By Dr. Becker
Today is National Dolphin Day!
Dolphins are extremely intelligent marine mammals belonging to a family of toothed whales that also includes orcas and pilot whales. The name “whale” actually identifies several different types of marine mammals, including whales, dolphins, and porpoises. The three groups make up what is known as the cetacean species.
In everyday use, the term whale does not include dolphins or porpoises to insure that the three groups are easily identifiable. All members of the cetacean species, regardless of their suborder, share several physical features, including flippers for swimming, a tail equipped with flukes that help navigate in the water, and blowholes for breathing.
Being mammals, all cetaceans are warm-blooded, breathe air, bear offspring and produce milk.
There Are Around 40 Different Species of Dolphin
Many people assume the familiar bottlenose dolphin is the only dolphin in existence, but there are actually around 40 different species of this aquatic mammal. The different species cover a wide range of sizes. The smallest is called Maui’s Dolphin, which grows to around 4 feet in length and 90 pounds. The largest dolphin is the orca, or killer whale. These big guys average about 25 feet in length and weigh just under 20,000 pounds.
The bottlenose dolphin grows to about 8 feet, weighs between 450 and 640 pounds, and can live to be over 40. Orcas are known to live twice that long.
Dolphins are found worldwide, and live primarily in shallow areas of tropical and temperate oceans. Five freshwater species live in the world's rivers, the Ganges and Indus river dolphin, the Amazon river dolphin, the Araguaian river dolphin, the Bolivian river dolphin, and the Chinese river dolphin, also called the Baiji dolphin, which may be extinct.
Some Dolphins Work with Fishermen to Bring in a Catch
Dolphins are highly social creatures that live in groups, called pods, of from a dozen to up to several hundred. When gathered in a location with an abundant source of food, pods sometimes merge together temporarily, forming superpods of over 1,000 dolphins. Wouldn’t that be a sight to see?
Dolphins are carnivores, dining primarily on fish, squid, and crustaceans, though some of the larger species like the killer whale also feed on other marine mammals.
Dolphins use echolocation to locate prey. They engage in a variety of hunting and feeding methods, including herding schools of fish into “bait balls,” chasing fish into shallow water where it is easier to catch them, and strand feeding in which fish are driven onto the beach. Dolphins will also tag along after seabirds, other whales and fishing boats to snatch up any fish that might get discarded.
Dolphins have also been known to team up with humans to catch fish. In Laguna, Santa Catarina, Brazil, dolphins drive fish toward shore, where fishermen wait to cast their nets. The dolphins actually signal the fishermen when it’s time to cast their nets. What the dolphins get out of the deal are fish that don’t make it into the nets.1
Dolphins Display Lively and Intriguing Social Behavior
Dolphins are agile and playful. Many species leap out of the water seemingly just for fun, and “spy-hop” (rise straight out of the water to have a look around). They also “bow-ride,” swimming alongside ships, probably to conserve energy.
Many dolphins establish strong social attachments and will stay with injured or ill members of the group, helping them to the surface of the water so they can breathe if necessary. There are also reports of dolphins protecting human swimmers from sharks by swimming in circles around them, or rushing the sharks to shoo them away.
On the flip side, dolphins can also be aggressive towards each other. Older males are typically covered with healed bite scars. Male bottlenose dolphins sometimes kill baby dolphins, and dolphins are also known to kill porpoises.
Dolphin communication involves a variety of clicks, whistles, and other vocalizations, along with non-verbal signals like posturing and touch.
Dolphin Reproduction and Sleep Habits
Dolphins mate throughout the year. Length of gestation depends on the species, and ranges from 9 to 17 months. When it’s time to give birth, the female temporarily separates from the pod and often stays near the surface of the water.
Mother dolphins usually have just one calf, and as soon as it is born, mom takes it quickly to the surface so it can take its first breath. Baby dolphins nurse from 1 to 2 years, and after weaning they stay with their mothers until they’re between 3 and 8 years old.
Another interesting fact about dolphins in the wild is that they deep-sleep with only one side of their brain. The other side remains awake to insure they continue to breathe and watch for predators and other threats. However, captive dolphins seem to enter a full sleep state with both eyes closed and no response to external stimuli. Somehow, their respiration is automatic, and a reflexive tail kick keeps the blowhole above the water if necessary.
One species of freshwater dolphin, the Indus river dolphin, sleeps differently than all other species. The waters these dolphins inhabit have strong currents and a lot of floating debris, so they must swim non-stop to avoid being injured. Sleep is accomplished in very short bursts of between 4 and 60 seconds.
Threats to Dolphins
Dolphins face a variety of threats to their existence, and most are the result of human activities. We’ve already lost the Yangtze (Chinese) river dolphin to habitat loss from construction of dams and boat traffic.
The small Maui’s dolphin is on the brink of extinction due to entanglement in fishing gear. Fewer than 100 of these dolphins remain. Two others listed as endangered are the Ganges river and the Indus river dolphins. Like the now-extinct Yangtze river dolphin, other freshwater species are losing their habitat to waterfront development.
Like the Maui’s dolphin, a number of other species are also threatened with extinction from entanglement in commercial and discarded fishing gear. The dolphins drown because once entangled, they can’t reach the surface to breathe.
Some dolphin species are used for food despite the toxic levels of contaminants like heavy metals in their flesh.
Climate change is impacting the oceans of the world, which can have an adverse effect on marine mammals. Changes in ocean temperatures and currents affect prey populations and distribution.
Other threats to dolphins include their capture for use in "swim with the dolphins" programs, ingestion of marine debris, oil and gas spills and subsequent clean-up activity, recreational watercraft, and noise pollution.