Anatomy

Dorsal Fin       

A dolphin's dorsal fin is made up of cartilage. The fin is supported not by skeleton, but by tough fibrous tissue inside it. A dorsal fin can be as distinctive as a human's face and is used by scientists to identify individuals. Dolphins that have dorsal fins seem to use them for stability.

Melon 

This is the fatty, rounded section of a dolphin's forehead. It is critical to its extremely sensitive and complex use of echolocation. The melon acts as an acoustical lens to focus sound waves into a beam that is projected in front of the animal. Dolphins seem to wear a permanent smile, but this is deceptive since the head, like the rest of the body carries significant blubber under the skin. Blubber prevents major muscles of the face reaching the surface. Therefore, whales and dolphins are capable of only a limited range of facial expressions.

Ear

The ears are visible only as tiny holes on the side of the head. The ear hole in a bottlenose dolphin is about 5-6cm behind the eye and is only 2-3 mm in diameter.

Eye     

Light travels slower in water than in air. Therefore, eyes adapted for focusing in air loses their focusing power in water. Humans have overcome the problem by wearing facemasks to keep our eyes in air when underwater. Dolphins however, have elastic lenses on their eyes that expand and contract to let them focus both above and below the water. The lenses and the cornea in a dolphin's eyes naturally auto-correct for the difference in refraction of light underwater and that above the surface. If a dolphin's eyes didn't have this built-in ability, they would be near-sighted every time he popped his head out of the water.   They also have strong muscles around the eye that can change the actual shape of the lens to suit either water or air vision. Cetaceans also have large pupils so that they can collect large amounts of light in even low-lit areas of water, and in bright areas the pupil can be closed right down to a small slit.  When whales and dolphins open their eyes underwater, they have special glands that produce greasy tears protect them from the stinging salt.

Dolphins are able to see beside them, in front of them and even behind them because their eyes are placed laterally, one eye on each side of the head. Unlike human eyes, which act together to view things in their range of vision, a dolphin's eyes act independently. One pupil may be staring intently at something while the other pupil is relaxed and constricted.

Teeth  

Like most mammals, newborn dolphin teeth are still embedded in the gums. The teeth will start erupting in the first 5 weeks of life. Although all dolphins are equipped with teeth not all species use their teeth for eating prey or tearing flesh. Depending on the dolphin species some dolphins (such as the killer whale) will use their teeth to rip apart the flesh of their prey in order to kill it and make it easier to consume while other species (such as the bottlenose dolphin) only use their teeth for grasping slippery prey as they don't chew their food, but swallow smaller fish whole. They may need to break up a large fish into smaller pieces before eating it. They achieve this by tossing the fish about, or knocking it against an object in the water.

The number of teeth a dolphin has can vary greatly depending on its species. Dolphins can have anywhere from a few dozen to over 250 teeth at a given time and some species (such as Risso’s dolphin) may only have teeth that are present on the lower jaw.

Unlike most land mammals toothed whales grow only one set of teeth their "milk" teeth, these are retained throughout their lives and in some older animals they can be very well worn. Instead of having different teeth types they only have one kind - in most this is a simple peg like structure with a single root. There is a little variation in teeth of different species, which is usually due to their diet or feeding strategy. For example, river dolphins have somewhat flatter teeth for crushing shell fish and porpoises have sharper edged teeth for slicing prey that is too large to be swallowed whole. However, in beaked whales that eat almost exclusively squid, (which are slippery), the female's teeth never emerge and the males only have one pair.

In toothed whales it is possible to determine the age of an individual by examining the alternate dark and light layers of dentine visible in teeth (rather like rings in a tree) inside their teeth. Broadly speaking, one complete layer is equal to one year of growth.

Blowhole        

The blowhole serves as a dolphin's nostrils, allowing it to breathe while swimming at top speed. Generally while asleep, female dolphins lie on the water's surface expose their blowholes to the air to breathe, Males however sleep just below the surface and a reflex action periodically raises their bodies. Toothed whales have only one blowhole but baleen whales have two blowholes.

Stomach

Like cows, dolphins have two stomachs split into three chambers.  Dolphins usually swallow their food hole. The first chamber or fore stomach stores the food where it can also be regurgitated. This first chamber grinds up and compresses the whole food into smaller pieces. The second is for digesting, digestive juices and gastric fluid is released and the digestion process starts. The third chamber absorbs the nutrients.

Dolphin skulls

Aspects of the internal skeleton vary greatly. The skull is different from that of the normal mammal structure as it has been ‘telescoped’ (compressed from front to back so that certain parts overlap each other). Dolphins with prominent beaks have elongated skulls with as many as 130 conical teeth in each jaw. However, dolphins without prominent beaks have shorter jaws and hence less teeth.