Taxonomy and Biological classification

        Taxonomy is the science of classifying organisms. Biological classification, or scientific classification in biology, is a method by which biologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy, but should be distinguished from folk taxonomy, which lacks scientific basis.

        The system currently used by taxonomists is called the Linnaean taxonomic system, in honor of Swedish biologist Carolus Linnaeus (1707 — 1778). Modern biological classification has its root in the work of Carolus Linnaeus, who grouped species according to shared physical characteristics. These groupings have since been revised to improve consistency with the Darwinian principle of common descent. Molecular phylogenetics, which uses DNA sequences as data, has driven many recent revisions and is likely to continue to do so. Biological classification belongs to the science of biological systematics.

Ancient through medieval times

        Current systems of classifying forms of life descend from the thought presented by the Greek philosopher Aristotle, who published in his metaphysical and logical works the first known classification of everything whatsoever, or "being". This is the scheme that gave such words as 'substance', 'species' and 'genus' and was retained in modified and less general form by Linnaeus.

        Aristotle also studied animals and classified them according to method of reproduction, as did Linnaeus later with plants. Aristotle's animal classification was eventually made obsolete by additional knowledge and was forgotten.

        The philosophical classification is in brief as follows: Primary substance is the individual being; for example, Peter, Paul, etc. Secondary substance is a predicate that can properly or characteristically be said of a class of primary substances; for example, man of Peter, Paul, etc. The characteristic must not be merely in the individual; for example, being skilled in grammar. Grammatical skill leaves most of Peter out and therefore is not characteristic of him. Similarly man (all of mankind) is not in Peter; rather, he is in man.

        Species is the secondary substance that is most proper to its individuals. The most characteristic thing that can be said of Peter is that Peter is a man. An identity is being postulated: "man" is equal to all its individuals and only those individuals. Members of a species differ only in number but are totally the same type.

        Genus is a secondary substance less characteristic of and more general than the species; for example, man is an animal, but not all animals are men. It is clear that a genus contains species. There is no limit to the number of Aristotelian genera that might be found to contain the species. Aristotle does not structure the genera into phylum, class, etc., as the Linnaean classification does.

        The secondary substance that distinguishes one species from another within a genus is the specific difference. Man can thus be comprehended as the sum of specific differences (the "differentiae" of biology) in less and less general categories. This sum is the definition; for example, man is an animate, sensate, rational substance. The most characteristic definition contains the species and the next most general genus: man is a rational animal. Definition is thus based on the unity problem: the species is but one yet has many differentiate.

        The very top genera are the categories. There are ten: one of substance and nine of "accidents", universals that must be "in" a substance. Substances exist by themselves; accidents are only in them: quantity, quality, etc. There is no higher category, "being", because of the following problem, which was only solved in the Middle Ages by Thomas Aquinas: a specific difference is not characteristic of its genus. If man is a rational animal, then rationality is not a property of animals. Substance therefore cannot be a kind of being because it can have no specific difference, which would have to be non-being.

        The problem of being occupied the attention of scholastics during the time of the Middle Ages. The solution of St. Thomas, termed the analogy of being, established the field of ontology, which received the better part of the publicity and also drew the line between philosophy and experimental science. The latter rose in the Renaissance from practical technique. Linnaeus, a classical scholar, combined the two on the threshold of the neo-classicist revival now called the Age of Enlightenment.

Early methodists

        An important advance was made by the Swiss professor, Conrad von Gesner (1516–1565). Gesner's work was a critical compilation of life known at the time.

        The exploration of parts of the New World produced large numbers of new plants and animals that needed descriptions and classification. The old systems made it difficult to study and locate all these new specimens within a collection and often the same plants or animals were given different names simply because there were too many species to keep track of. A system was needed that could group these specimens together so they could be found; the binomial system was developed based on morphology with groups having similar appearances.

        In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. Advances in using this knowledge to classify living beings bear a debt to the research of medical anatomists, such as Fabricius (1537–1619), Petrus Severinus (1580–1656), William Harvey (1578–1657), and Edward Tyson (1649–1708).

        Advances in classification due to the work of entomologists and the first microscopists is due to the research of people like Marcello Malpighi (1628–1694), Jan Swammerdam (1637–1680), and Robert Hooke (1635–1702). Lord Monboddo (1714–1799) was one of the early abstract thinkers whose works illustrate knowledge of species relationships and who foreshadowed the theory of evolution.

Early methodists

        Since late in the 15th century, a number of authors had become concerned with what they called methodus, (method). By method authors mean an arrangement of minerals, plants, and animals according to the principles of logical division. The term

        Methodists was coined by Carolus Linnaeus in his Bibliotheca Botanica to denote the authors who care about the principles of classification (in contrast to the mere collectors who are concerned primarily with the description of plants paying little or no attention to their arrangement into genera, etc). Important early Methodists were Italian philosopher, physician, and botanist Andrea Caesalpino, English naturalist John Ray, German physician and botanist Augustus Quirinus Rivinus, and French physician, botanist, and traveller Joseph Pitton de Tournefort.

        Andrea Caesalpino (1519–1603) in his De plantis libri XVI (1583) proposed the first methodical arrangement of plants. On the basis of the structure of trunk and fructification he divided plants into fifteen "higher genera".

        John Ray (1627–1705) was an English naturalist who published important works on plants, animals, and natural theology. The approach he took to the classification of plants in his Historia Plantarum was an important step towards modern taxonomy. Ray rejected the system of dichotomous division by which species were classified according to a pre-conceived, either/or type system, and instead classified plants according to similarities and differences that emerged from observation.

        Both Caesalpino and Ray used traditional plant names and thus, the name of a plant did not reflect its taxonomic position (e.g. even though the apple and the peach belonged to different "higher genera" of John Ray's methodus, both retained their traditional names Malus and Malus Persica respectively). A further step was taken by Rivinus and Pitton de Tournefort who made genus a distinct rank within taxonomic hierarchy and introduced the practice of naming the plants according to their genera.

        Augustus Quirinus Rivinus (1652–1723), in his classification of plants based on the characters of the flower, introduced the category of order (corresponding to the "higher" genera of John Ray and Andrea Caesalpino). He was the first to abolish the ancient division of plants into herbs and trees and insisted that the true method of division should be based on the parts of the fructification alone. Rivinus extensively used dichotomous keys to define both orders and genera. His method of naming plant species resembled that of Joseph Pitton de Tournefort. The names of all plants belonging to the same genus should begin with the same word (generic name). In the genera containing more than one species the first species was named with generic name only, while the second, etc were named with a combination of the generic name and a modifier (differentia specifica).

        Joseph Pitton de Tournefort (1656–1708) introduced an even more sophisticated hierarchy of class, section, genus, and species. He was the first to use consistently the uniformly composed species names that consisted of a generic name and a many-worded diagnostic phrase differentia specifica. Unlike Rivinus, he used differentiae with all species of polytypic genera.

Modern System

Linnaean taxonomy

        Carolus Linnaeus' great work, the Systema Naturae (1st ed. 1735), ran through twelve editions during his lifetime. In this work, nature was divided into three kingdoms: mineral, vegetable and animal. Linnaeus used five ranks: class, order, genus, species, and variety.

        He abandoned long descriptive names of classes and orders and two-word generic names (e. g. Bursa pastoris) still used by his immediate predecessors (Rivinus and Pitton de Tournefort) and replaced them with single-word names, provided genera with detailed diagnoses (characteres naturales), and reduced numerous varieties to their species, thus saving botany from the chaos of new forms produced by horticulturalists.

        Linnaeus is best known for his introduction of the method still used to formulate the scientific name of every species. Before Linnaeus, long many-worded names (composed of a generic name and a differentia specifica) had been used, but as these names gave a description of the species, they were not fixed. In his Philosophia Botanica (1751) Linnaeus took every effort to improve the composition and reduce the length of the many-worded names by abolishing unnecessary rhetorics, introducing new descriptive terms and defining their meaning with an unprecedented precision. In the late 1740s Linnaeus began to use a parallel system of naming species with nomina trivialia. Nomen triviale, a trivial name, was a single- or two-word epithet placed on the margin of the page next to the many-worded "scientific" name. The only rules Linnaeus applied to them was that the trivial names should be short, unique within a given genus, and that they should not be changed. Linnaeus consistently applied nomina trivialia to the species of plants in Species Plantarum (1st edn. 1753) and to the species of animals in the 10th edition of Systema Naturae (1758).

        By consistently using these specific epithets, Linnaeus separated nomenclature from taxonomy. Even though the parallel use of nomina trivialia and many-worded descriptive names continued until late in the eighteenth century, it was gradually replaced by the practice of using shorter proper names combined of the generic name and the trivial name of the species. In the nineteenth century, this new practice was codified in the first Rules and Laws of Nomenclature, and the 1st editionn. of Species Plantarum and the 10th edition. of Systema Naturae were chosen as starting points for the Botanical and Zoological Nomenclature respectively. This convention for naming species is referred to as binomial nomenclature.

        Today, nomenclature is regulated by Nomenclature Codes, which allows names divided into taxonomic ranks.

Evolutionary

        Whereas Linnaeus classified for ease of identification, it is now generally accepted that classification should reflect the Darwinian principle of common descent.

        Since the 1960s a trend called cladistic taxonomy (or cladistics or cladism) has emerged, arranging taxa in an evolutionary tree. If a taxon includes all the descendants of some ancestral form, it is called monophyletic, as opposed to paraphyletic. Other groups are called polyphyletic.

        A new formal code of nomenclature, the International Code of Phylogenetic Nomenclature, or PhyloCode for short, is currently under development, intended to deal with names of clades. Linnaean ranks will be optional under the PhyloCode, which is intended to coexist with the current, rank-based codes.

        Domains are a relatively new grouping. The three-domain system was first invented in 1990, but not generally accepted until later. Now, the majority of biologists accept the domain system, but a large minority use the five-kingdom method. One main characteristic of the three-domain method is the separation of Archaea and Bacteria, previously grouped into the single kingdom Bacteria (a kingdom also sometimes called Monera). Consequently, the three domains of life are conceptualized as Archaea, Bacteria, and Eukaryota (comprising the nuclei-bearing eukaryotes). A small minority of scientists add Archaea as a sixth kingdom, but do not accept the domain method.

        Thomas Cavalier-Smith, who has published extensively on the classification of protists, has recently proposed that the Neomura, the clade that groups together the Archaea and Eukarya, would have evolved from Bacteria, more precisely from Actinobacteria.


       There are 8 main taxonomic ranks: domain, kingdom, phylum, class, order, family, genus, species.

       There are slightly different ranks for zoology and for botany, including subdivisions such as tribe.

       The name of any taxon may be followed by the "authority" for the name, that is, the name of the author who first published a valid description of it. These names are frequently abbreviated: the abbreviation "L." is universally accepted for Linnaeus, and in botany there is a regulated list of standard abbreviations (see list of botanists by author abbreviation). The system for assigning authorities is slightly different in different branches of biology: see author citation (botany) and author citation (zoology). However, it is standard that if a name or placement has been changed since the original description, the first authority's name is placed in parentheses and the authority for the new name or placement may be placed after it (usually only in botany).

Marine mammals -- Dolphins

Dolphins are marine animals which live in water. They are vertebrate animals and are almost forty species of dolphin in seventeen genera. They vary in size from 1.2m and 40kg, up to 9.5m and 10 tones. They are mostly found at shallower seas of the continental shelves. Besides that, they are carnivores and mostly eating fish and squid. Dolphins are among the most intelligent animals and have made them popular in human culture. The dolphins are social living in pods of up to a dozen individuals. Individuals communicate using a variety of clicks, whistles and other vocalizations. Dolphins are also been known to kill porpoises for reasons which are not fully understood, as porpoises generally do not share the same food with dolphins and are therefore not competitors of food supplies. Dolphins occasionally leap above the water surface. Generally, dolphins sleep with only one brain hemisphere in slow-wave sleep at a time. Besides of humans, dolphins also have a few natural enemies such as larger species of sharks or dolphins.
        Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They vary in size from 1.2 m (4 ft) and 40 kg (90 lb) (Maui's Dolphin), up to 9.5 m (30 ft) and 10 tonnes (9.8 LT; 11 ST) (the Orca or Killer Whale). They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating fish and squid. The family Delphinidae is the largest in the Cetacean order, and relatively recent: dolphins evolved about ten million years ago, during the Miocene.

        Dolphins are among the most intelligent animals and their often friendly appearance and seemingly playful attitude have made them popular in human culture.
There are 32 different species of oceanic dolphins, 5 species of river dolphins, and 6 species of porpoises. The lifespan of bottlenose dolphins varies greatly between regions and populations. The average range is approximately 35-55 years, depending on the population..

        The name dolphin is originally from Ancient Greek δελφίς (delphís; "dolphin"), which was related to the Greek δελφύς (delphys; "womb"). The animal's name can therefore be interpreted as meaning "a 'fish' with a womb". The name was transmitted via the Latin delphinus, Middle Latin dolfinus and the Old French daulphin, which reintroduced the ph into the word.

        The word is used in a few different ways. It can mean:
• Any member of the family Delphinidae (oceanic dolphins),
• Any member of the families Delphinidae and Platanistoidea (oceanic and river dolphins),
• Any member of the suborder Odontoceti (toothed whales; these include the above families and some others),
• Used casually as a synonym for Bottlenose Dolphin, the most common and familiar species of dolphin.

        Porpoises are belong to suborder Odontoceti, family Phocoenidae. Orcas and some closely related species belong to the Delphinidae family and therefore qualify as dolphins, even though they are called whales in common language. A group of dolphins is called a "school" or a "pod". Male dolphins are called "bulls", females "cows" and young dolphins are called "calves".

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Bottlenose Dolphin

Bottlenose Dolphin

Bottlenose Dolphin can be divided into two species: Common Bottlenose Dolphin or Indo-Pacific Bottlenose dolphin. The specific name of Common Bottlenose Dolphin is Tursiops truncates, while the Indo-Pacific Bottlenose Dolphin is Tursiops aduncus. Bottlenose Dolphins lives in groups called pods that typically number about 15 dolphins. They eat mainly of forage fish. There is a blowhole on their head. Adults range in length between 2 and 4 meters and in weight between 150 and 650 kilograms. Newborn Bottlenose Dolphins range in 0.8 and 1.4 meters and in weight between 9 and 30 kilograms. The Common Bottlenose Dolphins live in mostly tropical to temperate countries, while the Indo-Pacific Dolphins live in around India, northern Australia, South China The eastern coast of Africa.

Common Dolphin

Common Dolphin

Common Dolphins can be divided into two species: Short-beaked Common Dolphin or Long-beaked Common Dolphin. The specific name of short-beaked Common Dolphin is Delphinus delphis, while the long-beaked Common Dolphin is Delphinus Capensis. Common Dolphins travel in groups of around 10-50 and are frequently gather in schools that are about 100-2000 individuals. Besides that, Common Dolphins face a mixture of treats due to the human influence such as water pollution. Common Dolphins are also not common in captivity. They live in pacific oceans and can be found at northern Japan, California, and Australia.

Commerson's Dolphin

Commerson's Dolphin

The Commerson’s Dolphin can be divided into two species: Skunk Dolphin and Piebald Dolphin. The specific name of this dolphin is Cephalorhynchus commersonii. This species is distributed in two locations. The larger population is near Falkland Island while the second is near the Kerguelen Islands. The Commerson’s Dolphin is very active. It spins ad twists as it swims. This dolphin feeds on a mix of coastal and pelagic fish and squid. Females reach breeding age at six to nine years. Males reach sexual maturity at about the same age.

Dusky Dolphin

Dusky Dolphin

The Dusky Dolphin is a highly gregarious and acrobatic dolphin found in coastal waters in Southern Hemisphere. The scientific name of the Dusky Dolphin is Lagenorhynchus obscurus. The largest Dusky Dolphin is 210cm in length and 100kg in mass. The population of these dolphins is unknown. These dolphins are first discovered by John Edward Gray in 1825. Their throat and belly are white. They can move over great distances at about 1440km. However it is not believed that they follow a migatory pattern.

Amazon River Dolphin

Amazon River Dolphin

The Amazon River Dolphins can be divided into four groups. The specific name of these dolphins is Inia geoffrensis. They can be found at freshwater around Brazil, Bolivia, Peru, Ecuador and Colombia. They are able to turn their necks at 180 degrees. The Amazon River Dolphin has about 100 peg-like front teeth for catching prey and it eats crabs, turtles, catfish and other fish. They are also qualified as endangered animals. The pink Amazon River Dolphins are bigger than the grey Amazon River Dolphins. The brain of the river dolphin is 40% bigger than human’s brain.

Spinner Dolphin

The Spinner Dolphin is sometimes refers to the long-snouted dolphin. These dolphins can be divided into four species: Eastern Spinner Dolphin(S.I.orientalis), Central American Spinner Dolphin (S.I.centroamericana), Gray’s Spinner Dolphin (S.I.longirostris) and Dwarf Spinner Dolphin (S.I.roseiventris).Adults vary in length from 129-235 centimeters and weight from 23-78kilograms. Females reach maturity at four to seven years while males require seven to ten years. Spinner Dolphin occurs in deep tropical waters in all of the world’s tropical oceans. They eat fish, jellyfish, krill, squid, shell-less snails and copepods. They communicate using click-whistles and pulsed sounds

Taxonomic Classification for Dolphins

Chinese River Dolphin
Taxonomic Classification
Kingdom :Animilia
Phylum :Chordata
Class :Mammalia
Subclass:Eutheria
Order :Cetacea
Suborder:Odontoceti
Family :Lipotidae
Genus :Lipotes
Species :L. vexillifer

Bottlenose Dolphin
Taxonomic Classification
Kingdom : Animilia(Animals)
Phylum : Chordata(Chordates)
Class : Mammalia(Mammals)
Subclass: Eutheria
Order : Cetacea(Cetaceans)
Suborder: Odontoceti(Odontocetes)
Family : Delphinidae(Marine Dolphins)
Genus : Tursiops
Species : truncatus

        Most of the dolphins including Tursiops truncatus (bottlenose dolphins) are cetaceans. The order Cetacea includes whales, dolphins, and porpoises. Cetaceans are the mammals best adapted to aquatic life. The Order Cetacea is further divided into two suborders, Odontoceti (toothed whales) and Mysticeti (baleen whales). Mysticetes do not have teeth but have baleen plates suspended from the roof of their mouth and have two nasal openings (blowholes). Odontocetes (which include bottlenose dolphins) have anywhere from 2-250 teeth and a single nasal opening.

The Boto, or Amazon River Dolphin
• Suborder Odontoceti, toothed whales
o 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, Lissiodelphis 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 Cephalorynchus
 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 Lagenorhyncus
 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

o Superfamily Platanistoidea
 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
 Family Iniidae
 Amazon River Dolphin (or Boto), Inia geoffrensis
 Family Lipotidae
 Chinese River Dolphin (or Baiji), Lipotes vexillifer (possibly extinct, since December 2006)
 Family Pontoporiidae
 La Plata Dolphin (or Franciscana), Pontoporia blainvillei

         Six species in the family Delphinidae are commonly called "whales" but genetically are dolphins. They are sometimes called blackfish.
• Melon-headed Whale, Peponocephala electra
• Killer Whale (Orca), Orcinus orca
• Pygmy Killer Whale, Feresa attenuata
• False Killer Whale, Pseudorca crassidens
• Long-finned Pilot Whale, Globicephala melas
• Short-finned Pilot Whale, Globicephala macrorhynchus

Hybrid dolphins

Wolphin Kawili'Kai at the Sea Life Park in Hawaii.

        In 1933, three strange dolphins beached off the Irish coast; they appeared to be hybrids between Risso's and Bottlenose Dolphins. This mating was later repeated in captivity producing a hybrid calf. In captivity, a Bottlenose Dolphin and a Rough-toothed Dolphin produced hybrid offspring. A Common-Bottlenose hybrid lives at SeaWorld California Other dolphin hybrids live in captivity around the world or have been reported in the wild, such as a Bottlenose-Atlantic Spotted hybrid. The best known hybrid is the Wolphin, a False Killer Whale-Bottlenose Dolphin hybrid. The Wolphin is a fertile hybrid. Two Wolphins currently live at the Sea Life Park in Hawaii; the first was born in 1985 from a male False Killer Whale and a female Bottlenose. Wolphins have also been observed in the wild.

Anatomy of Dolphins

The Anatomy of a Dolphin showing its skeleton, major organs, tail, and body shape

Evolution of cetaceans


        Dolphins, along with whales and porpoises, are descendants of terrestrial mammals, most likely of the Artiodactyl order. The ancestors of the modern day dolphins entered the water roughly fifty million years ago, in the Eocene epoch.

        Hind Limb Buds on Dolphins. An embryo of a Spotted Dolphin in the fifth week of development. The hind limbs are present as small bumps (hind limb buds) near the base of the tail. The pin is approximately 2.5 cm (1.0 in) long.

        Modern dolphin skeletons have 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 a more pronounced development of these vestigial hind limbs.

        Dolphins have a streamlined fusiform body, adapted for fast swimming. The tail fin, called the fluke, is used for propulsion, while the pectoral fins together with the entire tail section provide directional control. The dorsal fin, in those species that have one, provides stability while swimming.

        Though it varies per species, basic coloration patterns are shades of grey usually with a lighter underside, often with lines and patches of different hue and contrast.
The head contains the melon, a round organ used for echolocation. In many species, elongated jaws form a distinct beak; species such as the Bottlenose have a curved mouth which looks like a fixed smile. Some species have up to 250 teeth. Dolphins breathe through a blowhole on top of their head. The trachea is anterior to the brain. The dolphin brain is large and highly complex and is different in structure from that of most land mammals.

        Unlike most mammals, dolphins do not have hair, except for a few hairs around the tip of their rostrum which they lose shortly before or after birth. The only exception to this is the Boto river dolphin, which has persistent small hairs on the rostrum.
Dolphin’s reproductive organs are located on the underside of the body. Males have two slits, one concealing the penis and one further behind for the anus. The female has one genital slit, housing the vagina and the anus. A mammary slit is positioned on either side of the female's genital slit.

         A dolphin’s dorsal fin has two, main functions: stability and thermoregulation. The fin acts similarly to the keel of a sailboat, helping the dolphin swim through the water in a straight line. The dorsal fin is primarily composed of cartilaginous tissue and lacks blubber. Therefore, it is often used as a “thermal window”, allowing dolphins to dump excess heat to their environment when their internal body temperature gets too high.

        Like a human fingerprint, there are no two dorsal fins that are exactly identical. Each has its own unique shape, height, thickness, and notches. Scientists utilize the appearance of dorsal fins to identify individuals in the wild. Many populations are extensively catalogued by dorsal fin photographs, which allow scientists to study specific groups of resident dolphins for many years.

Senses

        
Most dolphins have acute eyesight, both in and out of the water, and they can hear frequencies ten times or more above the upper limit of adult human hearing. Though they have a small ear opening on each side of their head, it is believed that hearing underwater is also if not exclusively done with the lower jaw, which conducts sound to the middle ear via a fat-filled cavity in the lower jaw bone.

        Hearing is also used for echolocation, which all dolphins have. It is believed that dolphin teeth function as an antenna to receive incoming sound and to pinpoint the exact location of an object. The dolphin's sense of touch is also well-developed, with free nerve endings densely packed in the skin, especially around the snout, pectoral fins and genital area. However, dolphins lack an olfactory nerve and lobes and thus are believed to have no sense of smell. They do have a sense of taste and show preferences for certain kinds of fish. Since dolphins spend most of their time below the surface, tasting the water could function like smelling, in that substances in the water can signal the presence of objects that are not in the dolphin’s mouth.

        Though most dolphins do not have hair, they do have hair follicles that may perform some sensory function.The small hairs on the rostrum of the Boto river dolphin are believed to function as a tactile sense possibly to compensate for the Boto's poor eyesight.

Behaviour

A pod of Indo-Pacific Bottlenose Dolphins in the Red Sea

        Dolphins are often regarded as one of Earth's most intelligent animals, though it is hard to say just how intelligent. Comparing species' relative intelligence is complicated by differences in sensory apparatus, response modes, and nature of cognition. Furthermore, the difficulty and expense of experimental work with large aquatic animals has so far prevented some tests and limited sample size and rigor in others. Compared to many other species however, dolphin behavior has been studied extensively, both in captivity and in the wild. See cetacean intelligence for more details.

Social behavior

Dolphins surfing at Snapper Rocks, Queensland, Australia

        Dolphins are social, living in pods of up to a dozen individuals. In places with a high abundance of food, pods can merge temporarily, forming a superpod; such groupings may exceed 1,000 dolphins. Individuals communicate using a variety of clicks, whistles and other vocalizations. They make ultrasonic sounds for echolocation. Membership in pods is not rigid; interchange is common. However, dolphins can establish strong social bonds. Dolphins will stay with injured or ill individuals, even helping them to breathe by bringing them to the surface if needed. This altruism does not appear to be limited to their own species however. The dolphin Moko in New Zealand has been observed guiding a female Pygmy Sperm Whale together with her calf out of shallow water where they had stranded several times.They have also been seen protecting swimmers from sharks by swimming circles around the swimmers or charging the sharks to make them go away.

        Dolphins also display culture, something long believed to be unique to humans (and possibly other primate species). In May 2005, a discovery in Australia found Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) teaching their young to use tools. They cover their snouts with sponges to protect them while foraging. This knowledge is mostly transferred by mothers to daughters, unlike simian primates, where knowledge is generally passed on to both sexes. Using sponges as mouth protection is a learned behavior. Another learned behavior was discovered among river dolphins in Brazil, where some male dolphins use weeds and sticks as part of a sexual display.

        Dolphins engage in acts of aggression towards each other. The older a male dolphin is, the more likely his body is to be covered with bite scars. Male dolphins engage in such acts of aggression apparently for the same reasons as humans: disputes between companions and competition for females. Acts of aggression can become so intense that targeted dolphins sometimes go into exile as a result of losing a fight.

        Male Bottlenose Dolphins have been known to engage in infanticide. Dolphins have also been known to kill porpoises for reasons which are not fully understood, as porpoises generally do not share the same diet as dolphins and are therefore not competitors for food supplies.

Reproduction and sexuality

        Dolphin copulation happens belly to belly and though many species engage in lengthy foreplay, the actual act is usually brief, but may be repeated several times within a short timespan. The gestation period varies per species; for the small Tucuxi dolphin, this period is around 11 to 12 months, while for the Orca the gestation period is around 17 months. They usually become sexually active at a young age, even before reaching sexual maturity. The age of sexual maturity varies by species and gender.

        Dolphins are known to have sex for reasons other than reproduction, sometimes also engaging in homosexual behavior.Various species sometimes engage in sexual behavior including copulation with other dolphin species. Sexual encounters may be violent, with male dolphins sometimes showing aggressive behavior towards both females and other males.Occasionally, dolphins behave sexually towards other animals, including humans.

Feeding

        Various methods of feeding exist among and within species, some apparently exclusive to a single population. Fish and squid are the main food, but the False Killer Whale and the Killer Whale also feed on other marine mammals.

        One common feeding method is herding, where a pod squeezes a school of fish into a small volume, known as a bait ball. Individual members then take turns plowing through the ball, feeding on the stunned fish. Coralling is a method where dolphins chase fish into shallow water to more easily catch them. In South Carolina, the Atlantic Bottlenose Dolphin takes this further with strand feeding, driving prey onto mud banks for easy access. In some places, Orcas come to the beach to capture sea lions. Some species also whack fish with their fluke, stunning them and sometimes knocking them out of the water.

        Reports of cooperative human-dolphin fishing date back to the ancient Roman author and natural philosopher Pliny the Elder. A modern human-dolphin partnership currently operates in Laguna, Santa Catarina, Brazil. Here, dolphins drive fish towards fishermen waiting along the shore and signal the men to cast their nets. The dolphins’ reward is the fish that escape the nets.

Vocalizations

Pacific White-Sided Dolphins breaching

        Dolphins are capable of making a broad range of sounds using nasal airsacs located just below the blowhole. Roughly three categories of sounds can be identified: frequency modulated whistles, burst-pulsed sounds and clicks. Dolphins communicate with their whistles and burst-pulsed sounds, though the nature and extent of that ability is not known. At least some dolphin species can identify themselves using a signature whistle. The clicks are directional and are for echolocation, often occurring in a short series called a click train. The click rate increases when approaching an object of interest. Dolphin echolocation clicks are amongst the loudest sounds made by marine animals.

Jumping and playing

        Dolphins occasionally leap above the water surface, sometimes performing acrobatic figures (e.g. the Spinner Dolphin). Scientists are not certain about the purpose(s) of the acrobatics. Possibilities include locating schools of fish by looking at above-water signs like feeding birds, communicating with other dolphins, dislodging parasites or simple amusement.

        Play is an important part of dolphin culture. Dolphins play with seaweed and play-fight with other dolphins. At times they harass other local creatures, like seabirds and turtles. Dolphins enjoy riding waves and frequently surf coastal swells and the bow waves of boats, at times “leaping” between the dual bow waves of a moving catamaran. Occasionally, they playfully interact with swimmers.

Sleeping

        Generally, dolphins sleep with only one brain hemisphere in slow-wave sleep at a time, thus maintaining enough consciousness to breathe and to watch for possible predators and other threats. Earlier sleep stages can occur simultaneously in both hemispheres. In captivity, dolphins seemingly enter a fully asleep state where both eyes are closed and there is no response to mild external stimuli. Respiration is automatic; a tail kick reflex keeps the blowhole above the water if necessary.

        Anesthetized dolphins initially show a tail kick reflex.Though a similar state has been observed with wild Sperm Whales, it is not known if dolphins in the wild reach this state. The Indus river dolphin has a different sleep method from other dolphin species. Living in water with strong currents and potentially dangerous floating debris, it must swim continuously to avoid injury. As a result, this species sleeps in very short bursts which last between 4 and 60 sec

Natural Threats

       Except for humans (discussed below), dolphins have few natural enemies. Some species or specific populations have none, making them apex predators. For most smaller species, only a few larger species of shark such as the bull shark, dusky shark, tiger shark and great white shark are a potential risk, especially for calves. Some of the larger dolphin species such as Orcas may also prey on some of the smaller species, but this seems rare. Dolphins also suffer from a wide variety of diseases and parasites

Human Treats

     Dead Atlantic White-Sided Dolphins in Hvalba on the Faroe Islands, killed in a drive hunt


          Some dolphin species face an uncertain future, especially some river dolphin species such as the Amazon River Dolphin, and the Ganges and Yangtze River Dolphin, which are critically or seriously endangered. A 2006 survey found no individuals of the Yangtze River Dolphin, which now appears to be functionally extinct.

         Pesticides, heavy metals, plastics, and other industrial and agricultural pollutants that do not disintegrate rapidly in the environment concentrate in predators including dolphin from their prey. Injuries or deaths due to collisions with boats, especially their propellers, are also common.

         Various fishing methods, most notably purse seine fishing for tuna and the use of drift and gill nets, unintentionally kill many dolphins. Accidental by-catch in gill nets and incidental captures in antipredator nets that protect marine fish farms are common and pose a risk for mainly local dolphin populations.In some parts of the world such as Taiji in Japan and the Faroe Islands, dolphins are traditionally considered as food, and killed in harpoon or drive hunts. Dolphin meat is high in mercury and may thus pose a health danger to humans when consumed.
Dolphin safe labels attempt to reassure consumers that fish and other marine products have been caught in a dolphin-friendly way.

         Loud underwater noises, for example resulting from naval sonar use, live firing exercises or certain offshore construction projects such as wind farms may be harmful to dolphins, increasing stress, damaging hearing and causing decompression sickness by forcing them to surface too quickly to escape the noise.