins are social animals, often considered to be one of the most intelligent animals. In areas of high food abundance, groupings may exceed 1000 individuals. Their behavior has been extensively studied both in captivity and in the wild.
They communicate using a variety of clicks, whistles and other vocalizations. They make ultrasonic sounds for echolocation. The strong social bonds in this species is shown by the way in which individuals will stay with injured or ill individuals, helping them breathe by bringing them to the surface if needed. Acts of aggression also take place among individuals of the species. An example is that which takes place among males due to the competition for females.
Play is an important part of dolphin culture. They sometimes play with seaweed or play-fight with other dolphins. At times seabirds and turtles are also harassed by this playing behavior. They are often seen riding waves and occasionally leaping above the water surface, sometimes performing acrobatic figures. This behavior is still a wonder for scientists who to this day are unsure of the purpose of this acrobatic behavior.
REPRODUCTION AND SEXUALITY
Dolphin copulation happens belly to belly; though many species engage in lengthy foreplay. The gestation period varies with species ranging between 11-12 months in the Tucuxi dolphin to 17 months in the orca. Dolphins usually become sexually active at a young age, even before reaching sexual maturity, which varies by species and gender.
Various feeding behaviors exist among and within species, some being exclusive to a specific species. Fish and squid are their main food source. There are a number of different feeding strategies used by dolphins. One of the latter is known as “herding”, where a group of dolphins squeeze a school of fish into a small volume. “Corralling” is another method where dolphins chase the fish into shallow water in order to catch them more easily.
Cooperative human-dolphin fishing has also been reported dating back to ancient Roman times. Curiously enough a modern human-dolphin partnership can presently be observed in in Brazil, where the dolphins drive the fish towards the fishermen waiting along the shore and signal the men to cast their nets.
Dolphins are capable of making a broad range of sounds. They communicate with each other through whistles and burst pulsed sounds. Dolphin vocalizations are directional and are used for echolocation. These often occur in a series of short clicks whose rate increases when approaching an object of interest. Dolphin echolocation clicks are amongst the loudest sounds made by marine mammals.
JUMPING AND PLAYING
Dolphins on occasions leap above the water surface, performing acrobatic figures. Scientists to date are not able to explain this behavior. There have been suggestions however that it is for communication with other dolphins, the dislodgement of parasites or for mere amusement.
Play is especially important for dolphins, they often play with seaweed or play-fight with other dolphins. At times they harass other local creatures, like seabirds and turtles. Captive dolphins have been observed in aquariums to engage in complex behavior, which involves the creation and manipulation of bubble rings.
NATURAL THREATS AND DOLPHIN CONSERVATION
Human beings are probably the worst natural enemies for dolphins. Some dolphin populations are dominant predators and lack of any natural enemies. Some smaller dolphin species are only threatened by a few shark species. Calves are especially at stake. Some of the larger dolphinic species may prey on smaller dolphins, however this is rare.
Some dolphin species are considerably threatened and face an uncertain future, as are the Amazon or Ganges river dolphins. Pollutants that do not disintegrate rapidly in the environment are important threats to the species and so are the injuries or deaths caused by collisions with boats. There are a number of fishing methods that kill many dolphins unintentionally. Accidental by-catch in gill nets and incidental captures in antipredator nets that protect marine fish farms pose an important threat to local dolphin populations. In some countries dolphin meat is traditionally considered as food and consequently increases the number of dolphins being hunted for this purpose. Dolphin safe labels, an initiative, which dates back to the 1980s, ensures that consumer fish and other marine products have been caught in a dolphin-friendly way.
Lastly, loud underwater noises, such as those resulting from naval sonar use or from certain offshore construction projects, such as wind farms may prove harmful to the dolphins, increasing their stress and damaging their hearing.
1. Common dolphin
2. Bottlenose dolphin
3. Spotted Dolphin
4. Commerson’s Dolphin
5. Dusky Dolphin
6. Killer Whales, also known as Orcas
7. The Boto, or Amazon River Dolphin
A. Suborder Odontoceti, toothed whales
a. Family Delphinidae, oceanic dolphins
- Genus Delphinus
- Long-Beaked Common Dolphin, Delphinus capensis
- Short-Beaked Common Dolphin, Delphinus delphis
- Genus Tursiops
- Common Bottlenose Dolphin, Tursiops truncatus
- Indo-Pacific Bottlenose Dolphin, Tursiops aduncus
- Genus Lissodelphis
- Northern Rightwhale Dolphin, Lissodelphis borealis
- Southern Rightwhale Dolphin, Lissodelphis peronii
- Genus Sotalia
- Tucuxi, Sotalia fluviatilis
- Costero, Sotalia guianensis
- Genus Sousa
- Indo-Pacific Hump-backed Dolphin, Sousa chinensis
* Chinese White Dolphin (the Chinese variant), Sousa chinensis chinensis
- Atlantic Humpbacked Dolphin, Sousa teuszii
- Genus Stenella
- Atlantic Spotted Dolphin, Stenella frontalis
- Clymene Dolphin, Stenella clymene
- Pantropical Spotted Dolphin, Stenella attenuata
- Spinner Dolphin, Stenella longirostris
- Striped Dolphin, Stenella coeruleoalba
- Genus Steno
- Rough-Toothed Dolphin, Steno bredanensis
- Genus Cephalorhynchus
- Chilean Dolphin, Cephalorhynchus eutropia
- Commerson’s Dolphin, Cephalorhynchus commersonii
- Heaviside’s Dolphin, Cephalorhynchus heavisidii
- Hector’s Dolphin, Cephalorhynchus hectori
- Genus Grampus
- Risso’s Dolphin, Grampus griseus
- Genus Lagenodelphis
- Fraser’s Dolphin, Lagenodelphis hosei
- Genus Lagenorhynchus
- Atlantic White-Sided Dolphin, Lagenorhynchus acutus
- Dusky Dolphin, Lagenorhynchus obscurus
- Hourglass Dolphin, Lagenorhynchus cruciger
- Pacific White-Sided Dolphin, Lagenorhynchus obliquidens
- Peale’s Dolphin, Lagenorhynchus australis
- White-Beaked Dolphin, Lagenorhynchus albirostris
- Genus Orcaella
- Australian Snubfin Dolphin, Orcaella heinsohni
- Irrawaddy Dolphin, Orcaella brevirostris
- Genus Peponocephala
- Melon-headed Whale, Peponocephala electra
- Genus Orcinus
- Killer Whale (Orca), Orcinus orca
- Genus Feresa
- Pygmy Killer Whale, Feresa attenuata
- Genus Pseudorca
- False Killer Whale, Pseudorca crassidens
- Genus Globicephala
- Long-finned Pilot Whale, Globicephala melas
- Short-finned Pilot Whale, Globicephala macrorhynchus
- Genus †Australodelphis
- Australodelphis mirus
b. Family Platanistidae
- Ganges and Indus River Dolphin, Platanista gangetica with two subspecies
- Ganges River Dolphin (or Susu), Platanista gangetica gangetica
- Indus River Dolphin (or Bhulan), Platanista gangetica minor
c. Family Iniidae
- Amazon River Dolphin (or Boto), Inia geoffrensis
d. Family Lipotidae
- Baiji (or Chinese River Dolphin), Lipotes vexillifer (possibly extinct, since December 2006)
e. Family Pontoporiidae
- La Plata Dolphin (or Franciscana), Pontoporia blainvillei