Dolphins are the most varied family of cetaceans and range in size from the smallest cetacean at 1.7m for the Heaviside’s dolphin, through to the Killer Whale or Orca at 9m in length. They are extremely streamlined in order to swim fast to catch their prey and smaller species are very light so that leaping is possible.
Dolphins are social animals, and most species are usually found swimming in pods (schools or groups). Whilst coastal species tend to be found in smaller numbers, pelagic (open ocean) pods have been known to form aggregations of up to 1000 individuals. These groupings also allow the pods to hunt fish cooperatively as a team. They will often use a herding technique whereby members of the pod take turns in keeping the school of fish together whilst others have the opportunity to feed.
Pods of dolphins will often experience changes in membership, as individuals move between the social groups. Despite this interchange, the bond between these animals can be quite strong, and there have been reports of groups helping to bring and keep an injured animal to the surface to breathe. Aggression between males can be common due to sexual or territorial disputes, however mothers have been observed teaching their daughters how to use such things as sea sponges as protective tools whilst foraging for food. Dolphins have also been known to display aggressive sexual behaviour towards other species, including humans on occasions.
The feeding habits of dolphins are dependent on several factors of the individual species; size of prey, the distance from shore, and the depth to which they dive to. Most dolphins will only dive for short periods of time and stay within the upper 200m depths. The general content of most dolphin diets are small fish and squid, though orcas will prey on seals, other marine mammals and cetaceans, birds and sharks as well as fish.
The female bears a single calf, which is borne tail first. This limits the potential for loss of life through drowning. When born the calf immediately swims to the surface for air. Soon after birth, the calf suckles for the first time. As suckling takes place under water, it takes place in short bouts, between rising to the surface for air.
Dolphin milk is four times richer than human milk. This enables the calf to grow fast. Suckling lasts for eighteen to twenty-four months. At six months they supplement the milk with fish and squid. During suckling, the mother teaches the calf how to hunt and behave. Their parental instincts aid the care of the pod, by ensuring a greater chance of survival for the young and sick.
Dolphins do not have a sense of smell, but they have a high sense of taste and their skins are very sensitive. With these senses they are acutely aware of where they are through picking up differences in water temperature. They do this by tasting water types. These are very useful when sight and echolocation are restricted. They also use a range of different vocalisations in the form of clicks, whistles and burst-pulsed sounds. Whilst the whistles and burst-pulsed sounds seem to be used as primarily communication tools, dolphins use the clicks for echolocation.
Threats to Dolphins
There are very few natural predators for a majority of the dolphin species. Whilst some species of shark such as the tiger, great white and bull shark and some orca’s have been known to pose a threat, especially to calves, a majority of the problems associated with declines in populations can be attributed to human activities. These include:
- Human consumption - some countries such as areas in Japan and the Faroe Islands still kill and eat dolphins as part of their traditional feasts.
- Pollution - plastic bags, toxic waste products, sewage, etc.
- Agriculture - Pollutants such as DDT, dieldrin and lindane and other pesticides are known to be
- harmful to most marine life.
- Industrial – Polychlorinated biphenyls (PCBs) pollutants accumulate in trophic levels and can severely affect those at the top of the food chain.
- Drift and Gill nets – fishing techniques for fish and squid which are highly dangerous to dolphins and porpoises, as many become unwilling victims of by-catch.
There are two families of dolphins; Delphinidae, or the oceanic dolphin, and Platanistidae which includes the four genera and five species of river dolphins. The river dolphins are generally found in the coastal waters and fresh-water rivers of South America and Asia. Whilst little is known about their social behaviour and activities, researchers know much more about the lives of the following species of oceanic dolphins (not all species are listed).
- Spinner Dolphin (Stenella longirostris) - It can dive down to 60m. Requires tropical and warm temperate seas.
- Indo-pacific bottlenose dolphin (Tursiops aduncus) and Bottle Nose Dolphin (Tursiops truncatus) - Best known species, adapt to captivity, can reach 4m and live for 37 years. Worldwide distribution in tropical and warm temperate seas.
- Common Dolphin (Delphinus delphis) – lengths up to 2.7m long. Tend to prefer closed bodies of water such as the Red and Mediterranean Seas.
- White-beaked dolphin (Lagenorhynchus albirostris) – acrobatic and social animals endemic to the North Atlantic Ocean
- Atlantic white-sided dolphin(Lagenorhynchus acutus) – has a distinctive yellow patch on either side of the dorsal fin, endemic to the North Atlantic Ocean. Still hunted as a source of traditional food for the Faroe Islands
- Pacific white-sided dolphin (Lagenorhynchus obliquidens) – a three toned dolphin that is mainly found in the cool to temperate waters of the north Pacific
- Dusky dolphin (Langenorhynchus obscurus) – highly active residents of the oceans in the southern hemisphere, these dolphins remain targets of Peruvian fishermen and populations are considered to be vulnerable
- Peale's dolphin (Lagenorhynchus australis)- a smaller dolphin with a limited distribution, being noted only in the waters around Tierra del Fuego at the base of Chile.
- Hourglass dolphin (Lagenorhynchus cruciger) – a distinctly black and white dolphin whose distribution occurs in the very southern oceans of Antarctica
- Hector's dolphin (Cephalorhynchus hectori) – endemic to the waters surrounding New Zealand, these small dolphins have a distinctively rounded dorsal fin
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