Dolphins, porpoises, and whales all belong to a group of marine animals known as cetaceans. Like all mammals, cetaceans are warm-blooded, breathe air, and suckle their young. But how do you distinguish one from the other?
The approximately 86 species of cetaceans in the world are placed in two main groups. Cetaceans range in size from the vaquita porpoise (found in the Gulf of California) growing to 1.4 meters long, up to the 30 meter long blue whale who is found in all the waters of the world. The blue whale is the largest animal that has ever existed on earth.
One group, the Mysticetes or baleen whales, with 14 species, includes the largest whales, namely the blue and fin whales. Instead of teeth, they have long horny plates edged with bristle-like fibres that strain small marine animals from seawater.
The other group includes the toothed whales or Odontocetes, with around 72 species including the beaked whales, killer whales, pilot whales, dolphins and porpoises.
The term whale can refer to any cetacean but it is mostly used for the baleen whales and larger toothed whales.
In the past, the terms porpoise and dolphin have been used interchangeably, with one or the other favoured in different parts of the world. Today, porpoises refer to a group of six species with characteristic spade-shaped teeth and the absence of a distinct beak. Dolphins may or may not have a beak, and usually have many small pointed teeth.
Approximately thirty-five species of dolphins and other small cetaceans are known to occur in Australian waters.
The number of recognised cetacean species is likely to increase as new species are identified based on new genetic information. For example the Australian snubfin dolphin Orcaella heinsohni, was formally described and recognised as a new species endemic to the Australian region in 2005. This dolphin was previously classified within the Irrawaddy dolphin species Orcaella brevirostris.
Distribution and abundance
The waters around Australia range from the warm tropical waters of northern Australia to the cooler waters around the southern coastline. To the south of the continent are the colder subantarctic waters and off the Australian Antarctic Territory, the very cold, sometimes ice-strewn, Antarctic waters.
The killer whale occurs in all waters though it is most abundant in the southern oceans. The other wide ranging species, the bottlenose dolphin, occurs in all but the colder waters.
On the basis of their distribution the small cetaceans can be placed in three groups: those mostly restricted to shallow waters close inshore and to rivers and estuaries; those occurring most commonly in nearshore waters over the continental shelf; and the truly open ocean species seldom encountered close to land.
In general, the further offshore a small cetacean occurs the less is known about its biology, with the exception of those species taken commercially, and those incidentally taken in fishing nets. Some species are known only from stranded specimens.
Adaptations for life in the sea
Bottlenose dolphins have the general body form of toothed whales: a streamlined body, paired pectoral fins, and in most species a single dorsal fin.
Movement through the water is by up and down movements of the horizontally flattened tail fin. The skin contains a complex system of nerve endings that probably help cetaceans swim more efficiently. At high speeds they need to ensure a smooth flow of water over their body. If turbulence develops anywhere on the body surface then this flow is interrupted. Hence there is a need to constantly adjust the body while swimming. In some species, sustained speed is important in the pursuit of fast prey near the surface. In others, such as the deep-diving beaked whales, speed may be essential to enable the longest possible hunting time at the required depth, and to avoid predators.
The skin on the lower jaw is highly sensitive and is used to investigate small objects in the same way that people use their hands. Taste buds are present on the tongue but the sense of smell is believed to be poorly developed in most species.
The blowhole on top of the head enables air to be taken in rapidly when they surface, some species taking only one fifth of a second to exhale and fill their lungs.
Sight in most small cetaceans appears to be well developed. Fine detail is discerned both above and below the water in species such as killer whales. Although the bottlenose and common dolphins have binocular vision they often use only one eye to examine objects.
The external ear openings of toothed whales are small, only two to three millimetres in diameter in the bottlenose dolphin. Although they can hear well in air, hearing underwater is mainly by the conduction of sound through bones in the skull and lower jaw.
In many species there are distinct breeding seasons. Males of some species may fight when competing for females, often resulting in distinct scarring on the body as in the case of Risso's dolphin. Gestation may be ten months for the smaller species, and from twelve to sixteen months for some larger species. The young are usually born tail first. Often other dolphins will nudge the newborn to the surface for its first breath.
Calves may be suckled for eight to sixteen months in the small species, to several years for the larger pilot whales. In most species there appears to be a strong and prolonged bond between mother and young.
Bottlenose dolphins may breed every two to three years. Other species may breed over different time scales.
Most small cetaceans feed on fish, squid or cuttlefish. Sperm whales and pilot whales feed mainly on squid taken at great depths. Some of the inshore species such as bottlenose dolphins eat fish, squid, cuttlefish, eels, and even prawns and starfish. A layer of blubber or fat under the skin acts as a reserve of energy and provides insulation.
In warmer waters, species such as bottlenose dolphins may be able to catch sufficient food in a few hours of active hunting. In colder waters the smaller species need more time to catch enough food to provide for their increased energy needs.
Diving abilities vary greatly with some species taking most of their food in relatively shallow water, diving for only a few minutes at a time. Bottlenose dolphins dive deeper and are able to remain underwater for up to fifteen minutes. One of the large beaked whales has been known to dive to 2400 metres and remain submerged for over an hour.
The toothed whales have developed a remarkable extra sense, which they use for locating food and for navigation. A variety of sounds are produced underwater by moving air between air-spaces or sinuses in the head. When swimming normally, the sounds that are emitted are generally lower frequency; the echoes from these providing information about bottom topography, shorelines, underwater obstacles, water depth, and the presence of large animals.
When hunting or examining objects at close range, a series of clicks is produced which are conducted and focussed through the melon, a specialised fatty structure in the skull. Sounds reflected back from objects are thought to be collected by an oil filled channel in the lower jaw and conducted to the middle ear.
Echolocation is extremely sensitive and may provide cetaceans with a three dimensional view of their world. A recent hypothesis suggests that very high intensity focussed bursts may be used to stun or disorient prey.
The wide variety of whistles, clicks, groans and other noises produced by many toothed whales are thought to be important in communication between individuals.
Many of the small toothed cetaceans are social animals; others are usually seen only in pairs or small groups. The largest groupings are formed by some of the open ocean species, such as the spinner and spotted dolphins, which may gather in schools of several thousand. Pilot whales are also encountered in schools numbering more than a thousand.
Bottlenose dolphins may form groups of five to fifteen animals that move within a relatively restricted range. Larger groups may occur in open waters.
Killer whales form small cohesive and stable pods composed of a dominant male or 'bull' and other sexually mature males, mature and immature females, and calves.
Threats to dolphins
Those fortunate enough to see these remarkable sea creatures in the wild form lasting impressions of the encounter. Whether patrolling close to shore or riding the bow wave of a vessel, dolphins are a source of excitement. From as early as 1500 BC instances of dolphins making voluntary contact with people have featured in folklore. Dolphins commonly appeared in Greek and Roman art, often depicted as friendly and intelligent creatures. Early Greek coins featured dolphin motifs, apparently because the animals were thought to provide safety for travellers.
Apart from the artificial environment of aquaria, probably the closest encounters with dolphins take place on many surf beaches, where the dolphins swim and surface alongside human surfers. Many dolphin encounter tours operate along the Australian coastline.
Many encounters with people have not been beneficial for the animals. Throughout the world, dolphins have been hunted for food; slaughtered for fish bait; killed because they were seen as competing for fish destined for the table; and hunted simply for 'sport'. Many have fallen victim to fish netting and to shark meshing operations.
In response to growing public concern at the way humans have treated the oceans and sea creatures, the Australian Parliament passed the Whale Protection Act 1980, which prohibits killing, capturing, injuring, or interfering with whales, dolphins and porpoises in Australian waters. The Act also prohibits such actions by Australians anywhere in the world.
The provisions of the Whale Protection Act were replaced and strengthened by the introduction of the Environment Protection and Biodiversity Conservation Act 1999, which came into effect on 16 July 2000. This Act designates all Australian Commonwealth waters as the Australian Whale Sanctuary.
Unfortunately dolphins and other small cetaceans continue to be caught in fishing nets, albeit unintentionally. Under these circumstances penalties do not apply provided that live animals are released immediately; that captures are reported as soon as possible to the relevant State authorities or the Department of the Environment, Water, Heritage and the Arts. Dead animals should be retained and the authorities notified so that if necessary, arrangements can be made for the scientific study of the animals.
From reports compiled under the requirements of the Whale Protection Act, a significant incidental catch of cetaceans was found to be occurring in the Arafura Sea in Northern Australia. An estimated 14 000 dolphins had been killed in drift nets from June 1981 to December 1985. With the introduction in 1986 of restrictions to net lengths, the fishery ceased operations.
More recently, Korean and Japanese fishing fleets were using very long drift nets in the Tasman Sea and in the South Pacific, placing dolphins, turtles, and seabirds at considerable risk. International pressure has resulted in moves to ban drift netting commencing in 1992. Australia, New Zealand, and other South Pacific nations are signatories to a convention banning drift net fishing in the region.
There is mounting evidence that small cetaceans are affected by industrial waste products including heavy metals, polychlorinated biphenyls (PCBs), and by bacterial and viral pollution from sewage outfall. Many small cetaceans die from entanglement in plastic waste and fishing gear discarded or lost at sea. An unknown number die from the effects of ingesting plastics and other refuse. A killer whale that stranded and died in Tasmania in 1997 was found to have six plastic bags in its stomach. Thousands of kilometres of nets and line, and hundreds of thousands of tonnes of plastics are dumped into the world's oceans each year.
River dolphins throughout the world are affected by dams that disrupt their movements and the movements of the fish they eat. The Baiji or Chinese river dolphin is now believed to be functionally extinct, its population having fallen victim to fishing lines, nets and propellers as numbers of vessels in the Yangtze River increased.
Dolphins and other toothed cetaceans are the species that most often strand on Australian beaches. Pilot whales and sperm whales are two of the species that have been known to strand in very large numbers. There are many theories as to why such strandings occur. These include beach topography or composition; sick or trapped animals calling to the rest of the pod for help; and problems with echolocation. The exact cause has not been determined.
Report a stranding
There are ways in which humans can help in strandings to ensure some animals may return to the sea.
Report any stranded animals to the local authorities as soon as possible.
Most State conservation departments will have Stranding Contingency Plans to follow in the case strandings, whether small or large. Volunteers often attend strandings and assist the local authorities to get many cetaceans back out to sea.