Anatomy of Dolphins

## WEDNESDAY, JANUARY 20, 2010.

**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.

Posted by Liau Sheau Chang at 2:29 PM.