Dolphin
A dolphin is any one of the 40 extant species of aquatic mammal from the cetacean families Delphinidae, Platanistidae, Iniidae, Pontoporiidae, and the probably extinct Lipotidae. All these families belong to the parvorder Odontoceti, i.e., toothed whales, which also include the closely related families Monodontidae and Phocoenidae, as well as the more distant families Physeteroidea and Ziphiidae.
Dolphins range in sizes from the man-sized and Maui's dolphin; to the and apex predator, the orca. Various species of dolphins exhibit sexual dimorphism where the males are larger than females. They have streamlined, fish-like bodies with the two forelimbs evolving into flippers, complete loss of hindlimbs, and the tail forming a horizontal fluke. Though not quite as flexible as the semiaquatic seals, they are faster and more agile in water; some dolphins can briefly travel at speeds of or leap about. Dolphins use their conical teeth to capture fast-moving prey such as forage fish, coleoids and shrimps. They have well-developed hearing which is adapted for both air and water; so well developed that some can survive even if they are blind. Some species are well adapted for diving to great depths. Like other aquatic mammals, they have a thick layer of subcutaneous adipose tissue, i.e., blubber, to keep the body warm in the cold water.
Dolphins are widespread, mostly in pelagic marine environments but some thrive in brackish and freshwater bodies. Most species prefer the warm waters of the tropic zones, but some, such as the right whale dolphin, prefer colder climates. Like most nektonic animals, dolphins are carnivorous, feed largely on fish and squid, but a few large-bodied dolphins, such as the orca, prey upon larger marine animals such as seals, other dolphins and porpoises, large bluewater fish such as sharks, billfish and bluefin tuna, and sometimes even seabirds. Male dolphins typically mate with multiple females every year, but females only mate every two to three years. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively long period of time.
Dolphins are highly social animals living in complex "fission-fusion" societies, forming fluid groups that constantly change in size and composition. Like whales, dolphins produce a variety of vocalizations, usually in the form of high-pitched clicks and whistles, and are very keen with echolocation.
While relatively uncommon compared to whaling, dolphins are sometimes hunted in places such as Japan and parts of Latin America, in an activity known as dolphin drive hunting. Besides drive hunting, they also face threats from bycatch, ghost net entanglement, habitat loss and marine pollution. Due to their grace and high intelligence, dolphins feature prominently in various cultures worldwide, such as in art or folklore. Dolphins are sometimes kept in captivity within dolphinariums for research, conservation or to be trained to perform tricks in marine mammal parks; the most common dolphin species in captivity is the bottlenose dolphin, while there are also around 60 orcas in captivity.
Etymology
The name is originally from Greek δελφίς, "dolphin", which was related to the Greek δελφύς, "womb". The animal's name can therefore be interpreted as meaning "a 'fish' with a womb". The name was transmitted via the Latin delphinus, which in Medieval Latin became dolfinus and in Old French daulphin, which reintroduced the spelling ph into the word dolphin. The term mereswine is also used.The term dolphin can be used to refer to most species in the family Delphinidae and the river dolphin families of Iniidae, Pontoporiidae, Lipotidae and Platanistidae. Meanwhile, the mahi-mahi fish is called the dolphinfish. In common usage, the term whale is used only for the larger cetacean species, while the smaller ones with a beaked or longer nose are considered dolphins. The name dolphin is used casually as a synonym for bottlenose dolphin, the most common and familiar species of dolphin. There are six species of dolphins commonly thought of as whales, collectively known as blackfish: the orca, the melon-headed whale, the pygmy killer whale, the false killer whale, and the two species of pilot whales, all of which are classified under the family Delphinidae and qualify as dolphins. Although the terms dolphin and porpoise are sometimes used interchangeably, porpoise usually refers to the Phocoenidae family, which have a shorter beak and spade-shaped teeth and differ in their behavior.
A group of dolphins is called a school or a pod. Male dolphins are called bulls, females are called cows and young dolphins are called calves.
Evolution
Dolphins are descendants of land-dwelling mammals of the artiodactyl order. They are related to the Indohyus, an extinct chevrotain-like ungulate, from which they split approximately 48 million years ago.The primitive cetaceans, or archaeocetes, first took to the sea approximately 49 million years ago and became fully aquatic by 5–10 million years later.
Archaeoceti is a parvorder comprising ancient whales. These ancient whales are the predecessors of modern whales, stretching back to their first ancestor that spent their lives near the water. Likewise, the archaeocetes can be anywhere from near fully terrestrial, to semi-aquatic to fully aquatic, but what defines an archaeocete is the presence of visible legs or asymmetrical teeth. Their features became adapted for living in the marine environment. Major anatomical changes include the hearing set-up that channeled vibrations from the jaw to the earbone which occurred with Ambulocetus 49 million years ago, a streamlining of the body and the growth of flukes on the tail which occurred around 43 million years ago with Protocetus, the migration of the nasal openings toward the top of the cranium and the modification of the forelimbs into flippers which occurred with Basilosaurus 35 million years ago, and the shrinking and eventual disappearance of the hind limbs which took place with the first odontocetes and mysticetes 34 million years ago. The modern dolphin skeleton has two small, rod-shaped pelvic bones thought to be vestigial hind limbs. In October 2006, an unusual bottlenose dolphin was captured in Japan; it had small fins on each side of its genital slit, which scientists believe to be an unusually pronounced development of these vestigial hind limbs.
Today, the closest living relatives of cetaceans are the hippopotamuses; these share a semi-aquatic ancestor that branched off from other artiodactyls some 60 million years ago. Around 40 million years ago, a common ancestor between the two branched off into cetacea and anthracotheres; anthracotheres became extinct at the end of the Pleistocene two-and-a-half million years ago, eventually leaving only one surviving lineage: the two species of hippo.
Anatomy
Dolphins have torpedo-shaped bodies with generally non-flexible necks, limbs modified into flippers, a tail fin, and bulbous heads. Dolphin skulls have small eye orbits, long snouts, and eyes placed on the sides of its head; they lack external ear flaps. Dolphins range in size from the long and Maui's dolphin to the and orca. Overall, they tend to be dwarfed by other Cetartiodactyls. Several species have female-biased sexual dimorphism, with the females being larger than the males.Dolphins have conical teeth, as opposed to porpoises' spade-shaped teeth. These conical teeth are used to catch swift prey such as fish, squid or large mammals, such as seals.
Breathing involves expelling stale air from their blowhole, in an upward blast, which may be visible in cold air, followed by inhaling fresh air into the lungs. Dolphins have rather small, unidentifiable spouts.
All dolphins have a thick layer of blubber, thickness varying on climate. This blubber can help with buoyancy, protection to some extent as predators would have a hard time getting through a thick layer of fat, and energy for leaner times; the primary usage for blubber is insulation from the harsh climate. Calves, generally, are born with a thin layer of blubber, which develops at different paces depending on the habitat.
Dolphins have a two-chambered or three-chambered stomach that is similar in cellular structure to that of terrestrial carnivores. They have fundic and pyloric chambers.
Dolphins' reproductive organs are located inside the body, with genital slits on the ventral side. Males have two slits, one concealing the penis and one further behind for the anus. Females have one genital slit, housing the vagina and the anus, with a mammary slit on either side.
Integumentary system
The integumentary system is an organ system mostly consisting of skin, hair, nails and endocrine glands. The skin of dolphins is specialized to satisfy specific requirements, including protection, fat storage, heat regulation, and sensory perception. The skin of a dolphin is made up of two parts: the epidermis and the blubber, which consists of two layers including the dermis and subcutis.The dolphin's skin is known to have a smooth rubber texture and is without hair and glands, except mammary glands. At birth, a newborn dolphin has hairs lined up in a single band on both sides of the rostrum, which is their jaw, and usually has a total length of 16–17 cm. The epidermis is characterized by the lack of keratin and by a prominent intertwine of epidermal rete pegs and long dermal papillae. The epidermal rete pegs are the epithelial extensions that project into the underlying connective tissue in both skin and mucous membranes. The dermal papillae are finger-like projections that help adhesion between the epidermal and dermal layers, as well as providing a larger surface area to nourish the epidermal layer. The thickness of a dolphin's epidermis varies, depending on species and age.