Whale of the Month - Bottlenose Dolphin

Species fact file

Common names: common bottlenose dolphin, Atlantic bottlenose dolphin

Icelandic name: stökkull, höfrungar

Scientific name: Tursiops truncatus

Family: Delphinidae (dolphins)

Max length: 3.5m (females), 3.8m (males)

Distribution: global, except polar regions

The common bottlenose dolphin is one of the most well-known and familiar of all cetaceans. With their light grey colouring, short beak, and that distinctive smile, this is what most people immediately think of when they hear the word ‘dolphin’. Bottlenose dolphins are popular in marine parks all around the world and have been very well-studied; most of our knowledge on dolphin cognition and behaviour comes from research on captive bottlenose dolphins.

Bottlenose dolphins are found throughout the world’s temperate and tropical seas. They feed on a range of squid and fish, with a general preference for bottom-dwelling species such as mullet. Research in the eastern Pacific Ocean found that bottlenose dolphins hunt mostly at dusk and dawn – at this time of day, their prey species are transitioning between resting and feeding, making them easier for the dolphins to catch. Bottlenose dolphins do spend a lot of their time underwater, but they rarely dive deeper than 10 metres and on average return to the surface every 30 seconds.

Bottlenose dolphins are not known for lengthy dives, but they can spend as much as 87% of their time submerged! Photo by Ted Cheeseman via Happywhale.

As well as significant variation in diet between different locations, bottlenose dolphins display spectacular variation in their hunting techniques. All around the world, bottlenose dolphin populations have developed unique methods of catching food that often make use of various features of the surrounding environment and ecosystem. These strategies can be highly specialised to a particular region, prey species, or even to one single family group!

Perhaps one of the most famous of these hunting techniques is the so-called ‘mud ring’. This is a strategy used by bottlenose dolphins in Florida, who inhabit very shallow, muddy coastal plains. Mud rings are a team effort: once the group has located a shoal of mullet, one dolphin will encircle the shoal by stirring up the mud with its powerful tail. The rising clouds of silt create a barrier which the fish are unwilling to swim through, so they try to escape by leaping right out of the water – where the rest of the pod are waiting patiently, with open jaws! The dolphins all take it in turns to create the mud ring so that everyone gets an equal chance to catch their dinner. Click here to see some mud rings in action!

The cold seas off the north coast of Scotland are roamed by the most northerly resident pod of bottlenose dolphins in the world. These dolphins are known to hunt many fish species throughout the year depending on what is most available, but their favourite food is Atlantic salmon. From late spring through to early autumn, salmon gather in the Moray Firth to begin their migration upstream. Atlantic salmon is one of the largest salmon species on the planet, and they have been known to reach lengths of over one metre – this makes for quite a formidable opponent! But the bottlenose dolphins have worked out how to use the Moray Firth itself to help them catch their prey. The wide mouth of the estuary narrows in between two opposite peninsulas; when the tide turns, huge volumes of water are forced through this gap with astounding force. The combination of strong water currents racing through a bottleneck in the estuary creates the ideal conditions for hunting salmon! As the salmon swim through the narrowest point of the channel, the dolphins wait in ambush – then chase the fish into the strong tidal flow.

Breaching bottlenose dolphins in the Moray Firth, between Chanonry Point and Fort George. Photo by the author.

In the Moray Firth, bottlenose dolphins can easily be seen using this hunting strategy from land, sometimes swimming unbelievably close to shore and the crowds of eager dolphin watchers! They use this area so frequently and reliably that the whole population is often simply referred to as ‘the Moray Firth dolphins’ – even though, in reality, they are known to inhabit much of Scotland’s east coast. All throughout their range, these dolphins seem to prefer hunting in the narrow channels of estuaries, just as the tide is beginning to rise.

Perhaps most impressive of all, however, is the group of dolphins in Brazil who have learned to work with human fishermen in a way that increases the catch for both parties! Both dolphins and humans are hunting the same prey: mullet. The fishermen stand in the shallows with nets while the dolphins swim slightly further out. Because they can use echolocation, the dolphins are better at detecting shoals of fish underwater– when they find them, they give the fishermen their cue: slapping their heads or tails against the water’s surface to indicate when and where the nets should be cast.

Complex hunting strategies like these are not innate: they have to be learned. Most often it is mother dolphins who teach their calves how to successfully hunt and capture prey, but in tight-knit pods hunting tactics can be shared between all members.

Although there is some variation between different populations, bottlenose dolphins generally have what is known as a ‘fission-fusion’ social system. Fission-fusion societies are made of very small, tight-knit social units, or pods, that periodically merge with and then separate from other pods. Scientists have found that bottlenose dolphin societies are often divided by particular hunting strategies. For example, the dolphins in Brazil that cooperate with fishermen are part of a larger resident population which is comprised of three pods; one pod is made up entirely of cooperative dolphins, one is made up entirely of non-cooperative dolphins, and the third is mostly non-cooperative except for one individual. So, it seems that dolphins like to hang out with others that hunt in the same way as them! A similar pattern occurs in other parts of the world; for example, bottlenose dolphins in northern Spain live in separate pods depending on whether they use man-made shellfish farms as feeding grounds, and in Queensland, whether they hunt for fish in and around the trawling equipment used by local fishing vessels.

A large pod of bottlenose dolphins seen from above off the coast of Florida. Photo by FWC.

Because bottlenose dolphins are particularly well-researched, many aspects of cetacean ecology and behaviour have first been noted in this species. A good example of this is the “signature whistle”. Each individual bottlenose dolphin has a unique whistle which it uses to identify itself. When multiple pods meet or merge, dolphins from each group exchange signature whistles, as if by means of an introduction! These whistles are not passed from mother to calf: calves develop their own identifier at a young age, and mothers and daughters in particular seem to have very distinct signature whistles. Recently, researchers have discovered that signature whistles are in fact not unique to bottlenose dolphins – there is evidence that other dolphin species and even narwhals also share this ability.

Sharks are the primary predator of bottlenose dolphins, particularly in tropical regions. In some areas, killer whales are also known to sometimes hunt dolphins, but we are not sure how often this might occur. Bottlenose dolphins can be highly antagonistic towards other species that they don’t prey on – particularly harbour porpoises, but also seals and some other, smaller dolphin species.

Although currently considered one species worldwide, the common bottlenose dolphin exhibits so much geographical variation in ecology that most biologists agree there are probably multiple species. One major division is between bottlenose dolphins that live in coastal regions and those found in the pelagic open ocean. Coastal populations are generally very well-studied, but much less is known about the offshore bottlenose dolphins. We know that, while coastal dolphins tend to have very stable home ranges and generally quite small pods, offshore populations can range over huge areas and form groups of hundreds or even more!

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How hunting in cooperation with fishermen shapes bottlenose dolphin society

Bottlenose dolphin signature whistles