Dolphin

A dolphin is any one of the 40 extant species of aquatic mammal from several cetacean families. These include the oceanic dolphins, Indian river dolphins, New World river dolphins, and brackish dolphins. Dolphins belong to the group of toothed whales, which also includes belugas, narwhals, and porpoises.

Dolphins vary greatly in size, from the small Maui's dolphin to the large orca, which can weigh as much as ten tons. They have streamlined bodies with flippers and a tail fluke that help them swim quickly and agilely through the water. Dolphins use their teeth to catch fast-moving prey like fish and shrimp, and they have excellent hearing that works both in air and water.

Dolphins live in many parts of the world, mostly in warm ocean waters but also in some rivers and brackish environments. They are social animals that live in groups called pods and communicate with each other using a variety of clicks and whistles. Because of their intelligence and graceful movements, dolphins are popular in many cultures and are sometimes kept in special facilities called dolphinariums. However, dolphins face threats from hunting, getting caught in fishing nets, and pollution in their habitats.

Etymology

The word "dolphin" comes from the Greek word delphís, meaning "dolphin," which is linked to delphus, meaning "womb." This gives the idea of a "fish with a womb." Over time, the word traveled through Latin and French before becoming the English word we use today.

Today, the term "dolphin" refers to many species, including oceanic dolphins and river dolphins. Sometimes people also call the mahi-mahi fish a "dolphinfish," which can be confusing. A group of dolphins is called a school or a pod. Male dolphins are called bulls, females are cows, and their babies are calves.

Evolution

Main article: Evolution of cetaceans

Dolphins evolved from land mammals that walked on all four legs. About 48 million years ago, they began living in water. Over time, their bodies changed to help them swim better. Their ears changed so they could hear underwater, and they grew fins and a tail to move faster. Today, dolphins are most closely related to hippos, which also have a shared ancestor that lived in water long ago.

Anatomy

Dolphins have torpedo-shaped bodies with non-flexible necks, limbs changed into flippers, and a tail fin. They range in size from the 1.7-metre-long Maui's dolphin to the 9.5-metre orca. Their skulls have small eye orbits and long snouts.

Dolphins breathe by expelling stale air from their blowhole and inhaling fresh air. They have a thick layer of blubber for insulation, protection, and energy. Their teeth are conical, used for catching swift prey like fish and squid.

Dolphins have two pectoral flippers, a dorsal fin for stability, and a fluke for propulsion. They swim by moving their fluke and rear body vertically, using their flippers mainly for steering. Some dolphins can swim very fast and dive deep, slowing their heart rate to conserve oxygen.

Dolphins use echolocation to navigate and locate objects. They generate sounds using recycled air and special organs. Though they have good eyesight, their vision is different from humans, with eyes placed on the sides of their heads. They lack a sense of smell and taste.

Intelligence

Further information: Cetacean intelligence and Cetacean surfacing behaviour

Dolphins are very intelligent animals. They can teach, learn, work together, and even show emotions. Some parts of their brains have special cells that are also found in humans, which may help them think and feel in complex ways.

Scientists often measure animal intelligence by comparing brain size to body size. Orcas have one of the largest brains of any animal, and their brain-to-body size ratio is very high, similar to humans. One way to test if an animal is self-aware is by using a mirror. When dolphins see their own reflection and notice a mark on their body, they show behaviors that suggest they recognize themselves, although some scientists debate these results.

Behavior

Dolphins are highly social animals, often living in groups called pods. These pods can vary in size, sometimes merging into larger groups called superpods when food is plentiful. Dolphins form strong bonds and often help injured or ill members stay afloat to breathe. They also communicate using clicks, whistles, and body language.

Dolphins display many interesting behaviors, including using tools and learning from each other. For example, some dolphins in Australia use sponges to protect their snouts while searching for food. They also show care for other species, like helping a stranded whale and protecting swimmers from sharks. Dolphins can be playful, jumping out of the water and interacting with objects or other animals. They sometimes chase and capture small animals, possibly out of curiosity.

Threats

Dolphins have few natural enemies. Many species are apex predators, meaning they are at the top of the food chain. Smaller dolphins may be at risk from larger sharks, such as bull, dusky, tiger, and great white sharks, especially when they are young. Larger dolphins, like orcas, might occasionally prey on smaller dolphins, though this is rare. Dolphins can suffer from various diseases and parasites. The Cetacean morbillivirus has caused significant die-offs in some areas, leading to hundreds of deaths. Symptoms often include pneumonia, encephalitis, and a weakened immune system.

Dolphins face many dangers from humans. Some species, especially river dolphins like the Ganges and Yangtze river dolphins, are critically endangered. Pollution from pesticides, heavy metals, and plastics can harm dolphins. Collisions with boats, particularly from propellers, are also common dangers. Fishing practices, such as seine fishing for tuna and the use of gill nets, often result in dolphins being caught accidentally. In some places, dolphins are hunted for their meat, which can be dangerous for people to eat due to high mercury levels. Efforts to make fishing "dolphin-safe" have helped reduce some of these risks.

Climate change also affects dolphins. Warmer waters force many dolphin species to move to new areas to find cooler temperatures. This change can reduce their food supplies and lead to population declines. For example, in the Mediterranean, rising sea temperatures have caused a steep decline in short-beaked common dolphins. River dolphins are especially vulnerable to changes in water temperature and quality, which can further threaten their survival.

Relationships with humans

In history and religion

Dolphins have long been important in human culture. In Greek myths, they were seen as helpers of people. They were also important to the Minoans, as shown by art from the palace at Knossos. Ancient Greeks liked dolphins and thought seeing them was a good sign. Dolphins appear in many stories and artworks, including ones with Cupid and the poet Arion.

In Hindu mythology, the Ganges river dolphin is linked to the goddess Ganga. In Amazon River cultures, dolphins are thought to be shapeshifters.

In captivity

Dolphins became very popular in the 1960s, leading to many places where people could see them. Today, many places still keep dolphins, with bottlenose dolphins being the most common. These dolphins are kept in places like SeaWorld and are often trained to perform tricks.

Some organizations work to rescue sick or injured dolphins, while others focus on protecting them. India has made the dolphin its national aquatic animal and created a sanctuary for them.

Military

Some militaries have used dolphins for tasks like finding mines or helping lost people. The United States Navy has trained dolphins for different jobs, though they say the dolphins were never trained to hurt anyone.

Therapy

Dolphins are sometimes used in therapy to help people with mental or developmental issues. However, studies on this have had problems, and it’s not clear if it really works better than other types of therapy.

Controversy

There is debate about keeping dolphins in places where people can see them. Some argue that it’s stressful for the dolphins and doesn’t mimic their natural habitat. There have been rare cases where dolphins have hurt people, both in places where they are kept and in the wild. Some scientists think dolphins should have special rights because of their intelligence.

Consumption

In some places, like Taiji, Japan and the Faroe Islands, dolphins are caught for food. The meat is often eaten raw or cooked, but it can contain harmful substances like mercury, which can be dangerous to health, especially for children and pregnant women.

Main article: Military dolphin

See also: Captive orca