General notes: Dolphins are a diverse group of animals, ranging from the small Māui’s dolphins (1.7 m) to large Orcas (8 m). While most have a general dolphin-shaped body, the shape of the head, pectoral fins, tail flukes and dorsal fin can vary enormously across the different species. In some species the dorsal fin is even absent. Their color patterns also vary across the 43 known species. As such the notes below will discuss general dolphin biology but species specific information can be accessed through our Marine Species Database.
Brief overview: Dolphin Anatomy video
Rostrum: The elongate mouth of dolphins is known as the rostrum. The rostrum contains interlocking conical teeth which are well suited to catching prey such as fish and squid. The number of teeth varies between species and they retain a single set of teeth for their whole lives. Dolphins lay down a new layer of enamel on their teeth each year which leaves growth rings that can be counted to determine an individual’s age. The gap at the front of the rostrum where we might expect to see the front teeth allows infant dolphins to suckle. Some species of dolphin use their rostrums to probe in substrates for food. The lower jaw contains an enlarged foramen which supports a fatty pad. This fatty pad is used to receive sounds, transmitting vibrations to the middle ear and aiding the dolphin in hearing underwater. Additionally, dolphins’ teeth act like antennae, resonating in response to sounds. As such they play an important part in both dolphin communication and echolocation. The size and shape of the rostrum varies between species – in some, such as the bottlenose dolphin, it is pronounced while in others, such as orcas, it can barely be seen. The Amazonian river dolphin has sensory whiskers on its rostrum to help it detect food in turbid river waters.
Melon: The bulge above the rostrum and eyes that precedes the blowhole is called the melon. The melon is a fatty structure on the forehead that is used to help amplify and project sounds as well as helping to detect sounds. It facilitates dolphin communication and echolocation.
Blowhole: Nasal drift during evolution has resulted in dolphins’ nasal opening being located on top of their heads which allows them to breathe while swimming or resting underwater with their blowhole above the surface. Dolphins have a single nostril which has a crescent-shaped muscular flap and can seal shut to prevent water entering the respiratory system. A complex network of nerve endings detect changes in pressure around the blowhole allowing dolphins to interpret when their blowhole is above the surface so that they can breathe. Inexperienced new-born dolphins which have not yet developed the ability to interpret these pressure signals from their blowhole must instead raise their heads out of the water to breathe safely. When these young dolphins drop back down onto the surface of the water the resulting splash is known as a ‘chin slap’. When dolphins surface to breathe they first exhale in an explosive fashion which results in an audible ‘blow’ and visible spray as water resting on the blowhole, vapor in the exhaled gas and mucus from the respiratory system are expelled into the air. As well as breathing, the blowhole is used for vocalizations. On the surface air is released through the blowhole to generate sound – making use of the musculature of the blowhole to change the size and the shape of the opening and thereby alter the sounds produced. Underwater the dolphin uses the nasal sacs located inside the blowhole to produce sounds (see internal anatomy below).
Eyes: Dolphins’ eyes are located on the sides of their head and can be moved independently – giving them two fields of vision. This allows dolphins to have up to a 300 degree field of vision though they can position themselves to overlap their fields of vision in order to generate bi-scopic 3D vision. Due to the rostrum and melon obscuring the forward field their best option for overlapping their fields of vision is by looking downwards with both eyes. This is why dolphins sometimes position themselves with their rostrums pointed upward, looking at you ventrally in order to get a better picture of how you look.
The eyeballs themselves are adapted to underwater vision. The part of the eyeball we can see, the cornea, is flattened and does not refract light like ours do. Instead they have a spherical lens which they can move backwards and forwards in the eye allowing them to see clearly above and below water. In water light attenuates quickly and when diving dolphin eyes must quickly adapt to changes in illumination. To this end dolphins have unusual irises with a flap that hangs down from the top. This flap, called the operculum, can be raised in low light conditions giving the appearance of a circular iris. However, as light levels increase the operculum can be lowered to partially cover the iris and reduce the amount of light entering the eye. As the operculum is lowered it gives the iris a distinctive horseshoe or U shape. When fully lowered the operculum extends from the top of the iris to the bottom of the iris leaving a small opening to either side. This effectively looks like two small irises. Their eyes are further adapted for low illuminations with the tapetum lucidum, a reflective layer, located behind the retina. Extracellular collagen fibrils and the multiple reflections of light from these layers of fibrils significantly increase the amount of light hitting the retina and thereby drastically improve their vision in low light conditions.
While their eyes are highly adapted (allowing them to view the world above and below the water as well as compensating for shifts in illumination and low illumination) it is likely they have poor, if any, color vision. Color vision is controlled by photoreceptors called cone cells in the retina which are sensitive to specific spectrums or colors of light. Humans have three different cone cells – red, blue and green. The combination of these three types of cone cell allow us to see in color. By contrast dolphins only have the green cone cells, an evolutionary adaptation that is likely to have happened when cetaceans were a shallow water coastal species, as the loss of the blue cone cells is not beneficial to animals living in deeper water where blue light travels deepest. This means that dolphins are effectively color blind, seeing only in a greenish-blue spectrum if any color at all.
The visual acuity of dolphins does vary between species but generally they can see up to a distance of 150 ft.
Ears: The external ears of dolphins are gone with the remaining small openings barely visible behind their eyes. These earholes are relics that no longer serve a function in dolphins, in fact the ear canal itself is blocked with organic material such as fibrous tissue and ear wax. Instead the dolphin detects sound using its jaw and melon.
Flanks: The flanks are the sides of the dolphin and in many dolphins these display distinctive markings that can be used to help identify the species.
Pectoral fins: The pectoral fins are modified webbed hands where the individual digits are no longer visible. These pectoral fins are used to steer dolphin as it moves through the water. Dolphins can regulate the flow of blood to their pectoral fins in order to reduce heat loss in cold conditions. The size and shape of pectoral fins varies across the different species of dolphins.
Dorsal fin: The dorsal fin helps to stabilize the dolphin’s movement – reducing roll and yaw to give the dolphin greater control. While the size and shape of the dorsal fin vary across the different species of dolphin some lack a visible dorsal fin altogether.
Dorsal ridge: The dorsal ridge is a firm crest which is more prominent in some cetacean species. It is thought they are useful to whales such as belugas to help break up ice at the surface.
Peduncle: The peduncle starts behind the dorsal fin and is the muscular region that powers dolphin movement. The muscles of the peduncle provide the up and down motion of the tail.
Tail flukes: Dolphin tails flatten out to form the two tail flukes which are separated by the median notch. The movement of these fins through the water generates the forward thrust which dolphins use to swim.
Anus & genital openings: Dolphins have a series of slits on the ventral surface (underside) near the tail. These slits house the reproductive organs and anus. In male dolphins a long slit houses the genitals and a small slit behind it is the anus. In female dolphins the genitals and anus share one long slit with the anus located behind the genitals. Female dolphins also have two more slits, one to either side of the genital-anal slit, which house the mammary glands.
Full dolphin necropsy: Dolphin Necropsy video
Skin: Dolphin skin is adapted to aquatic life in a number of ways. The hair follicles and their associated glands have been lost. The epidermis is extremely smooth and relatively thick varying from 1-3 mm depending on the area of the body. The epidermis is very compact and the layers are difficult to discern clearly. Within the epidermis there are deep ridges (40-50% of the epidermal thickness) which connect it with the dermis – a layer containing blood vessels, nerves and free cells. The dermis is connected to an extremely thick hypodermis filled with fat cells. The outermost layer of the skin of dolphins rapidly sheds skin cells, up to 12 times a day, which allows it to maintain a smooth, self-cleaning surface.
Blubber: Dolphin blubber is a complex, active spongy layer consisting of a collagen fiber matrix or frame filled with fatty adipose tissue. Blubber covers the entire body of dolphins except for their pectoral fins, dorsal fins and tail flukes. This specialized layer serves a number of important functions including thermoregulation, buoyancy, energy storage, increasing swimming efficiency and healing.
Blubber can be up to 93% fat, since the conductivity of lipids is a third of that of water this makes blubber an effective insulator. Because blubber generally has a lower density than seawater it generates buoyancy helping dolphins to conserve energy. Blubber also acts as an important energy store that can help to sustain dolphins between meals if necessary. The blubber is not uniform in structure – the blubber around the mid-section is quite elastic while the blubber around the end of the tail (near the flukes) is three times stiffer. The elastic mid-section and stiff tail work like a spring with the tail ‘bouncing back’ from the extreme of each upward and downward stroke. By acting like a spring, the blubber can both increase power and conserve energy. Blubber also plays a key role in recovering from injuries. Blubber contains organohalogens which have antimicrobial and antibacterial properties which prevent both infection and decomposition around injuries. At the same time dolphins’ ability to restrict blood flow through the blubber layer, which aids thermal regulation particularly when diving, reduces blood loss from injuries.
The thickness of the blubber layer varies with species, those in colder climates tend to have thicker layers than those in warmer climates.
Nervous system: Dolphin brains vary enormously between species with the Ganges River dolphin having a 200 g brain and Orcas having a 5600 g brain. While some differences relate to body size the body brain ratio differs too with some much smaller species having brains that are larger relative to their body size. Bottlenose dolphins, for instance, have brains weighing between 1200 and 2000 g (human brains weigh between 1000 g and 1800 g).
Dolphin brains are structured differently to ours. Their neuronal morphology is poorly differentiated with fewer specialized cells, their neuron to glial cell ratio is lower and some structures such as a distinct prefrontal cortex have not been identified. However, dolphins have a complex neocortex which is responsible for problem-solving and self-awareness. They have Von Economo neurons which are linked to emotions and social cognition as well as other spindle neurons which are linked to recognizing, remembering, reasoning, communicating, perceiving, adapting to change, problem-solving and understanding. Dolphin brains have a high ratio of glial cells to neurons. These glial cells provide structural and nutritional support for neurons as well modulating and coordinating neural activity. Dolphins also have a specific region of the brain dedicated to echolocation.
The structure of dolphin brains not only supports complex social interaction and echolocation but also the rapid processing of auditory and visual information. In fact, it seems that dolphin vision is attuned to movement which makes sense in an animal which rarely remains static and has echolocation to scan its environment.
The spinal cord is almost cylindrical in shape and extends from the neck to the flukes providing nerves to all the areas between including the pelvic and pectoral girdles, the fins and the genitals. The dorsal somatosensory roots (detecting pressure, pain and warmth) are thin relative to the ventral somatomotor roots (controlling movement). The dorsal white columns and dorsal horns of the grey matter are reduced in dolphins and part of this, the substantia gelatinosa, is almost missing if not absent. These reduced areas are associated with internal and external nociception (sensing pain). The reduction in somatosensory reception and nociception may correlate in reduction of the body surface and simplification of the extremities with the body becoming torpedo like fusiform in shape, the hind-limbs being lost and the fore-limbs becoming fins. This may also explain how dolphins seem able to ignore significant bite wounds to the soft tissue from sharks. Conversely the well-developed somatomotor roots match the extremely muscular swimming body of dolphins.
Respiratory and circulatory systems: Dolphins breathe air like terrestrial mammals but their respiratory system is adapted to aquatic life. While breathing is automatic in terrestrial mammals, breathing in dolphins is voluntary – meaning they must actively choose to breathe. This adaptation prevents them from accidentally taking water into the lungs. The respiratory system is paired with the circulatory system to optimize breathing, gas exchange and gas storage for swimming and diving. A complex network of nerves around the blowhole allow the dolphin to sense when it is clear of the water and able to breathe. Exhaling forces any water pooled on the blowhole out of the way, clearing it for breathing in air. If the dolphin needs to clear mucus or other matter out of the airways it can forcefully exhale to expel these in exhalations known as ‘chuffs’ or ‘honks’. Exhaling and inhaling takes just 0.3 seconds, air is drawn into the blowhole, down the larynx and along the trachea to the bronchioles of the lungs. Typically dolphins breathe 1.5 to 4 times per minute. In one single breath a dolphin is able to replace up to 95% of the air in its lungs (humans can exchange around 65%). In the lungs gaseous exchange occurs across the walls of the alveoli as with other mammals but with greater efficiency, removing up to 80% of the oxygen from each breath. This is facilitated by higher volumes of blood (around 10 – 15% of adult body weight) and higher concentrations of red blood cells and hemoglobin within the blood. Dolphins are able to store oxygen within their muscle tissue and release oxygen rich blood from the muscles to oxygenate the internal organs. This is important when dolphins dive. When diving, dolphins can experience considerable pressure changes. To accommodate this dolphins’ lungs can collapse alongside specially hinged ribs. As the pressure increases, the air in the lungs is compressed and the volume reduced. The alveoli collapse as the air is forced out of them, once the air has escaped the lungs up into the trachea, muscular sphincters at the end of the bronchioles squeeze shut to keep gases within the trachea and larynx. The trachea and larynx are reinforced with irregular intertwined cartilaginous rings which prevent these from collapsing. It should also be noted that the upper respiratory tract is also where the nasal sacs are located. These nasal sacs are used to generate vocalizations underwater. Air from the nasal sacs is expelled into the nasal passages where it passes across the phonic lips – creating the range of sounds used to communicate and echolocate. Pulmonary surfactants in the alveoli likely help to lubricate the lungs and facilitate their reopening after collapsing. The heart, which is four-chambered like ours, can slow from 100 beats per minute to 10 beats per minute during diving and dolphins can hold their breaths for up to 7 minutes (though there are some rare records of 15 minutes). The change in blood pressure caused by these changes in heart rate would likely lead to a human having a stroke. However, dolphins have special tissue called the retia mirabilia underneath their rib cage – between the blowhole and the dorsal fin. This tissue is a dense mass of blood vessels. Arteries in the dolphin feed into the retia instead of directly into the brain. This allows the retia to act as a buffer – preventing surges or dips in the blood flow and maintaining a constant controlled flow of blood to the brain.
The size of the lungs varies between the different species of dolphins. However, it seems that shallow water species who perform shorter shallower dives may have larger lungs which more often remain partially open during diving and can act as the primary site for gaseous exchange.
Digestive system: Dolphin teeth are conical and well suited to capturing prey but are unable to masticate (chew) the food. Instead, when dolphins capture a prey item they expel the water from their mouths and swallow their food whole. The elastic esophagus assists in the swallowing of whole food, transporting it down to the stomach. Dolphin stomachs have three distinct chambers. The esophagus opens into the saccular forestomach. The forestomach is lined by stratified squamous epithelium which help to protect the forestomach from abrasions. The presence of a forestomach allows dolphins to store food ready for digestion which in turn allows them to feed more infrequently. The forestomach connects to the second, globular main stomach through a small opening. The main stomach has a mucous membrane with mucous, parietal and chief cells. The mucous cells produce the protective mucosal lining of the stomach, the parietal cells help to produce the stomach acid which is around 1.5 to 3 pH (depending on the dolphin species) while the chief cells produce the digestive enzymes. As such the main stomach is the main site of digestion. The partially digested food passes through a narrow channel controlled by a sphincter into the pyloric stomach where the dolphin finishes digesting the food. Dolphin intestines lack external subdivisions with minimal changes in its diameter or external appearance. There is no caecum (the blind pouch found at the beginning of large intestine), vermiform appendix (the tube that hangs down from the caecum) or discernible large intestine. Structures such as the duodenum can be seen because of their connection the stomach and the pancreatic (pancreas) and hepatic (liver) ducts. Indeed, most areas of the intestine can only be identified in situ when other organs, ligaments and connective tissues can be used to discern the different parts of the intestine. This simplified intestine, which is similar to the human small intestine, is related to the dolphin’s diet of primarily animal protein. Water is extracted from the food as are nutrients but the simplified structure of the intestines – particularly the lack of any bowel suggests that dolphins do not store any undigested residue. Instead, fecal matter is expelled as a liquid which is normally seen as a cloud of matter in the water.
Renal System: Dolphin renal systems allow them to survive in a broad range of salinities and to cope with systemic hypoxia during diving. The kidneys are highly reniculated with each kidney consisting of many renules or lobes, each one with its own cortical tissue and medullary pyramid acts as an individual miniature kidney. While reniculation should allow dolphins to filter out large amounts of salt they are relatively poor at concentrating urine and it is possible that reniculation actually evolved as an adaptation to increased body size and diving. Hormonal regulation and the rate of urine formation play a more important role in osmoregulation. Dolphin kidneys have glycogen reservoirs, concentrated bundles of medullary blood vessels and a layer of elastic fibers and smooth muscle (sporta perimedullaris musculosa) which separate the cortex from the medulla. The enlarged glycogen reservoirs allow anaerobic glycosis of the kidney cells in the hypoxic conditions experienced when diving. The concentrated bundles of medullary blood vessels may work in a similar manner to the retia mirabilia (which maintains constant blood flow to the brain during diving) and maintain the metabolic demands of the kidneys during diving though the renal system. Since the renal system is suppressed as part of the diving reflex maintaining the metabolic demand of the kidneys is important because it protects them against reperfusion, free radicals and the cytoxic effects of concentrated urea. The sporta perimedullaris musculosa is thought to support the structure of the renicules preventing them and their blood vessels from collapsing under pressure during diving activity.
Dolphins have small bladders and as such urinate frequently.
Reproductive system: At a basic level dolphin reproductive systems are similar to our own. Male dolphins have penises and testes and they produce sperm. Female dolphins have vaginas, uteruses and ovaries and produce eggs. However, dolphin genitals are tucked away inside the genital slit of males and genital-anal slit of females. The size and shape of dolphin genitals varies enormously across species. Male dolphin penises range in length from something almost as long as an arm to a penis as long as a table in killer whales. Female dolphins have vaginas with twists and folds which may help to keep seawater out of uterus and be used by females to prevent fertilization by unwanted sperm. Potentially this means that by altering their position during copulation females may be able to misdirect the penis and prevent reproduction. They also have large well-developed clitorises just inside the vagina and may well experience pleasure during intercourse.
As mentioned, male dolphin genitals are kept internally, including their testicles. Since the higher temperatures inside the body would inhibit sperm production dolphins have evolved a network of blood vessels around the testes that lead to the extremities of the pectoral fins and tail flukes thereby drawing heat away from the testicles. The penis and the ability to ejaculate are also adapted in dolphins. Dolphins do not need stimuli to become erect and ejaculation is voluntary, allowing dolphins to ejaculate rapidly when necessary.
Female dolphins have a bicornate uterus – separating into two horns that can each act as a uterus. They can switch between using their right or left uterus and ovaries, which they seem to do as they age. Dolphins are not thought to have a reproductive cycle but may instead spontaneously ovulate. Annular folds on the cervix and a mucosal plug between these folds protect deposited sperm from seawater. In order to prevent calves overheating in utero the uterus has a network of blood vessels which draw heat away by carrying blood to the extremities such as the pectoral fins and tail flukes.
Skeletal system: Dolphin skeletal structure is adapted in a number of ways. Their skulls are compact and have a parabolic dish shape. This accommodates the space needed for the melon as well as assisting echolocation. The bones of the skull are thick which allow for strong muscular attachment. The arms are modified into pectoral fins with a static elbow joint though the bones of the arm are analogous with those in human arms. Their spinal cords are thick and long with more vertebrae than ours and help to support their muscular frames. They have hinged ribs for diving and reduced hind-limbs and a reduced pelvic girdle. The pelvic girdle has become an attachment point for the muscles that control the penis.
Dolphins are all carnivorous and occupy a high trophic niche feeding primarily on fish but also opportunistically feeding on squid and in some cases crustaceans. The largest dolphins, orcas, are also known to feed on whale calves, large sharks, pinnipeds and penguins.
Specific ecologies vary between the different species but also within species across different populations. For instance some orcas are known to specialize in feeding on large sharks, attacking their fins or turning them over to induce tonic immobility before feeding on the oil-rich livers while others will beach themselves to hunt sea lions. Needless to say that dolphins have adapted to feed on a wide range of prey in a wide range of habitats and represent an important predator. As apex or near apex predators they play an important role in asserting top-down control on ecosystems. Smaller species of dolphins are also a prey item for larger species of shark such as great whites, tiger and bull sharks.
Beyond this, dolphins are important to the feeding habits of other animals in the oceans. When orcas hunt gray whale calves they typically consume only the oil rich tongue which leaves a sizeable carcass for other marine fauna to scavenge on as well as a whale-fall for deep sea benthic communities. Similarly, discarded white shark and six gill carcasses also provide scavenging opportunities for other marine life. It should also be noted that dolphin hunting behavior can provide opportunities for fish and seabirds. When dolphin pods hunt down a school of fish they will often herd them into tightly-grouped bait balls and force these close to the surface. These compacted schools of fish offer an easier meal for large predatory fish such as tuna and sharks as well as a wide range of seabirds. In one particularly interesting example a population of bottlenose dolphins off the coast of Laguna, Brazil have learned to cooperatively fish with humans.