Dolphins are very clever marine cetaceans, comprising the suborder Odontoceti. The largest taxonomic family is that of Delphinidae. There are nearly 40 distinct species of dolphin existent in the world today, spread between 17 genera. They are all carnivorous predators, made distinct from sharks with which they share the status of apex predator in their habitat by their intelligence and other mammalian features. Physiologically, dolphins are very similar to whales, except that they are usually much smaller and have markedly different habits such as schooling and other dolphin-specific behaviors.
Many scientists and behavioral researchers consider the dolphin to be the most intelligent aquatic creatures and some go as far as to claim that they are the second most intelligent animal of any kind, after human beings. This is made evident by their receptiveness to training, social behaviors, and means of communication. While it is evident that dolphins are highly intelligent, actually measuring this intelligence is very difficult due to biological differences between humans and marine mammals. Designing applicable tests for intelligence is a very demanding task that researchers are currently developing solutions to.
While dolphins generally have excellent eyesight, their hearing is also very sharp and many species communicate through vocal clicks and whistles. Bottlenose dolphins have been observed to develop specific signature tunes that identify individuals, functioning as a parallel to naming in humans. Dolphins even call for one another using the signature whistle developed by the target dolphin and can remember individual’s signatures for very long time spans – over decades. The extent to which this communication is used to describe information is currently undergoing intense study by marine biologists and linguists alike.
Many dolphins like to jump and play with one another or other creatures, even human beings. While some of these activities form part of a social function, there are certain behaviors that they seem to engage in purely for the sake of entertainment. The idea of entertainment places dolphins on a unique level in terms of their cognitive abilities, because only humans are known to engage in playful behavior without direct function or purpose. Dogs and cats, for example, only play in mock combat that sharpens their skills, while dolphins blow bubble rings and make other games for which the only observable purpose seems to be fun.
Bolivian River Dolphin
The Bolivian river dolphin is a freshwater dolphin that lives in the Madeira River. While in the past it was considered to be a separate species, today most of the scientific community classifies it as a subspecies of the Amazon river dolphin.
Older publications as well as some modern ones used to classify Bolivian dolphins as a separate species of the Amazon river dolphin, or Inia geoffrensis. Today, most of the scientific community recognizes the Bolivian variant as a subspecies with the scientific name Inia geoffrensis boliviensis, along with I. geoffrensis and I. humboldtiana – the Amazon and Orinoco river dolphin, respectively. The species belongs to the genus Inia and family Iniidae.
The Bolivian river dolphin is the larger freshwater river dolphin in the world, with adults reaching lengths of over 2,5 meters and weighing in excess of 185 kg. The species presents sexual dimorphism, with females being 16 percent shorter and up to 55 percent lighter than males. The body is strong and flexible, with the cervical vertebrae not fused. This allows the dolphin to turn its head 90 degrees in both sides. The snout is long as is the case with other river dolphins.
The flippers are long and flexible – they further provide the dolphins with great maneuverability, which allows them to swim through flooded forests in search of food. The eyes are small, yet the dolphins have accurate eyesight. However, they strongly rely on their eco sonar to hunt which is performed by using various sounds and the melon structure in the front of the head.
Due to the rapids and waterfalls of the Madeira River, the populations have been isolated into a narrow area of the river, most likely being responsible for the appearance of the distinct Bolivian subspecies. These prefer water that is at least 1 meter deep, yet they also adventure into flooded forests during the rainy season. These dolphins have among the most diverse range of food, consuming dozens of species of fish, crustaceans, and even turtles.
Females give birth to calves during the flood season, or between May and June, after a gestation of about 11 months. Thanks to the smaller size of the females, these can stay behind with the calves in flooded forests after water retreats for far longer than males, enabling young dolphins to get the energy needed for development. Bolivian river dolphin calves usually stay with their mothers for two to three years.
Indus River Dolphin
The Indus river dolphin is a subspecies of the river dolphin which can only be found in the Indus River in Pakistan and India. It is the first cetacean that swims on its side ever discovered.
Until 1998, this dolphin was considered to be the same species with the Ganges River dolphin. Further studies proved that, in fact, these are two subspecies of the freshwater river dolphin. The Indus river dolphin, or Platanista gangetica minor, belongs to the genus Platanista along with its cousin, the subspecies Platanista gangetica. The latter lives in the Ganges river.
This dolphin swims on the side and has adapted to living in freshwaters of large rivers. The main characteristic of this dolphin is its long snout which becomes thicker towards the end. The teeth are visible even when the jaws are closed. The teeth of young individuals are one inch long and very sharp, and they undergo transformation at maturity, becoming square and fat.
Since this species has no crystalline lens in the eyes, individuals are essentially blind, relying entirely on echolocation for hunting and navigation. However, it is believed that the eyes can still distinguish between various intensities of light as well as establish direction. The tail and flippers are large in proportion to the body.
Sexual dimorphism is present, with females being slightly larger than males. Females also grow longer rostrums after reaching 150 cm total length, growing 20 extra centimeters compared to males. Females can measure 2.6 meters and males 2.2 meters.
These dolphins inhabit only the Indus River, which runs mostly through Pakistan. While, in the past, they occupied most of its 3,400 km length, nowadays they only live in small subpopulations occupying a fifth of the river. The preferred habitat is in waters which are deeper than one meter and have more than 700 square meters in sectional area.
The diet of these dolphins mainly consists of fish and crustaceans. Individuals hunt using echolocation only since they cannot rely on their eyesight. The species usually feed on prawns, catfish, carp and a wide range of freshwater crustaceans.
Females have a gestational period of about nine to 10 months, giving birth to calves between January and May. Calves usually do not stay with their mothers for more than a few months. Indus river dolphin individuals can live for more than 20 years, with the oldest known specimen being 28 years old.
The Irrawaddy dolphin is an oceanic dolphin species that predominantly inhabit brackish waters. This euryhaline dolphin can be found in rivers and estuaries in Southeast Asia and especially in the Bay of Bengal.
The Irrawaddy dolphin belongs to the Cetacea order and Delphinidae family. Alongside its cousin, the Australian snubfin dolphin, it belongs to the Orcaella genus. The scientific name of this dolphin is Orcaella brevirostris, which is Latin for short beak. The species was first described in 1866 by Sir Richard Owen, with the description being based on a specimen from 1852.
This dolphin closely resembles the beluga, but genetically, it is much closer to the killer whale. The dolphin has a large melon, a structure made of adipose tissue that is used for communication as well as echolocation. The head is round and the beak is short and barely distinguishable. It has a short, blunt triangular dorsal fin and long and broad flippers. The body colors are usually light shades of grey, with whiter areas on the underside.
This dolphin has a unique U-shaped blowhole which opens to the front of the dolphin rather than vertically, as is the case with most other species. The short beak has jaws with 12 to 19 teeth each. Adults can weigh from 90 to 200 kg and can reach lengths of over 2.3 meters.
Even though it is named after the Irrawaddy river in Myanmar, it is not a true river dolphin, but an oceanic dolphin. However, this species is euryhaline, meaning that is has evolved to be able to live in waters of various salinities, from brackish waters found in estuaries to marine waters. There are subpopulations living in the Bay of Bengal waters, from Bangladesh to India, Cambodia, Philippines, Indonesia, Thailand, and Myanmar.
This species has a diverse diet, ranging from cephalopods to crustaceans and fish. The dolphin catches its prey by using echolocation, emitting sounds at a frequency of 60 kilohertz which bounce back and are picked up by the melon structure.
Individuals reach sexual maturity between seven and nine years of age. Females and males search for a mate for several minutes, copulate for 40 seconds after which they part ways. Cows have a gestational period of 14 months and they give birth to a calf which weighs 10 kg and is one meter long. Weaning occurs after about two years. Irrawaddy dolphin individuals can live for up to 30 years.
Pygmy Killer Whale
The pygmy killer whale is a small whale belonging to the oceanic dolphin family. It is a cetacean that is rarely spotted, with fewer facts known about its biology, most of them being derived from indirect data.
With the scientific name Feresa attenuata, these dolphin species belong to the Delphinidae family of oceanic dolphins. It belongs to the subfamily Globicephalinae alongside the common killer whale, the long and short-finned pilot whale, and the melon-headed whale. The species was first described by John Gray in 1874. This rare cetacean proved to be elusive to scientists until 1954, a period when there were only two known skulls of the species in museums. While being in the same group as the killer whale, this species is not in the same genus as its larger and more popular cousin.
Specimens are average in size, with a weight similar to that of humans. Due to its small size and rare occurrence, individuals of these species are usually confused with others such as the melon-headed whale. The body is of a similar shape to that of the killer whale, with the narrowing from the head to the tail earning its name F. attenuate, which is Latin for tapering. The individuals are dark-colored, with a dark cape. The round head does not present a beak. The animal is not acrobatic, but breaching and spy-hopping have been observed.
There is an estimated population of 38,900 individuals in the tropical regions of the eastern Pacific Ocean, however, distribution of populations is wide and ranges from tropical waters to sub-tropical waters around the world. Pygmy killer whale individuals can be seen near Hawaii or Japan, and they are also present in large numbers in the Indian Ocean. There are also recordings of populations in the Atlantic, with sightings from South Carolina to Senegal. Individuals of this species can only be found in oceans.
It is believed that this species is actually more aggressive than its larger relative – the killer whale. Behavior such as snapping of jaws, flipper-beating, and flukes on water has been observed. They are aggressive towards other cetaceans. Groups of up to 25 individuals have been seen, and on rare occasions, even counting as many as a couple hundred individuals. Females achieve sexual maturity when reaching approximately 2.2 meters. Most calves are born during the warm summer months. Usually, pygmy killer whale females give birth to a single calf.
Risso’s dolphin is one of the largest dolphins in the world with populations present in most oceanic waters near continental coasts.
The scientific name for the species is Grampus griseus, and it is the only known member of the genus Grampus in the oceanic dolphins family Delphinidae. It was first described in public in 1812 by Georges Cuvier – the description was based on an original written by Nicard naturalist Antoine Risso, hence the name of the species.
This dolphin has a large body, a well-defined dorsal fin, and a narrow tail. The head is bulbous and presents a crease in the front. Calves are born with lighter body colors which will darken as they age only to lighten up again at maturity. Scars fill most of body, most of which are caused by interactions with other individuals. These dolphins have two to seven pairs of teeth that are embedded into the lower jaw. Adults usually grow to be about three meters in length, but specimens measuring four meters have been found. Just as with most other dolphin species, males are slightly larger than females. Individuals can weigh up to 500 kilograms, making them the largest dolphins.
The species can be found in most parts of the world, typically in coastal regions. They live in temperate and subtropical waters, from the Atlantic Ocean to the Pacific, from India, the Persian Gulf, to the Gulf of Alaska to Tierra del Fuego to the south. They prefer deeper waters that are close to the continental shelf. These dolphins are usually found at depths ranging between 400 and 1000 meters. They live in waters of at least 10 degrees Celsius with preference for 15 to 20 degrees water temperatures. There is an estimated population of 60,000 individuals living near the continental shelf of the United States and a total of 175,000 in the Pacific Ocean.
Risso’s dolphin individuals are more specialized than most other dolphins, with a diet consisting almost exclusively of neritic and oceanic squid. These are usually hunted during the night with the help of echolocation.
These dolphins usually travel in groups of 10 to 50 individuals with larger groups being observed as well. Females reproduce once every two years and a half, with a gestation of around 13 months. Sexual maturity is reached at eight to 10 years for females and 10 to 12 for males. Risso’s dolphin individuals can live for up to 40 years.