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Dolphin fossils have been found dating back to 45-50 million years ago, which was during the early Eocene epoch era. The early dolphin or Protosetidae, back then did not resemble today’s dolphin. It is suggested that a primitive mammal called Mesonychidae roamed the planes of Africa. This mammal started to become more dependent on the ocean and swamps abundance of food. Researchers believe the dolphin went through an amphibious stage, similar to the seals. They would feed in the ocean and come upon land to breed. Roughly 30 million years ago the early dolphin split into two main species, the toothed whale and the baleen whale. The group called Delphinidae is where the modern day dolphin arose. These fossils date back to 25 million years ago. Modern day dolphins live in rivers and oceans throughout the world. There are 33 known species of ocean dolphins and five river dolphins. The bottle-nosed dolphin inhibits mainly temperate and tropical waters, in the Pacific, Atlantic, Indian ocean and Mediterranean Sea. They normally stay close to shore in warmer water, but some stay offshore in the cooler waters. Dolphins migrate due to the movement of food supply and the change in water temperature. The size of a dolphin varies from 3.9 feet to 32 feet long and 110 pounds to 18,000 pounds. The tucuxi dolphin is the smallest known dolphin. This dolphin frequents and has been spotted 1250 miles up the Amazon River. Bottle-nosed dolphins average 8 feet and 500 pounds. The color of the dolphin ranges from black, white, gray, tan to even pink. Some have strips. Dolphins have a well-developed sense of hearing. The auditory nerve may have 67,900 or more cochlear fibers, which is twice as many as the human. They can detect sound frequencies of less than 1kHz if they are loud enough. The dolphin’s sense of sight is also very good. A dolphin can see particularly well in the water. The sense of touch of a dolphin is also well developed. The dolphin has limited sense of smell. There is not a lot known about a dolphin’s sense of taste. They do have taste buds, though they have not been studied enough to come to any conclusions. Dolphins do show preferences toward different types of fish though. They eat 4% up to 33% of their weight of fish, squid and shrimp daily. Dolphins feed by chasing fish onto mud banks and snapping them up while they are beached or by herding prey into clusters. Sometimes dolphins also follow shrimp boats to eat what was thrown overboard. Dolphins don’t chew their food they just swallow it whole. Dolphins swim up to 19 miles per hour with bursts of up to 35 miles per hour. The tail of the dolphin moves in an up and down motion for movement with the flippers to stabilize. They swim in schools. Bottle-nosed dolphins have few members, under twenty. Others species have up to thousands of members in their school. Dolphins can dive up to 1795 feet. Bottle-nosed dolphins rarely dive more than 150 feet. They can dive for up to ten minutes. They slow their heartbeat down while diving, which helps them to stay down longer. Dolphin’s breath threw a blowhole at the top of their head. Every two minutes they make a short, explosive inhalation and then a longer inhalation before submerging again. They exhale just before reaching the surface, which is what makes the little splash in the water. Exhaling and inhaling takes the dolphin approximately .3 seconds. Dolphins have to be conscious to breath. This means that they cannot go into a full deep sleep, because then they would suffocate. Dolphins have "solved" that by letting one half of their brain sleep at a time. This has been determined by doing EEG studies on dolphins. Dolphins sleep about 8 hours a day in this fashion. REM (Rapid Eye Movement) sleep, usually associated with dreaming has been recorded only very rarely. Some scientists claim dolphins do not have REM sleep at all. Dolphins reach sexual maturity at different ages. Males become sexually mature at 10 to 12 years, while females become sexually mature at 5 to 10 years. This is similar to the human. The female initiates courtship and breeding. The gestation period is roughly 12 months. Calves are born throughout the year. A dolphin can have a calf every two years, but normally wait three. Calves are born in the water. The calf is normally 42 to 52 inches long and 44 pounds, defering between species. The mother nurses the calf for up to 18 months. The calf normally stays with its mother for three to six years, or possibly longer. Sound and communication is one of the most important aspects of the dolphin. Dolphins identify themselves with a signature whistle. A mother dolphin will whistle to her calf over and over again so the young can identify the mother. Whistles are single-toned squeals that come from deeper in the larynx then clicks. Whistles communicate alarm, sexual excitement and other emotional states. Sound varies in volume, wavelength, frequency and pattern. The frequency of sound ranges from .25 to 150 kHz. The lower frequency is used mainly for social communication and take more energy. The higher frequency is used for the echolocation system. Dolphins produce clicks that resemble moans, trills, grunts, squeaks, or creaking doors. Clicks last about 50 to 128 microseconds. Clicks pass through the melon which consists of lipids(fats). The melon then acts as an acoustic lens to focus these sound waves into beams. Sound waves travel through the water at 1.5 km/sec which is four times faster than sound traveling through air. Sound waves bounce off objects and return in the form of an echo. The sound is received and counducted through the lower jaw to middle ear, inner ear and then hearing in the center of the brain. Brain receives sound waves in the from of nerve impulses and relay messages of sound. This enables the dolphin to interpret the sounds meaning. This system is known as the echolocation system which is the ability that dolphins possess to “see” with their ears by listening for echoes. They can determine size, shape, speed, distance and direction. A dolphin’s average life span is 25 to 65 years. A dolphin dies for many reasons. One is the variety of diseases and parasites that a dolphin can transmit. Some of these are tapeworm, roundworm, and viral and bacterial infections. Morbillivirus is also a huge killer of the dolphin. Sharks also eat dolphins. Rare, but occasionally killer whales eat dolphins as well. Humans have a huge impact on the death of dolphins. They are taken in for food, leather, oil and fertilizer uses. The river dolphins are the majority of dolphins becoming extinct. The River dolphins frequent the rivers in Asia and South America. These dolphins are affected from our pollution and habitat destruction. Fisherman also catch dolphins in their nets. The tuna industry has killed as many as 6 to 12 million dolphins in the eastern Pacific in the last 30 years. Dolphins have been studied for quite some time to discover there intelligence level. One test done by Pieter Arend Folkens was to teach the dolphin to pick up trash in his cage. If he brought the trainer a piece of trash, he would get a fish. Every day the dolphin would be able to find trash in his tank. The trainers went down below the tank, only to discover that the dolphin was stashing trash in the corner of the tank. The dolphin would also only bring up a ripped off piece of the trash to receive his reward. “This behavior is particulary interesting because it shows that the dolphin had a sense of the future and delayed gratification. He had enough presence to realize that a big piece of trash got the same reward as a small piece, so why not deliver only small pieces to keep the extra food coming? He in effect had trained the humans.” Another experiment done by Dr. Javis Bastian found that dolphins oculd communicate about abstract ideas. Two dolphins were trained to push different switches to earn a fish. That was simply, but then they had to go in sequence. First one dolphin would hit the switch and then the other dolphin would hit the same switch. Finally, the separated the dolphins so they could only communicate what to do through sound. Dr. Bastian concluded that dolphins can communicate abstract ideas, left and right. Dolphins are amazing animals. Society needs to become more aware of the risks that dolphins face each and every day so my kids, and my kids’ kids can experience dolphins in nature like I have had the chance to. Bibliography: Dolphin Evolution. 3 May 2001. Dolphin Species. 18 May 2001. Donoghue, Michael, and Annie Wheeler. Save the Dolphins. Dobbs Ferry, New York: Sheridan House. 1990 Word Count: 1526
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