The body of the bottlenose dolphin is highly streamlined, torpedo-shaped, and well adapted for living the marine environment.They have a number of physical and physiological features, which reflect these adaptations.
An adult bottlenose dolphin and grow up to 4 m long and weigh up to 650 kg. There are large differences between bottlenose dolphinpopulations in different areas. For instance, the dolphins living in the coastal waters of Northeast America (the Northwestern Atlantic), especially Florida, are on average 2.5-2.6 m long and weigh up to 250 kg. The more oceanic (pelagic) bottlenose dolphins in the same area are generally somewhat larger. These different types of bottlenose
dolphins are called eco-types.
In the Northwest Atlantic there is a coastal and a pelagic eco-type.
These eco-types also have physiological differences: for instance the blood composition is different, probably reflecting the need to dive deeper in the pelagic type. The dolphins in the North Sea, the Northeast Atlantic and the Pacific Oceans are larger than the Northwest Atlantic dolphins.
---Skin and colour
The dorsal side of the body is usually dark grey in colour. The ventral side is whitish or slightly pink. In some areas bottlenose dolphins can be slightly spotted ventrally. The dark dorsal side and light ventral side are an example of counter-shading, which helps the dolphins blend in with their environment.
Seen from above the water, the dark back against the dark water makes them difficult to spot and also from under water, their light bellies blend in with the light sky above. This makes it more difficult for predators to see them and they can also approach their prey more easily without being seen. The extend of the grey colour and the presence of some stripes and bands, especially in the head area, are unique for each individual, although differences can be subtle.
The dolphin skin has a rubbery feel to it. It feels rather smooth, because the top layer of cornified cells is very thin whereas the epidermis itself is relatively thick. (The human skin has a much thicker cornified top layer). Also, the dolphin skin sloughs easily, so that a lot of cells are lost every day. This is compensated for by a rapid growth rate of the epidermis.
If the skin is damaged, dark scar tissue is formed, which later turns white. These scars may remain visible for a very long time. Below the skin, there is a massive storage of fat: the blubber layer. This blubber layer has a number of functions: it acts as an energy storage, it helps insulate the body and prevent heat loss to the cold water and it helps the dolphin maintain its streamlined body shape.
The skeleton of whales and dolphins has changed from a land mammal layout to one adapted for life at sea. Because whales and dolphins in water are essentially weightless, the bones do not have to carry a lot of weight and are therefore lighter than those of land mammals. The rib cage is rather flexible. Those ribs that are attached to the breastbone have an elastic connection. This way the thorax can collapse under high pressure without damage when a whale or dolphin dives deep.
The hind limbs have all but disappeared. Internally, there are still a few rudimentary bones, but externally, no hind limbs are visible. The front limbs have been flattened and shortened into pectoral fins or flippers. The neck is shortened because some of the neck vertebrae fused together. The vertebrae in the tail stock area have well-developed chevron bones, to which the large dorsal muscles are attached.
The bottlenose dolphin has a well-defined beak. The lower jaw extends slightly beyond the upper jaw. The corners of the mouth are turned upwards, giving the dolphin a fixed, permanent "smile". They have 18-26 pairs of sharp, conical teeth in each jaw. All teeth have more or less the same shape. A dolphin does not chew its food and therefore has no use for molars. The teeth are used to grab the often slippery prey. Dolphin teeth are permanent. Unlike many other mammals, dolphins do not have milk-teeth they need to change later in life.
The bulbous forehead of the dolphin is formed mainly by a large fatty body, called the melon. This melon sits in front of the skull and is connected to a system of air sacs. It plays a role in echolocation (see the echolocation section).
The skull of dolphins is slightly asymmetrical, especially in the area around the nasal passage. If you look closely, you can see that the blowhole of dolphin is not placed exactly on the length axis of the body.
The blowhole is what the nasal opening is in other mammals. Baleen whales have a double blowhole, just like humans have 2 nostrils. In toothed whales, there is only one blowhole. In
dolphins, the blowhole is crescent-shaped. When a dolphin or whale breathes, it opens the blowhole and exhales powerfully. This will clear away any water which may be on the blowhole. Immediately afterwards, the whale inhales and closes the blowhole. Once the blowhole is closed, the whale can safely dive under water. The "lid" of the blowhole closes tightly and needs to be opened actively with a muscle. Directly below the blowhole there is a system of air sacs, which play a role in sound production. Via the larynx, the air sac system is connected to the windpipe and the
lungs. The larynx passes through the esophagus, so that the airways and the esophagus are completely separated, making it almost impossible for water or food to get into the lungs through the mouth.
An arrow pointing at dolphin's ear
The eyes are located on the side of the head, close to the corners of the mouth. A tear duct is present in the forward corner of the eye. Dolphin tears are very viscous are secreted in large quantities. Their main function is probably protection and lubrication of the cornea. The tears contain slightly less salt than blood, which in turn contains less salt than the surrounding sea water. See Senses for more information about the eye. An arrow pointing at dolphin's ear.The small pinholes behind the eye are the dolphin's ear openings. A dolphin has no auricles. See Senses for more information about the ear and hearing.
---Pectoral fins or flippers
The dolphin's front limbs are the flippers or pectoral fins. These fins are made of collagen-rich connective tissue and they have bones which are similar to those in the bones in the front limbs of land mammals, but they are shortened. The joint that is comparable to the elbow is located at the body surface. The humerus (upper arm) has a large globular head where it meets the scapula in the shoulder joint.The carpal bones are recognisable and there are 5 digits (although these cannot be distinguished externally).
The flippers act as stabilisers and as rudders: they are used for steering, while swimming and, together with the tail flukes, can act as effective brakes. The flippers are also used to establish brief bodily contact in social interactions between dolphins.
The flippers contain a number of blood vessel. These vessel come rather close to the surface and play a role in thermoregulation. When the body core heats up, more blood is pumped through these veins, so that heat can be transferred to the colder sea water. If the body temperature drops, the blood flow through these vessels is reduced or even shut down.
The tail of the dolphin consists of 2 blades, with a clear notch between the blades. The tail flukes contain no bones, but are made of collagen-rich connective tissue. The tail flukes are horizontal. They are the main source of propulsion. When swimming the tail flukes are moved up and down, unlike a fish's tail, which moves from side to side. The tail flukes are connected to some groups of powerful muscles in the back and the belly of the dolphin. With just a few flicks of the tail, the dolphin can accelerate to a speed high enough to lift its entire body out of the water.
The dolphin's fast cruising speed (a travelling speed they can maintain for quite a while) is about 3-3.5 m/s (11 - 12.5 km/hr). They can reach speeds of up to 4.6 m/s (9.3 knots, 16.5 km/hr) while
travelling in this fashion. When they move faster, they will start jumping clear of the water (porpoising). They are actually saving energy by jumping. When chased by a speedboat, dolphins have been clocked at speeds of 7.3 m/s (26.3 km/hr), which they maintained for about 1500 meters, leaping constantly. The most efficient travelling speed for dolphins is between 1.67 and 2.27 m/s (6.0-8.2 km/hr). There have been reports of dolphins travelling at much higher speeds, but these refer to dolphins being pushed along by the bow wave of a speeding boat. They were getting a free ride.
Apart from its function in swimming, the tail flukes also play a role in thermoregulation, like the flippers. The tail flukes contain a number of large blood vessels.
Not all dolphin species have a dorsal fin. Some species have a small dorsal ridge only and others, like the killer whale have a very large dorsal fin. The bottlenose dolphin has a falcate (hook-shaped) dorsal fin. The main purpose of the dorsal fin is probably that of a stabiliser,like the lee-board on a boat.
Like the tail flukes, the dorsal fin is made of connective tissue. It also contains some blood vessels and plays a role in thermoregulation, as do the flippers and the tail flukes.
---Physiological adaptations to life in the sea
A dolphin's normal body temperature is like that of other mammals, around 37 C. The sea water in which dolphins live is much colder. Depending on the region where they live, the water can be anywhere from 5 to 25 C. Water also absorbs heat much faster than air so a dolphin needs to be well insulated to prevent excessive heat loss.
A dolphin's main protection against heat loss is the thick blubber layer. This fatty tissue is a very poor heat conductor and thus it helps keep the heat inside the body. In addition, the body surface area in dolphins is relatively small, because of their torpedo-shape and the lack of large extremities. Blood vessels do not come close to the skin, except on the pectoral, dorsal and tail fins. In these extremities, blood flow can be regulated to prevent both hyper- and hypothermia.
The metabolic rate in dolphins is higher than in land mammals. Also the long intervals between breaths may help reduce heat loss, because cold air comes in contact with the blood stream in the lungs less frequently.