Dolphin Research Institute (DRI)
EDUCATION >> FACTS: BOTTLENOSE DOLPHINS

Bottlenose Dolphins

Bottlenose dolphins

The Dolphin Research Institute conducts research on bottlenose dolphins (Tursiops truncatus). This is the ever-smiling species that most people have come to know and love due to "Flipper". The name bottlenose dolphin originated in England from the resemblance of the short rounded snout to the swollen form of an old-fashioned bottle. They are widely distributed in cold temperate to tropical waters worldwide. Within this distribution are inshore and offshore populations. More is known about the inshore populations, which can be found in many enclosed seas including the Red, Black and Mediterranean seas and the Gulf of California, as well as our bays in Victoria. Inshore populations are generally found in water less than 30m deep, occupying a diverse range of habitats including open coasts with strong surf to sheltered bays and waterways, lagoons, large estuaries and the lower reaches of rivers. It is possible that many inshore populations are resident year-round, whilst offshore populations are highly transient, undertaking seasonal migrations.

Bottlenose dolphins are powerful swimmers with a foraging and travelling speed of about 5km/hour, and a cruising speed of about 10km/hour, although they can also swim at speeds around 37km/hour. In coastal waters, dives rarely last more than 3-4 minutes, but may be longer in oceanic animals, which can dive to depths of 600m for 15 minutes. When they surface, their head will show, but generally not their beak. Dolphins are conscious breathers, which means that they have to be awake or semi-conscious to breathe or else they will drown. They therefore sleep in a semi-conscious state at the surface with their blowhole exposed and only a few minutes at a time. They have the remarkable ability to rest one half of their brain whilst the other remains alert for approaching danger, and then are able to switch which side they are resting.

Group size is highly variable but with inshore populations, groups generally number around 1-10. Offshore groups may number in the hundreds. In Port Phillip Bay, we can see group size range from 1-60, but they commonly number around 5-15. In a resident coastal group, the size and composition of subgroups may vary seasonally or diurnally in response to a number of factors, including distance offshore, prey type and availability, and protection/avoidance against predators. We believe that the population in Port Phillip Bay is resident with occasional transient individuals coming in. Our research programs will now help us confirm if this is so and allow us to identify what affects their presence and distribution.

Appearance

The bottlenose dolphin varies greatly in appearance, including their size, shape and colour, over its geographic range. Much of the variation observed between populations probably reflects differences in their respective environments. There are currently different opinions on the classification of the species in terms of possible sub-species. There may be at least two ecotypes recognised - the smaller and slimmer inshore form (Tursiops truncatus aduncus) and the larger form (Tursiops truncatus truncatus).

Birth weight = 15-30kg Birth length = 70-130cm Adult weight= 150-650kg Adult length = 190-390cm


The dorsal cape of the bottlenose dolphin is usually a dark bluish grey with a prominent curved-back dorsal fin. The sides of the head and body are paler grey or brownish grey, which shades gradually into an off-white or pinkish underbelly. The flippers are long and slender with pointed tips and the outer surface is a dark greyish black. Bottlenose dolphins may appear to be always smiling, however it is not a sign of happiness as their face is fixed in that expression even when in distress or pain. The short beak usually has a scarred white tip, although its length and thickness is variable. There is a sharp crease between the beak and the forehead, the latter of which is quite rounded, and there may be dark lines from the forehead (melon) to the eyes and the blowhole, as well as a dark eye-to-flipper stripe. Some populations develop dark spots and blotches on the underbelly at sexual maturity. The tail is quite thick, this being the most muscly part of their body. The flukes are a dark greyish black and have a distinct notch in the middle. A dolphin will often lift its flukes above the water surface when about to dive.

Life History

Bottlenose dolphins can live to over 30 years of age and some studies have aged individuals at 50 years old when they died. Having a long life span means that they have a long adolescence and only begin breeding at 9-10 years old for females and 10-13 years in males. The genitals are concealed in both males and females, but they can be sexed if you are able to catch a glimpse of their belly. Both sexes have a naval and genital slit, but the females also have a mammary slit on either side of the genital slit.

Dolphins have many partners over a lifetime and mate all year round. Calves are born throughout the year, although most are born in spring and summer after a gestation period of 12 months. Calves suckle for up to 18 months, although they begin eating fish at about 6 months of age. Their teeth begin to erupt at about 5 weeks of age. Adult females have calves at intervals of two to three years and will not have twins. It is difficult to see a birth in the wild, as the female group is very protective of the mother and new baby. Most of what we do know about births comes from studies on captive dolphins. Calves are born tail first so that they do not drown and are quickly pushed up to the surface for their first breath.

Newborns can be identified by a number of features. Firstly, they are very small at about 50-75% of the mother's length. They will consistently surface in the "calf position", which is very close to the mother's side. They often have darker colouration than the mother and surface differently by putting most of their head out of the water. When they are only hours old their dorsal fin, which is cartilage, will still be floppy as it takes a little while to firm up. For several weeks, calves will have what look like pale vertical stripes on their sides. These are referred to as neonatal stripes or foetal folds, which are creases in the skin and are caused from the way the baby is curled up in the mother's womb.

A calf will stay with its mother for about 5 years or more. If it is a female, it is likely to remain with the group it was born into, whilst males will leave the area once old enough. Females with calves stay together, using the most productive areas of the community home range, whilst males form long-term bonds with each other and will range further afield as they age. It is possible in Port Phillip that the population mainly consists of female groups whilst the males come into the Bay from spring through to summer. It is difficult to sex dolphins in the field and we currently sex females based on their association with calves.

Intelligence

Bottlenose dolphins possess one of the highest ratios of brain size to body mass in the animal world, but does this mean they have a high level of intelligence? Animal intelligence is an area of research that still has a long way to go before we fully understand it. How do you define intelligence and how do you measure it? Dolphin intelligence still requires further research, but so far studies have shown that dolphins are smart, highly social animals that can do some pretty tricky things! A lot of intelligence research has been conducted on captive animals, in which dolphins have shown that they are self-aware. This means that they are able to recognise themselves in a mirror, whereas other species would think they are looking at another animal.

Dolphins also have shown language comprehension abilities. In work completed in Hawaii, two types of language systems were tested. One of those used whistle-like computer generated sounds to represent objects and actions. The other test trained the dolphins to answer either yes or no in response to simple questions. The captive dolphins had a high average (75%) of correct responses in these tests and were able to learn and respond correctly to the artificial language. Whether the tests show language ability is still debated however, and intelligence studies continue.

Behaviour and Social Lives

Bottlenose dolphins are highly active and can be frequently seen tail slapping, bow and wake-riding, and breaching. They will chase one another, roll over each other and carry objects such as seaweed. They can be very active when chasing fish, sometimes breaching and playing with them. We often see the dolphins of Port Phillip playing with their food, including bowriding vessels with a fish in their mouth. On one particular occasion a dolphin was throwing a fish it had just caught back over its head and catching it again and again, landing on its back each time. Quite an impressive display!

Researchers must be careful when undertaking behavioural studies, as we should not try to assign human behaviour categories to the dolphins' behaviour. The Dolphin Research Institute uses 5 main behavioural categories which are travelling, feeding, social, milling and human interaction. We may observe a combination of behaviours and often will be unable to assign a category when they are doing something quite different.

Groups of dolphins cooperate in a number of ways, including in feeding, defence and childcare. Group dynamics vary and individuals may be seen with a variety of other dolphins each day. In past summers, we have been able to identify a number of key individuals that we could almost guarantee were associated with calves each time. But there would not always be the same dolphins within the group every day. Their social structure is not as tight-knit as that of other cetaceans such as the Orca, where a family group will stay together for many years. There have also been cases of wild, lone dolphins (generally males) seeking out swimmers and small boats, sometimes remaining in the same area for many years.

Bottlenose dolphins can be quite aggressive, particularly the males towards each other. Males have been known to fight so violently that they have killed each other, they will also attack other species of dolphins and porpoises. Males have been documented killing juveniles of their own species if another male has fathered them, a behaviour known as infanticide. This sort of behaviour has also been observed in other mammals from rodents to lions. There has recently been a lot of press about the "dark side of dolphins" which involves some of the behaviours identified above, but it is important to remember that they are wild animals.

The senses

Bottlenose dolphins have the sense of sight, hearing, taste and touch, but not smell. It is likely that they did not require a sense of smell and therefore never had it or it was reduced over years of evolution. The senses are used in every facet of dolphin life as in other mammals, including navigation, feeding, breeding and communication.

Sound

Dolphins use a feature called echolocation to create acoustical pictures of their surroundings. They are able to produce intense, short, broadband pulses of ultrasonic sound (often referred to as clicks) which then bounce off objects in their path. Their hearing is also excellent, even though you have to look very hard to see any evidence of an external ear opening. This confused scientists working with captive dolphins as to how they could hear so well when the external ear opening was so small, and they found it may be that sound passes through the dolphins' lower jaw, as well as through the ear canals. The inner ear itself is adapted for hearing ultrasonic frequencies far beyond the range of human hearing - well over 100kHz, which is greater than that of bats.

By sending out clicks a dolphin can learn much about its surroundings by how long it takes for the echo to come back. This tool is used primarily in the detection and tracking of prey. The intensity of sound as a dolphin closes in on its prey is so great that it is thought that they may be actually able to stun fish! So if their echolocation is so powerful, why do they get caught in fishing nets? One theory is that when they know an area well, they don't bother to use their echolocation powers, using some sort of mental map to travel through. Thus when something new is put in their path such as a driftnet, they literally don't see it till it is too late.

Sight

The bottlenose dolphins' eye is very good at quickly detecting moving objects. The eye has evolved to give them a faster projection of a larger image, as well as large nerve fibres to rapidly send the information to the brain. The bottlenose dolphin has a visual range of 180o forwards, backwards and to the side, but it cannot see up. This is often why we see dolphins chasing fish bellyside up and why when bowriding they turn on their sides and back to look at those humans watching them! Bottlenose dolphins also have a very interesting adaptation in that they can move each eye independently of the other!

Aerial vision is important in foraging. It is not uncommon to see dolphins look around above the surface of the water when they surface to breathe. One form of this behaviour is called spy-hopping, when the dolphin rises vertically out of the water, head-first. A common behaviour seen in Port Phillip is a dolphin chasing a single fish that is leaping and skittering across the surface of the water. The dolphins are visually tracking these fish and this sort of behaviour has been documented elsewhere. Sight is also important in looking out for danger, particularly if a predator is rising from below or approaching from the side or rear. However, it requires the dolphin to be paying attention!

Taste

Dolphins have some ability to taste and are able to detect the four basic stimuli - sweet, sour, bitter and salt. It is thought that taste buds are like a chemoreception system that can be used for locating other dolphins, finding food, orientation, reproduction and stress sensitivity. Dolphins are especially sensitive to substances found in mammalian urine and faeces, so it is possible that they use information from this chemical trail to locate other dolphins. A female dolphin that is ready to breed releases some potent chemical stimuli that attract males. Bodily products, especially urine, are also thought to contain chemical indicators of physiological stress, which may alert others to the physical condition of a cetacean that has passed through the same waters before them. Taste buds can also be used to find food, as a large school of fish can leave a chemical trace that lingers for hours. In terms of orientation, many of the ocean's currents have distinct chemical traces that dolphins may use to navigate.

Touch

A dolphin is able to "sense" the water's movement over its body to use to its own advantage, but how exactly it does this is still unknown. Aside from the dolphins' external sensitivity to water flow, all cetaceans use the sense of touch in social and sexual situations. Touch is a major component of a dolphin's day, as much as foraging for fish is. They stroke or pat one another with pectoral fins or flukes, rub bodies together, and press their genitals against a neighbour, who doesn't always have to be of the opposite sex. At the other end of the scale, touching comes under aggressive behaviour also. Dolphins, as do many other cetaceans, use tooth raking as a sign of displeasure. They may also smack others with their tail and hit with their snout or flipper. It is thought that they may also use sound on others to irritate them - imagine if they used echolocation on other dolphins at the intensity thought used to stun fish!

Feeding

Bottlenose dolphins have small conical teeth with about 40-52 in the top row and 36-48 in the bottom. Their diet and hunting behaviour varies greatly as they adapt to the local prey availability and conditions. Coastal dolphins eat a variety of fish species, cephalopods (squid and octopus), and krill or other crustaceans occasionally. Their diet will generally consist of 4 or 5 locally abundant prey species and they eat around 15kg of food per day. Distribution patterns are strongly influenced by prey availability. If for some reason (e.g. El Nino) their primary prey move out of an area, the dolphins will follow. Even within a permanent resident population, there may be substantial differences in where individuals look for food. Within an area, many dolphins have favoured "core" areas where they spend most of their time.

Once dolphins spot food, they use a variety of methods to catch it. They often cooperate as a group to herd fish shoals up against a shoreline or use their own bodies as a wall. The main aim is to slow down and trap the fish school, which they then attack from all sides. Another method often used involves forcing the fish to travel in a spherical mass that is gradually tightened by the dolphins, which continuously swim around the fish. The dolphins will then begin to dive, either under or into the fish, where they can easily take fish each time. Living in groups is advantageous for foraging in areas rich in prey, as individuals can save energy. The more individuals there are to hunt, the easier it is to catch large shoals of fish. However, there may be cases where there is not always enough prey available to feed a large group of dolphins. Although they often hunt in groups, solitary dolphins can also be seen hunting using a variety of methods.

How dolphins hunt for prey depends largely on what they are hunting and its availability. Dolphins are opportunistic feeders and have been known to follow shrimp boats to take advantage of the prey stirred up by the trawls. They have also learned to take advantage of trash fish dumped by fishermen once they have sorted out the commercially valuable fish. A technique has been observed in South Carolina and Baja, California where bottlenose dolphins create a pressure wave so strong that is pushes the fish up onto mudbanks. The dolphins slide out of the water and beach themselves temporarily to capture the fish and then slide back into the water, always on their right side. There are also historical cases of beneficial relationships between Aborigines and dolphins in Moreton Bay, where the dolphins would herd fish schools onto the shore enabling humans to catch them. There were enough fish for both humans and dolphins.

Communication

Much research has been conducted on dolphin communication and it was once thought that dolphins had a complex language of their own. However, it would seem that this is not quite the case. There is no evidence to support the theory that dolphins talk in the same way humans do. Nevertheless, it is possible that they have the basic elements of language by using specific sounds for different objects and they are probably able to communicate with sounds this way. They produce high frequency whistles, clicks and squeaks, and do so singly, in bursts or in continuous streams. What we hear though, is not what the dolphins would hear. Firstly, most of the energy in a typical squeak or click is far outside our hearing range. Dolphins' brains also process sound very fast and would probably hear a lot more detail within a squeak than we do.

It is thought that whistles may have evolved to help a group's ability to maintain contact when foraging over a large area, as whistles can travel greater distances than pulsed sounds such as clicks. A dolphin is able to echolocate and whistle at the same time, so that as it is searching for food it is also able to communicate whilst foraging. Whistling can accompany a variety of behaviours and situations. Dolphins whistle more when arriving at a familiar place, when feeding, under stress or when bowriding. Bottlenose dolphins also emit pulsed sounds at other dolphins that seem to be unfriendly, and they may scream and growl at each other!

It would seem that dolphins are not just making noise when they whistle, as there is evidence that a whistle can tell others about who you are. Not long ago, it was thought that each individual dolphin had a signature whistle that was sort of like a name. However, current theory suggests that whistles are not that specific and that it may be more of a group signature. Cetaceans not only use clicks, squeaks and whistles to communicate, they also use body language. Bottlenose dolphins have been known to clap their jaws together in conflict situations, and splashes and slaps made with the body may also be used as a form of communication. A common behaviour that we observe is tail slapping, which seems to be used in a number of situations. If a dolphin repeatedly tail-slaps towards a boat, be it our research vessel or a commercial tour vessel, it would seem that they may be "telling" the boat to "back off"! When they tail-slap in this manner and display other avoidance behaviour, we take that as our cue to leave them. On the other hand, dolphins have been seen to repeatedly tail-slap when there are no boats near them or even around. This could be linked with feeding or social behaviour. On one occasion, we noted a dolphin rapidly tail slap, which caused the others to quickly go to where that dolphin was. It is possible that it was trying to tell the others that there was food there.

Australian bottlenose dolphins

Distribution

Within Australia, the bottlenose dolphin is found in all states and the Northern Territory, and Norfolk Island. Based on the theory that there are two ecotypes, the inshore (Tursiops truncatus aduncus) and the offshore (Tursiops truncatus truncatus). T. t. aduncus occurs in New South Wales north of Port Macquarie through to Queensland, the Northern Territory, and Western Australia south to Perth. T. t. truncatus occurs in southern Queensland, through New South Wales, Victoria, Tasmania, South Australia, and in Western Australia (to Albany). Bottlenose dolphins can be migratory within this range.

Their general habitat includes coastal, estuarine, pelagic and oceanic areas that are tropical to temperate, and occasionally in subantarctic waters. The species has been studied in several key locations around the country such as Shark Bay and Cockburn Sound in Western Australia, Moreton Bay in Queensland, Jervis Bay in New South Wales, Port Phillip in Victoria, and Adelaide in South Australia.

Status

The status of the bottlenose dolphin worldwide is insufficiently known, although the overall numbers appear to be substantial. However, there have been population declines recently in parts of northern Europe, the Black Sea and the Mediterranean. In The Action Plan for Australian Cetaceans, published by the Australian Nature Conservation Agency (ANCA), the bottlenose dolphin, has been placed in "No category assigned" due to insufficient information. The IUCN (International Union for the Conservation of Nature) has also not assigned the species a status for this reason.

Accurate population estimates are not available for any bottlenose dolphin community in Australia. There have been a number of studies that have provided minimum local population estimates through photo-identification studies, but there are no estimates for rates of change.

Research

Because cetacean species are rarely limited to one nation or state, there are a number of organisations that are concerned with the conservation and management of cetaceans. Within Australia, all cetacean species are managed by ANCA, which is a Federal Government body responsible for management of cetaceans within our exclusive economic zone (EEZ). This zone extends from our coasts to 200 nautical miles seaward. For waters falling within the jurisdiction of the individual states of Australia (ie. coastal waters), cetaceans are managed by the wildlife agencies within those states.

There are a number of research programs into bottlenose dolphins underway around Australia and worldwide. In Australia, there have been aerial surveys conducted off the Northern Territory, through the Great Barrier Reef region, the northern waters of Western Australia, and Shark Bay. General ecological and photo-identification studies have been done, and many are still going, in Moreton Bay, southeastern Shark Bay, Jervis Bay, Port Phillip, Bunbury, Adelaide and Perth. There have been behavioural studies into the impacts of feeding programs at Moreton and Shark Bays. Taxonomy studies involving molecular genetics to determine relationships have been conducted in Tasmania. General sighting records have also been kept in a number of places around the country.

There are large gaps in the information we have about bottlenose dolphins and research is required for general baseline information on population numbers around Australia. This includes identifying habitat requirements and if there are processes threatening their habitat. Other research required is more aerial surveys, pollution levels and their impacts, mixture between the two ecotypes, the effects of tourism, and the causes of stranding.

Threats facing dolphins

There are a number of threats that currently face not only bottlenose dolphins, but also all cetaceans within Australian waters. Incidental capture is a significant threat and is associated with the prawn fishery industry in the Gulf of Carpentaria, with aquaculture nets (significantly high rates in South Australia), trawl nets, drift-nets and gillnets. Dolphins are also getting trapped in shark nets that have been erected to protect humans from the risk of shark attack. Habitat destruction and degradation is a current threat, but even more concerning is that we don't know enough about dolphin habitat needs. Illegal killing of dolphins has been occurring far too often, particularly in the Adelaide River where recently a number of dolphins were shot and a baby was stabbed to death. This may be someone's idea of sport, or due to the perception that dolphins are "stealing" fish from the commercial fish stocks. This highlights another significant issue, that being the overfishing of many of our fish stocks. It is possible that the dolphins' key prey species are being fished out, thus reducing the amount of food available to them.

The tourism industry is also booming on the back of cetaceans. Dolphin and whale watching has become very popular, as have dolphin swims. Dolphin swim operations currently occur in Jervis Bay and Port Stephens in New South Wales, Hervey and Tincan Bays in Queensland, and Port Phillip Bay. In Port Phillip Bay, the number of commercial dolphin swim boats increased from two in 1991 to five in 1998. Recent legislation in Victoria (2002) aims to manage the licensed tourism industry on a "precautionary principle" basis. Over the summer months there is significant pressure on dolphins from tour boats, as well as dolphin watching ferries and private vessels. At Monkey Mia in Western Australia, the behaviour of the dolphins has changed over the years and they are no longer wild dolphins as such. Mothers have been known to leave their young unattended as they get food handouts and there have been cases of calves being attacked by sharks. There are also feeding programs at Bunbury (in Western Australia also) and Tincan Bay (Queensland).

Besides the current threats, there are a number of potential threats to bottlenose dolphins including pollution (organochlorines), disease (Morbillivirus) and increased tourism on top of the current levels. It is clear that there is a lot of work to be done to ensure that bottlenose dolphins and other cetaceans continue to inhabit Australian waters.



References Used:

Bannister, J.L, Kemper, C.M. & Warnecke, R (1996) The Action Plan for Australian Cetaceans. Australian Nature Conservation Agency.

Carwardine, M; Hoyt, E; Fordyce, R.E. & Gill, P (1998) Whales, Dolphins and Porpoises. Reader's Digest

Connor, R.C.& Micklethwaite Peterson, D (1994) The Lives of Whales