Dive Dry with Dr. Bill

083: Common Dolphin

Common dolphins are found in all tropical and warm temperate waters. This is the species known in ancient Greece and Rome. There are actually two types found in our waters. The long-beaked common dolphin is found most frequently in coastal waters while the short-beaked common dolphin is more common offshore. These animals may form herds of hundreds and sometimes thousands of individuals, with some reports of hundreds of thousands on occasion.

This dolphin is more colorful than the bottlenose. The back is dark-gray to black. A distinctive hourglass pattern on the side changes from light gray at the rear to a yellow-tan near the head. They are sometimes called the hourglass or crisscross dolphin. The belly is white. Large dark circles surrounding the eyes are connected by a dark line across the head behind the beak. This species may reach 8 1/2 feet and nearly 300 pounds making them one of the smaller species. Females are slightly smaller than males. The dorsal fin is triangular or sickle-shaped (curved).

Squid, octopus and small schooling fish like anchovies, sardines and bonito are their primary food. Each jaw is lined with 80-120 sharp teeth that are curved slightly backward to prevent food from escaping once caught. Common dolphins in a group all feed at the same time, day or night. They may work together to herd fish into tighter schools to make feeding easier, and sometimes rise from below driving the fish into the air where they catch them. A United Nations study indicated that off the California coast dolphins eat about 300,000 tons of anchovies while commercial fishermen catch about 110,000 tons. Common dolphins will also eat fish escaping from fishing nets or discarded by fishers. Their enemies include sharks and killer whales.

Sexual maturity is at 3-4 years and about 6-7 feet. Courtship occurs in the spring and fall. Courting involves rubbing one another's body with the flippers and swimming along side one another. The male may rush the female, then veer away at the last moment. Actual mating occurs belly to belly (a modified missionary position?) and is rather brief (too bad).

Calves are carried for 10-11 months and are 30-34" and 25-35 pounds at birth (which occurs tail-first). Although single births are the norm, twins and even triplets have been found. Calves lack lips to suckle so, like many marine mammals, the mother squirts milk into its mouth. Dolphin milk has six times the protein content of human milk and is high in fat. It would probably make an interesting beer milkshake to celebrate Ed "Doc" Ricketts' birthday. The young nurse for about 18 months but will also take some solid food after six months. They have an estimated longevity of 35-40 years.

Dolphins will care for one another. Sick individuals are buoyed to the surface by others so they can breathe. They are also known to rescue humans in the water. They appear to display emotion when separated from one another or when reunited after such separations. Mothers may take turns caring for the young, and the adults all teach them and even discipline them when they do wrong. They navigate and communicate using whistle-like vocalizations.

These active marine mammals are very interesting to watch. The dolphins may leap out of the water, and do flips or somersaults. They frequently change course to ride the bow and stern wakes of fast-moving boats including those crossing our channel. It is believed this behavior started when dolphins rode the bow wakes of large whales. Like other toothed whales, dolphins have a single blowhole on top of the head. They surface several times a minute to breathe.

With all this activity the dolphin needs some time to rest. Since they cannot check in to the local Hilton for the night, they rest one side of the brain for 5-10 minutes and later the other. During a day, each side rests 3-4 hours. It is estimated that their intelligence is between that of a dog and a chimpanzee. I think its higher since they spend all their time in the ocean, my favorite place to be!

Some countries (Turkey, Russia, Japan) have actively sought the common dolphin for fish meal, oil or their meat. The population in the Black Sea was severely depleted by the fishery there. Conservationists have also worked to stop their take as incidental catch by tuna purse seiners in the eastern tropical Pacific which may reach 8,000 animals a year.

© 2004 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Common dolphin skull above larger bottlenose dolphin skull; single blowhole of a dolphin bow riding;
several bow riding the bow of Lindblad's MV Sea Lion; group of common dolphins

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia