Common dolphins (Delphinus delphis) are agile swimmers and jumpers and are often seen riding waves at the bow of ships. They usually travel in schools of a dozen to more than 1,000 individuals. Dolphins are capable of sustained swimming speeds of up to 18 mph (29 kmh). In shorter bursts dolphins can attain a speed of 23 to 25 mph (37 to 40 kmh).
Common dolphins can be found in tropical, subtropical and warm temperate oceanic regions, including the Mediterranean, Red and Black seas. While most dolphin species live in the ocean, there are five species of river dolphin including the Buoto (Amazon, Brazil) and the nearly extinct Baija (Yangtze, China) and Susus (Ganges, India and Bangladesh and Indus, Pakistan).
Although dolphins may look and behave similar to fish, their ancestors had four legs and lived on the land. About 50 million years ago the dolphin's ancestors, which probably looked somewhat like a modern otter, began to adapt to life in the water. The fossil record suggests that the earliest dolphins appeared about 10 million years ago.
Phylogenetic (evolutionary) order:
Dolphins are mammals of the order Cetacea which includes whales, dolphins and porpoise. The order Cetacea is divided into two suborders - the Mysticeti (baleen whales) and Odontoceti (toothed whales). The Odontoceti are divided further into nine families which include the Delphinidae, or oceanic dolphins.
Whale, porpoise or dolphin?
Common word usage tends to confuse the relationships between members of the Cetacea. Both the killer whale (Orca) and the pilot whale are actually dolphins (family Delphinidae). Additionally, the word "porpoise" - frequently used to describe dolphin - refers to the genetically distinct family Phocoenidae which have a blunter snout and more chisel-shaped teeth.
Adult common dolphin females reach 7.8 feet (2.4 m) and males 8.5 feet (2.6 m) in length and weigh 165 to 187 pounds (75 to 85 kg). Calves range in length from 30 to 34 inches (76 to 86 cm).
Common dolphins feed on herring, anchovies, sardines and squid.
The dolphin's body is highly streamlined and well adapted to life in the water. Its rubbery skin covers a dense layer of fat (blubber) that helps these mammals to maintain a high internal temperature of 97.9 degrees to 99.0 degrees F (36.5 degrees to 37.2 degrees C). Like all mammals, dolphins must breath air. A single nostril, or blowhole, situated almost directly on top of the dolphin's head, allows it to breath efficiently while mostly submerged. Dolphins normally come to the surface to breathe about every two minutes, though are capable of staying submerged longer during dives as deep as 1,000 ft (300m).
Nearly all dolphins have a bulging forehead which contains a large mass of fat and oil-containing tissue. This so-called "melon" is an important adaptation that allows dolphins to echolocate (use sonar for orientation) and communicate. The flippers and dorsal bones contain skeletal remnants of five-digit phalanges passed down from their terrestrial ancestors.
Dolphins are popularly characterized as highly intelligent creatures. While scientific studies are inconclusive, recent studies suggest that dolphins are at least as intelligent as dogs and probably more clever than chimpanzees. The dolphin brain shows many characteristics generally associated with intelligence: large size, high surface area/fold density/fissurisation, high neural density and a high ratio of cerebral cortex to motor cortex.
An average human brain is approximately 1550 grams. For comparison, the brain of an average size bottlenose dolphin is 1800 grams. Why the big brain? While not all scientists agree, many believe that a big brain may be required for sonar and sound processing or to meet the demands of social living.
Dolphins navigate using sonar (or "echolocation"). When moving, a dolphin will emit a series of high pitched clicks generated within its nasal sacs. If the sound waves of the clicks hit an underwater object, they will be deflected back toward the dolphin. The dolphin is able to determine its relationship to the object by evaluating the relative strength
of the echo at either side of its head and the time it takes for the sound wave to return to the dolphin.
Dolphins communicate to each other using a high pitched whistle language. Scientists believe that each species has a unique language while intraspecies regional dialects also exist.
Most information about the reproductive cycles of dolphins is based on studies of captive bottle-nose dolphins. Female bottle-nose give birth every four to five years. Attendant females have been observed helping the calf to the surface for its first breath. Young bottle-nose dolphins may nurse for 18 to 20 months and remain with their mothers
for three to six years.