Oceanic dolphins are a group of marine mammals that are found across all oceans of the world. A few species, like Irrawaddy dolphins, are also spotted in rivers and estuaries, in addition to coastal waters. Though most of these dolphins prefer warmer waters of the tropics, some, like the right whale dolphins, inhabit colder waters.
These dolphins belong to the family Delphinidae under Cetacea, an infraorder that also includes porpoises (family Phocoenidae). They are easily recognized by their torpedo-shaped, streamlined bodies, bulbous heads, and notched tails. Their smooth skin covers a thick layer of blubber, which provides insulation in cold waters. They also have an inflexible neck, which reduces drag and helps improve their hydrodynamic efficiency.
Highly intelligent and social, oceanic dolphins communicate using a complex system of high-frequency vocalizations, including echolocation, which helps them navigate and hunt. They typically live in pods, ranging from small groups to large aggregations, working together to forage for fish, small crustaceans, and squid.
There are currently 38 extant species of these dolphins.
Most oceanic dolphins have an average weight ranging between 110 and 440 lb (50 and 200 kg). However, the largest oceanic dolphin, the killer whale, measures around 30 ft (9 m) and weighs 10 metric-ton (11 short-ton), whereas the smallest species, Hector’s dolphin, measures between 3 ft 11 in and 5 ft 3 in (1.2 and 1.6 m) and weighs between 88 to 132 lb (40 to 60 kg).
In addition, some members of this group, like bottlenose dolphins, are sexually dimorphic, with the males being typically larger than the females.
These cetaceans have a torpedo-shaped, muscular body with a bulbous head, an inflexible neck, and a large, notched tail. Beneath their smooth skin lies a thick layer of adipose tissue, or blubber, which provides insulation in cold waters. Their body coloration varies from gray and white to black and even pink, with some species exhibiting countershading to enhance camouflage.
Most oceanic dolphins have long, slender beaks, though some species, such as orcas and Irrawaddy dolphins, have shorter, more rounded snouts. Almost all species possess a melon, a fatty organ in their foreheads that plays a crucial role in echolocation, allowing them to navigate and communicate through sound waves. Positioned just behind the melon is the blowhole, which enables these marine mammals to breathe by inhaling fresh air and exhaling through an upward, steamy spout. Further along the back, between the blowhole and the tail, lies the dorsal fin, which provides stability as they swim.
Their two pectoral flippers (modified forelimbs) contain four digits each. While they lack functional hindlimbs, some species retain rudimentary appendages.
In these dolphins, the ears and the eyes are specially adapted for life underwater. Other sensory organs, like taste buds and olfactory lobes, are either atrophied or absent altogether.
Unlike humans, who receive sound through an eardrum, oceanic dolphins (despite having tiny auditory openings behind their eyes) primarily receive sound through their lower jaw and throat. The lower jaw contains a specialized, low-impedance, fat-filled cavity that transmits sound vibrations to the middle ear.
The middle ear bones, or ossicles, are acoustically isolated from the skull bones by air-filled sinus pockets that enhance directional hearing underwater.
Though relatively small in comparison to their body size, the eyes on either side of the head provide a wide view of the surroundings.
To allow the entry of maximum light, they have flattened eyeballs, large pupils, slightly flattened corneas, and a retroreflective layer called the tapetum lucidum. Additionally, both rod and cone cells provide color vision.
According to the Society for Marine Mammalogy (2021), the 38 extant species of oceanic dolphins are divided into 5 subfamilies.
As evident from the above list, some species are commonly called ‘whales’ and not dolphins. This is because all dolphins belong to the parvorder Odontoceti (toothed whales) and are, in fact, whales.
A few extinct genera of these dolphins include Etruridelphis, Septidelphis, Platalearostrum, Australodelphis, and Rododelphis.
These dolphins are purely carnivorous and feed on fish, small crustaceans, and mollusks, such as squid. Some species, like the killer and the false killer whales, also eat larger marine mammals and birds.
They are highly social animals, often traveling in small, cohesive groups called pods. The members of these groups coordinate and forage across vast areas, sometimes covering hundreds of square kilometers.
However, as many as a thousand individuals may occasionally gather to form a large group, which often disperses into smaller, loosely associated subgroups.
To propel through water, these dolphins move their tail fins and the rear part of their bodies vertically while using their pectoral flippers for steering. Some species may even leap out of the water to increase speed, as air resistance is lower than water resistance. Additionally, their fused neck vertebrae contribute to inflexibility but enhance stability during underwater movement.
Dolphins are known for their impressive swimming speeds, with species like orcas traveling as fast as 55.5 km/h (34.5 mph).
Usually, a pod of dolphins squeezes numerous fish into a tight, spherical formation known as a bait ball (a process known as herding). Then, one by one, each member of the pod takes turns swimming through the ball to capture fish. These dolphins typically use their cone-shaped, sharp teeth, ranging from 100 to 200 in number, to tear their prey apart.
In addition to this cooperative hunting technique, dolphins may also drive fish into shallow waters to make capture easier (a strategy called corralling). Some species, such as the Atlantic bottlenose dolphin, even use their tail flukes to stun prey by striking them forcefully.
They communicate through a variety of sounds, including burst-pulsed sounds, frequency-modulated whistles, and clicks. In fact, their clicks are among the loudest among marine animals and are used in echolocating objects in their surroundings.
These sounds are produced through vibrations in two pairs of connective tissue folds (phonic lips) found in the nasal passages of these dolphins.
Some species, like bottlenose dolphins, produce signature whistles to identify individuals within their groups. Interestingly, they can recognize their conspecifics even if they have not met for over 20 years.
These dolphins are quite jovial and like to play around with bubble rings and objects like seaweed, seagrass, and sponges. They usually pass these items to group members or toss them playfully out of the water.
They have been spotted playing around with humpback whales and even humans.
The lifespan of oceanic dolphins varies by species. Most live between 10 and 20 years, but some, like female bottlenose dolphins, can survive up to 60 years in the wild. Orcas have the longest lifespan among all oceanic dolphins, with females living as long as 90 years.
It has been observed that, when kept away from their conspecifics in captivity, these dolphins tend to have a shorter lifespan due to limited social interaction.
Oceanic dolphins are polygynandrous, which means both males and females mate with multiple partners throughout their lives. They typically reproduce year-round, with males courting potential mates through acrobatic displays and behaviors like body rubbing.
After mating, females undergo a gestation period lasting between 10 and 18 months before giving birth to a single calf, typically in the spring and summer. The calf is weaned by the females at around 18 months but remains in the maternal pod for up to six years. As highly social animals, female dolphins often assist other pregnant females during birth.
Females reach sexual maturity between 5 and 10 years of age, while males take longer, maturing at around 8 to 14 years.
They are preyed upon by large shark species, such as tiger, dusky, and bull sharks. Moreover, some predatory oceanic dolphins, like killer whales and false killer whales, also kill other dolphin species.
These dolphins often fall victim to fishing bycatch, suffer from the impacts of naval sonar, firing exercises, and offshore construction projects, and are consumed in countries like Japan and Peru. Due to these factors, their populations keep declining with time.
Currently, the following species are listed in the IUCN Red List.
| Species | IUCN Category |
|---|---|
| Atlantic humpback dolphin | Critically Endangered (CR) |
| Hector’s dolphin, Irrawaddy dolphin, Indian Ocean humpback dolphin, and tucuxi | Endangered (EN) |
| Indo-Pacific humpback dolphin, Australian humpback dolphin, and Australian snubfin dolphin | Vulnerable (VU) |
| False killer whale, Indo-Pacific bottlenose dolphin, Heaviside’s dolphin, and Chilean dolphin | Near Threatened (NT) |
The remaining species are listed as either Data Deficient (DD) or Least Concern (LC).
These animals are extremely intelligent, having the ability to learn, teach, coordinate, cooperate, remember, and grieve. In fact, the killer whale has the second-highest brain mass among all animals.
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