Dolphin’s Anatomy

Anatomical and physiological modifications that enables them to swim, submerge, breathe, search for food, reproduce, etc., in few words, to thrive on an element 800 times more dense than air, soon appeared in the order’s history. The first conditioner was related to the need to keep body heat. Mammals’ internal temperature remains constant between 36 and 40º C according to the species.

They have a high metabolic rate; hot blood implies lots of advantages but also some inconveniences. Water conducts heat better than air, it has a greater calorific capacity; it absorbs energy to limit the loss of calories, wet mammals have to keep at a minimum the body ratio between surface and volume, in other words, they turn into more volume. Besides, they protect themselves with an insulating skin or with a layer of subcutaneous fat.

Hydrodynamism has its own demands; physics laws to improve speed are equal to all objects moving under water. Animals must be fast and swim during long periods of time to find food and escape from their predators.

Internal Anatomy

External Anatomy

In the dolphins, the silhouette is similar to fish; the body is elongated and acquires elegant curve lines of an absolute perfection. Head is rounded, the rostrum behaves as a wheel; limbs are reduced: even genitalia and mammary glands are lost in the general silhouette and are hidden inside the womb. Ears and hair or fur disappear.

Hind limbs are atrophied; in exchange, forelimbs are transformed in small paddles. Propulsion is enhanced with a flat horizontal tail. Dolphins’ anatomy is not different from the rest of marine mammals, they are identical.

Distribution of organs is not different in detail, changes are clearer in the skeleton and in the muscles. Ears are located in the middle of a sinus complex, right and left are pretty much separated between them, in this way it is easier to locate the origin of each sound.

Behaviour

Movement. Not all dolphins move at 30 knots (54 Km/h), some of them may do so during hundreds of meters, as the “mular” or bottlenose dolphin, common dolphin, the “killer whales”, false killer whales or longed finned pilot whales. The majority of the species are propelled for miles at 10 knots (18 Km/h). When slowly moving, as while entertaining, speed varies between 2 and 6 knots. It seems the absolute speed record belongs to the killer whale with 40 knots (74 Km/h).

Besides being more dense and viscous than air and making movement more difficult, the liquid environment is hostile for animals with lungs, which have to get oxygen from the surface. But the ocean offers abundance in food and is unavoidably enticing. In 1936, and again in 1948, biologist James Gray posted in Natures magazine, his famous “dolphins’ paradox”, according to which the potency of the propelling muscles of dolphins and whales do not explain the speeds they reach. As it is known, median potency of the mammal muscle is of 16.5 W/Kg, obtained by calculating the weight of the locomotive muscles and the cetacean’s total mass.

If the animal were any object, it would need 10 times more energy for the same amount of movement. Cetaceans defy physics laws. The truth is, it does not get ahead by rowing its fins: forelimbs with a spatula-shape have a secondary function in propulsion; they serve for vertical and horizontal stabilization, they facilitate turns, slow downs and accelerations, without forgetting they also serve to caress.

Dorsal fin is not present in all the species. This fin may be rounded or triangular. Bottlenose dolphins’ locomotion consists on a harmonious vertical balancing, whereas in the fish this is on a horizontal plane. The animal propels itself thanks to the action of the muscles inserted between de vertebrae. Dolphin starts to raise its tail, maintaining their caudal globes rigid, turned upside down and creating turbulence underneath. A depression under its body is formed, which is projected forwards and up. Open pectoral fins counterbalance the descent, feature that was described by Kenneth S. Norris as a “perfect biological hydroplane”. Caudal globes get relaxed, the tail falls and the thorax goes up due to a low density in the head region where lots of fats are kept. Then, the cycle repeats itself all over again.

Senses

Bottlenose dolphins’ senses give them a great looseness in the blue immensity, besides being able to dive at depths that humans will never reach. Senses are the result of a brilliant genetic “handcraft” of the mammals’ 5 senses, where their key features are: smell atrophy and the hypertrophy of the ear in the form of the echolocation sense.

The Sense of Touch

The sense of touch is not poor either. Their skin is highly specialized and possesses a complex and organized system of nervous ends highly abundant in the regions of greater sensitivity. Their skin is very soft and is easily damaged and healed, with a layer of fat. An aid for efficiency in swimming, since their skin works as a turbulence pressure sensor; if anywhere throughout the body’s surface there should be an excessive pressure, the skin will distort in order to slip the body with ease in the water.

Some species use the mandibular region to detect low frequency vibrations, using pressure receptors, which enables to calculate the speed they need to swim. The contour of the nasal opening is also filled with receptors, which send the signal of the need to emerge and start to breathe, just before reaching the water surface.

In bottlenose dolphins, these tiny hairs are not abundant, but instead, they possess sensorial ends which intensify the sensations on the rostrum. It seems they enjoy rubbing themselves against vessels, rocks and other peers. It seems they like their rostrum to be caressed. Mothers use their pectoral fins to caress their calves. Males explore the bodies of other peers with their penis erected, with no sexual intent.

The senses of Taste and Smell

Taste and smell are the less developed senses. Aerial mammals analyze first the dissolved molecules in the water, then the dissolved ones in the air. Taste is good in bottlenose dolphins, the base of the tongue is constituted by taste buds, chemo-receptors in nature. If we should give a bottlenose dolphin a rotten fish, it would throw it away in distaste.

Dolphins locate in liquids the elements of the basic flavours (sweet, salty, sour and bitter) and a variety of perfumes. They recognize, although diluted, their peers’ debris and maybe they are capable of detecting external hormones. In turn, they have almost lost their capability to analyze the air molecules. Their blowhole opens up too fast and too much so it allows a flux of air so huge that the cells in charge of smell just wouldn’t have enough time to function properly. So they have almost disappeared in bottlenose dolphins, just as the corresponding lobes in their brains.

The sense of Sight

The eye, about which we suspect a great intelligence, is just above the opening of the mouth. The eye is small, it is almost an atrophiated organ. It is surrounded by rigid eyelids, wit no lashes, rich in fat tissue. Out of the water, dolphins seem to be crying... Pupil is a little bit oval, the iris is dark and the lens is almost spherical. The retina has lots of rods, these neurons allows perception of black and white, and are sensible to dim light. But dolphins in captivity seem to like red and yellow. Usually, vision is monocular, the field is stereoscopic, except for the region before the mouth where it is narrowed in the bottlenose dolphin.

Major visual problem for them is due to the difference between air and water’s refraction indexes. Their eyes accommodate to both elements. They do possess potent intraocular muscles capable of modifying the lens shape. They are not bothered by the deformations caused by refraction.

Their eyes accommodate to both elements. They do possess potent intraocular muscles capable of modifying the lens shape. They are not bothered by the deformations caused by refraction.

The Sense of Hearing

The ear constitutes their main source of information, they perceive sounds scattered by the atmosphere. Above all, they use the water’s property of transmittance (5 times faster and farther than air).

They lack external ears, which would disturbe their body’s hydrodynamism. The collection of sounds is ensured by the bones of the skull and the fine fat layers protecting the two portions of the lower jaw. The ear opening opens slowly behind the eye and it is so thin that some anatomists didn’t find it. The auditory canal is thin, almost obstructed and the ear drum is thick. In bottlenose dolphins, this plug does not exist.

Internal ear continues to an enormous auditory nerve and ends in auditory lobes, also hypertrophiated. It is worth to mention that sound images perceived by left and right ears do not mix.

The Magnetic Sense

There is evidence of several organisms with the power to receive sensorial information from the Earth’s magnetic pole to position themselves. There are areas in their brains containing iron oxide crystals, which work as if they had an internal magnetic needle. Terrestrial magnetic head orients them on their long trips. For them, geomagnetic anomalies constitute points of reference that they should memorize. Random variations of this field, provoked ones, may disorient them.