Distinguishing Features
The different species vary in size from the tiny Maui dolphin to the huge Orcas or killer whales. The different species can be identified by their shape, colouring and markings and by certain physical features. Though it varies by species, basic colour patterns are shades of grey, usually with a lighter underside and often with contrasting lines and patches.
The head contains the melon, a round organ used for echolocation. In many species, elongated jaws form a distinctive beak; species such as the bottlenose have a curved mouth which looks like it is smiling. They have cone-shaped teeth and as many as 250 teeth in some species. The dolphin brain is large and highly complex, and is different in structure from that of most land mammals.
The main body parts used to identify a species are:
beak - the elongated part of the mouth and jaws
melon - the rounded structure in the top of the dolphin's head just in front of the blowhole
dorsal fin - the fin on the upper side of the body
fluke - one half of the tail
flipper - one of a pair of wide, flat forelimbs that is used for swimming
median notch - the indentation between the 2 flukes
As they are mammals, dolphins are warm-blooded and their internal temperature is around 36°C - about the same as a human's. To conserve this temperature they are surrounded by a thick layer of fat called “blubber” just below the skin. Unlike most mammals, their skin is hairless, thick and lacks glands. It is also kept smooth by constantly being sloughed off and replaced. A bottlenose dolphin for example, replaces its outermost layer of skin every 2 hours.
Dolphins breathe air through lungs just like humans do. When needing to breathe, they rise to the water surface and inhale and exhale out of the blowholes located on top of its head (its nostrils).
They can stay up to 15 minutes under water but usually stay only a few minutes under the surface at a time. Bottlenose dolphins can dive to depths of up to 30 metres the Risso can dive to over 1,000 metres. However, most species are mainly shallow divers as they need to reach the surface to breathe frequently.
Dolphins have a streamlined body adapted for fast swimming. Unlike fish which swim by moving their heads from side-to-side to swing their tail, they propel themselves by moving their tail in a smooth, up-and-down motion. The tail fin 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. They can swim at a rate of between 5 to 12 kilometres an hour.
These mammals have surprisingly good vision in and out of water which is binocular to a certain degree, like a human's. They don't have great colour vision, though; it's comparable to a severely colour-blind person. Some behaviours associated with their vision indicate high specialisation of the 2 sides of the brain, which is associated with intelligence. For instance, they tend to swim in a counterclockwise direction in tanks. Furthermore when presented with new visual stimulation, like new people, they tend to look at them with their right eyes.Behaviour
Dolphins are characteristically very social beings and live in long-lasting groups that range in size from a few animals (2-40) called pods, to larger groups of up to several hundred members, known as schools or herds. Large groups are often mixed in terms of age and sex, but smaller groups are generally one of 3 types:
A nuclear group, comprised of a single adult male and female;
A nursery group, consisting of a number of females and young;
A bachelor group comprised of adult and younger males.
Large groups may consist of more than one species. Spotted and spinner dolphins are found living in such an association. Scientists believe this is possible because spotted dolphins tend to feed on larger aquatic species living near the surface, while spinners tend to feed at night on smaller species found in deeper waters.
Groups often have a hierarchy of power with a few individuals considered dominant. Regardless of the type, all groups seem to have well-developed skills in working together for activities like finding food, defence, reproduction or caring for their young. Dolphins can establish strong social bonds and will stay with injured or ill individuals, even helping them to breathe by bringing them to the surface if needed.
They use sound in a technique called echolocation to find objects, navigate and hunt for fish. This technique uses the same principles as radar. Echolocation is a process where a dolphin emits a steady series of split-second, high frequency sounds or "clicks" through its blowhole. The clicks are pulses of ultrasonic sound (sounds repeated as rapidly as 800 times/second) produced in its nasal passages and focused in the large, lens-shaped organ in the melon. The melon concentrates the sound pulses into a directional beam.
When the outgoing sound waves or clicks bounce off objects in their path, a portion of the signal is reflected back. The bony lower jaw of the dolphin receives the incoming sound waves through special tissue and transmits them to the inner ear where they are converted into nerve impulses and then transmitted to the brain. This allows them to identify objects without having to touch them. They sometimes also use their sonar to stun fish.
Dolphins use very high sounds for echolocation, so they can hear sounds as high as 150 kHz. A human being's hearing ranges from 40 Hz to 20 kHz. They also use sound to communicate and can make a unique signature whistle; this may help individuals recognise each other. They also use sound to collaborate and perform group activities.Feeding Habits
They feed on most kinds of fish, including mullet, whiting, snapper, tuna, bream, and invertebrates such as squid. Their diets in the wild tend to depend on the area in which they're living as well as the time of the year. Some feed exclusively on either fish or cephalopods (the class of marine invertebrates including squid, octopus and cuttlefish), while others have a more varied diet including fish, squid, crabs, shrimps and lobsters.
Dolphins swallow their food whole, without chewing. They use the muscles at the back of their tongue and throat to squeeze the salt water out and the fish down. They tend to take the fish head first, a behaviour researchers believe that keeps the fins and spines folded back. That way a dolphin does not injure its throat.
When foraging for food, dolphins use their echolocation to act as radar and let them know exactly where the fish are. Some researchers also believe that they emit loud noises to stun and confuse the fish, making them easier to catch.
Dolphins usually feed in groups, forming a circle or a U shape and herding the fish, then they simply take turns to swoop through the school of fish, eating as many as they can. Certain species use different and often impressive group hunting techniques. These include:
Intentional beaching - coralling prey into shallow water;
Fish whacking - smacking fish out of the water with their flukes, often into the mouths of their pod mates;
Bubble nets - releasing a cloud of bubbles from their blowholes or creating bubbles through driving their flukes through the surface of the water repeatedly which causes the fish to school within the wall of bubbles;
Mud rings - similar to the above but involves the suspension of mud into the water by stirring up the bottom with their fins;
Marine sponges - used as tools to forage for organisms living in the substrate.
They have also been known to take fish right out of the nets of fishermen!
How much a dolphin actually needs to eat really depends on what it's eating. Certain types of fish for example, have higher fat content than others and consequently provide more energy at a lesser quantity than the fish that have a low fat content. Most research suggests that a dolphin needs to eat approximately 1/3 of its body weight in fish on a daily basis. Adults can eat a fish that weighs up to 5 kg but smaller, younger dolphins tend to eat smaller fish.Reproduction
Dolphins are among the most sexual of animals. They spend a great amount of time throughout their life in sexual play and are known to have sex for reasons other than reproduction; they enjoy it. They don't mate for life and when aroused, a male may mate several times in an hour, often with the same female but not always. They will mate with other dolphins regardless of size, age or family relationship and will sometimes engage in homosexual behaviour.
Sexual encounters may be violent and non-consensual, with males sometimes showing aggressive behavior towards both females and other males. Sexual behavior including copulation with other dolphin species has also been documented. Occasionally, dolphins behave sexually towards other animals, including humans. Even lifeless objects are used for masturbation.
Dolphins' reproductive organs are located on the underside of the body. Males have 2 slits, one concealing the penis and one further behind for the anus. The female has 1 genital slit, housing the vagina and the anus. 2 mammary slits are positioned on either side of the female's genital slit.
The male-female courtship ritual plays a large part in mating. Males make elaborate swimming patterns and calls and sometimes dolphins will court or play for days, swimming together and petting each other with their fins. The actual mating is brief; the male nudges the female from behind with his sex organ for several minutes and then copulation happens belly to belly. Although the act is brief, it may be repeated several times within a short time period. Dolphins copulate during the whole year with up to 50 copulations a day.
They usually become sexually active at a young age, even before reaching sexual maturity. The age of sexual maturity varies by species, gender and region. Females reach this point around 7 to 12 years of age, and males at around 10 to 15 years old.
Mature females typically give birth every 3 to 5 years until their death, although they may give birth as many as 8 times during their lifetime. Most species tend to bear 1 calf every other year or so during their reproductively active years.Life Cycle
After 10 to 12 months of pregnancy, depending on the species, a pregnant female gives birth. Dolphins are among the few animals that have assisted births where another female acts as midwife. The mother's pod will surround her protectively while she's in labour and giving birth, waiting to fend off any predators. The births are usually singular, with the exception of a few smaller species which tend to have 2 calves.
Babies are born tail-first with eyes open, senses alert and enough muscular coordination to follow the mother immediately. Dolphin mothers help their calves to reach the surface by swimming beneath them and gently lifting upward to get their first breath.
As they are mammals, dolphins nurse their young from mammary glands. Nursing continues for as long as 12 to 18 months. Calves must feed quickly, as they need to be able to get back up to the surface in order to breathe, however they don't have the anatomy of most mammals for suckling. So instead of babies sucking to stimulate milk flow, the mother has specialised muscle contractions that squirt milk into the baby's mouth.
The baby grows very quickly on the high-fat mother's milk, in some species doubling its weight within 2 weeks. Calves stay with their mother's for up to 5 years and sometimes longer during which time the mother is very protective and keeps the calf at her side at all times.
There is some variation in the life expectancy among the different species of dolphins, ranging from 17 years to about 35 years, with no significant difference between those in the wild and in captivity. Females tend to live longer than males. One female at the Dolphin Research Center in Grass Key, Florida is in her early 50s though it is very rarely known for any to reach that age.
Predation
Dolphins have few natural enemies and some species or specific populations have none. The only predators that the smaller species or calves have in the ocean are the larger species of sharks, such as the bull shark, dusky shark, tiger shark and great white shark. Some cases of killer whales eating dolphins have been documented, but it is not a common behaviour.
Unfortunately, despite our love for and fascination with these delightful marine creatures, we humans are the most dangerous predator dolphins have today. Although we don't live in the sea, we kill more dolphins than any other animal in the ocean and we have dramatically reduced their populations worldwide.Distribution & Habitat
Dolphins are found worldwide. They live offshore, in cool, deep waters; and in coastal waters, where it is warm and shallow. You can find them in tropical, subtropical, temperate, subarctic, and arctic waters near every continent: Asia, Africa, Australia, North America, South America, Antarctica, and Europe.
Some dolphin species migrate and this may be for a number of reasons such as changes in water temperature, seasonal changes, feeding habits or preferences and availability of food. Certain species follow the warm or cold weather, and others like to stay close to their home range.
The 'Sardine Run' that occurs every year between May and July off the east coast of South Africa is a must-do for dolphin lovers, as you will get to see thousands of them including several differnet species.Ecological Considerations
There are several species that face an uncertain future. The populations of several river dolphins have been drastically reduced by a combination of man-made factors including pollution, damming, increased boat traffic and shoreline development.
The critically endangered species are:
The Yangtze River dolphin (Baiji), China - declared functionally extinct in 2007;
The South Asian river dolphin - Indus River and Ganges River, India;
The Maui dolphin - New Zealand;
Amazon River dolphin (Boto);
La Plata Dolphin (Franciscana) - salt water rivers in South America;
Ayeyarwady River Irrawaddy dolphin - Myanmar;
Mahakam River Irrawaddy dolphin - Indonesia;
Malampaya Sound Irrawaddy dolphin - Philippines;
Mekong River Irrawaddy dolphin - Laos/Cambodia;
Songkhla Lake Irrawaddy dolphin - Thailand;
Eastern Taiwan Strait humpback dolphin.
Endangered Delphinidae species are:
Hector's dolphin
Mediterranean common dolphin
Black Sea bottlenose dolphin
Depressingly, there are other species considered potentially vulnerable but lack of scientific data excludes them from the list.
Industrial Fishing
Fishing technologies are responsible for killing thousands of dolphins every year, most notably purse seine fishing for tuna. Because of their association with certain types of smaller tuna, approximately 20,000 dolphins, particularly common and spinner are believed to die this way each year. This method is especially predominant in the Eastern Tropical Pacific Ocean, from southern California to northern Chile.
In the 1980's an agreement between marine activists and the major tuna companies, saw a change in the type of nets being used to catch the tuna. Incidental kills reduced by up to 50% and dolphin-safe labels were established to inform consumers that the fish has been caught in a dolphin-friendly way.
Drift nets and gill nets are other fishing methods responsible for killing thousands of dolphins each year. More dolphins are killed by being caught incidentally in these fishing nets than by any other means. The use of drift nets on the high seas was banned in 1993 but gill nets remain legal, and continue to pose a threat to coastal species. Accidental captures in anti-predator nets that protect marine fish farms are also common.
Traditional Food Source
Thousands of dolphins and small whales throughout the world continue to be hunted for food, oil, fertiliser and other products.
In some parts of the world, such as Taiji in Japan and the Faroe Islands, dolphins are killed in harpoon or drive hunts. The meat is sold for human or animal consumption, or made into fertiliser. Japan is believed to be the largest consumer of dolphin meat.
Taiji's annual dolphin hunting has received international criticism for both the cruelty of the killing and the high mercury levels of the meat. Over 20,000 of the creatures are caught each year; the prettiest ones are sold to dolphinariums while the rest are brutally slaughtered. 'The Cove' is a 2009 documentary film that analyzes and questions Japan's dolphin hunting culture. It is told from an ocean conservationist's point of view and was awarded the 2010 Academy Award for 'Best Documentary Feature'. The film calls for action to stop mass killings, change Japanese fishing practices and educate the unaware Japanese public about the risks of mercury poisoning from eating dolphin meat.
Subsistence hunting takes place in the Solomon Islands of the Pacific, in Greenland and in Arctic parts of Canada and the United States. The dolphins killed are used for food, oil or other traditional uses and cannot legally be sold. Unfortunately this has proven difficult to regulate. Again, eating the meat poses health risks for the consumer.
Man-made Interference
Loud underwater noises, such as those resulting from naval sonar use, live firing exercises, or certain offshore construction projects such as wind farms, may be harmful to dolphins. Side effects include increased stress, damaged hearing, and decompression sickness caused by surfacing too quickly to escape the noise.
In addition, many dolphin deaths have been attributed to swallowing man-made objects including balls, nets and pieces of plastic. Injuries or deaths due to collisions with boats, especially their propellers, are also common.
Toxic Marine Habitats
However, what may pose the greatest threat to the future of dolphins is man-made toxic pollution in their marine environment. Pesticides, PCBs, heavy metals, plastic particles, radio-isotopes and other industrial wastes are released into our oceans, bays and rivers in unknown amounts on a daily basis. Many of these pollutants do not break down easily in the environment.
Through a process known as biomagnification, these toxins are absorbed and accumulate in organisms in lower levels of the marine food chain. In feeding on these lower level organisms, animals further up the chain receive a higher dose of the toxins and accumulate even larger concentrations in their tissues. Dolphins are near the top of the marine food chain, and so receive higher doses of toxins. Some of these toxins are known to interfere with reproductive ability and weaken the immune system. Species that have a limited or localised distribution may be particularly vulnerable to the effects of pollution.
