Dolphin facts

Whilst thinking of a dolphin might conjure up an image from the film Free Willy, or a snapshot of a bottlenose dolphin from childhood visits to the local aquarium, there are actually 42 species of dolphins in existence on Earth. These various species have developed formidable adaptations in order to live in the world’s waters –  some dolphins are slim and pink, others are fat, black and white. Some have as few as four teeth, others have up to 240. Some dolphins are intensely social, whilst others stray from the group. But their cheeky nature, adaptation skills, grace and cunning have endeared dolphins to researchers for decades, helping to reveal new facets of these marine mammals’ lives.

• Six dolphin species are commonly called whales (think killer whales!), and technically, all dolphins are whales because they’re part of the category “toothed whales” in the order Cetacea.
• Dolphins understand their surroundings through a hearing-based mechanism called echolocation: they emit clicks into the water, and these clicks bounce off surfaces, creating echoes that the dolphin deciphers. This enables them to locate everything around them. 
• Some dolphin species have up to 240 teeth, but they don’t use them to chew on food: these teeth are used for holding onto prey before swallowing it whole.
• Dolphins have unique whistle signatures, similar to names, and they can recognise the whistles of old pod mates, even after not having seen them for 20 years.
• These sea mammals have elevated cognitive abilities. In experiments, some species were found to recognise their reflection in the mirror –  suggesting they may have some sense of self – and they can use small makeshift tools to hunt down prey. This enables them to locate everything around them.

Even though they live in the ocean, dolphins and whales, unlike sharks, are marine mammals, not fish.¹ They’re mammals because they breathe air through their lungs, and because they are warm-blooded animals and need to use their metabolism and energy to stay warm.² They also give birth to live young and feed their babies thick pasty milk through tiny nipples, much like humans do.

Dolphins are part of a large category of animals called cetaceans, which includes more than 80 species of whales, dolphins, and porpoises. Cetaceans evolved from mammals which, in prehistoric times, moved from the land to the sea, lost their legs and grew a tail.³ Their tails move up and down, not side to side like fish tails do.⁴

Today, there are at least 42 species of dolphins. While the most populous dolphins are common dolphins and bottlenose dolphins, there are a variety of unique species with distinct adaptations, such as the Irrawaddy dolphin with its flat nose, and the Commerson’s dolphin with its skunk-like black and white colour patterns.⁵ The Indo-Pacific humpback dolphin has a hunched-over posture and is coloured in shades of pink, while the melon-headed whale, also known as the electra dolphin, has a long, slim body but a rounded, melon-shaped head similar to that of a beluga whale.⁶ 

Dolphin taxonomy can seem a little confusing because six dolphin species have names with the word ‘whale’ in them  – like the killer whale and the false killer whale, which are just giant dolphins, as is the long-finned pilot whale.⁷ But technically, all dolphins are whales because they’re part of a group of animals called ‘toothed whales’, alongside whales such as the narwhal, in the order Cetacea ⁸

Most dolphins live in the salty waters of the sea, or brackish waters along the coast.⁹ They can be found almost everywhere on the planet.

Killer whales, also known as orcas and long-finned pilot whales have made homes in the frigid subpolar waters of the Arctic thanks to their large, fatty bodies.¹⁰ Pantropical spotted dolphins, spinner dolphins and spotted dolphins thrive best in the ecosystems of warm tropical waters or temperate climates. This is also true of other dolphin species including the bottlenose dolphin.

Six species of dolphin have evolved to live in freshwater streams and rivers instead.¹¹ This category includes the Amazon river dolphin, or ‘boto’, which sports a long thin beak and is bright pink in colour, and the Ganges river dolphin, which is famous for swimming on its side in order to stir up hidden crustaceans from the bottom of the river.¹² Overall, river dolphins are less agile acrobats and swimmers than their marine counterparts and have poorer eyesight; some are essentially blind, like the Indus river dolphin.

As dolphins are mammals, although they spend most of their time underwater, they do have to regularly resurface in order to breathe.¹³ They do this through the little hole on top of their head called a blowhole. The dolphin opens its blowhole up to inhale oxygen, and then tightens and closes the blowhole when it dives back into the water, an action similar to pinching a human nose. Dolphins breathe consciously –  not automatically, like humans  – meaning that they actively decide when to inhale and exhale.¹⁴

Dolphins have become good at holding their breath while they’re submerged: they only have to resurface for air every eight or 10 minutes on average, and some species can swim to depths of several hundred metres in between breaths.¹⁵ Scientists have suggested that dolphins can slow down their heart rate when they descend into the water, in order to make the most of the oxygen they have while submerged and to conserve energy.¹⁶

Dolphins have streamlined, torpedo-like bodies which are perfectly adapted to living in their watery environments. Apart from few quirks  – some have rounder faces, others have slimmer hips – dolphin species all look fairly similar.¹⁷ Their bodies are covered in buttery fat called blubber which keeps them warm and makes their skin look sleek and tight. Most dolphins have beak-like mouths containing tiny cylindrical teeth, plus two long, slim, powerful pectoral flippers, and a strong, fluke-like tail.

The average bottlenose dolphin is between two and four metres long and weighs between 130 and 630kg.¹⁸ Most of these dolphins are somewhere in between, at 2.5m, but sometimes bottlenose dolphins can vary in size according to where they live and the UK variety of bottlenose dolphins is considerably larger, often getting as long as 4m in length. The smallest species of dolphin is the Māui dolphin, also known as a subspecies of Hector’s dolphin, which is usually 1.5m in length, weighs just 50kg, and likes to hang out in 20m deep waters.¹⁹ The largest dolphin species is the orca, also known as the killer whale –  it can get to just below 10m long and a whopping 10,000kg heavy.²⁰

Their compact and sleek body enables dolphins to swim effectively and to glide quickly across water.²¹ When researchers recorded 2,000 different speed measurements for both wild and captive dolphins they found that bottlenose dolphins could swim at a speedy 8.2m/s, closely followed by common dolphins and false killer whales at 8m/s. When the U.S. Navy tried to find the fastest dolphins, they recorded Pacific spotted dolphins sprinting along at a brisk 39.7km/h.²²  Other studies have recorded bottlenose dolphins swimming at 32 km/h, and common dolphins moving at a swift 60km/h.²³

Dolphins can also leap and flip several metres out of the water with both fervour and grace. This is something they do quite often, and it is thought that the movements are performed as entertainment, for communication, and as a way to get a better view of what’s going on around them.²⁴ Swimming upwards, preparing for a vertical leap out of the water, bottlenose dolphins could lunge at up to 11.2m/s.

Looking at the ‘speech’ of dolphins has kept scientists busy for decades.²⁵ Dolphins are sophisticated communicators and use an elaborate range of pulses and sounds. They can make a variety of whistles, clicks, and squeaks in order to ‘chat’ with each other. As dolphins produce these sounds without actually having any vocal cords, it’s believed that they blow air through the sacs under their blowhole instead.²⁶

Scientists have found that bottlenose dolphins have unique signature whistles which represent them as an identifier or name.²⁷

Most of these sounds are higher-pitched than the sounds that humans can hear. Dolphins can hear frequencies as high as 150 kHz, whilst humans can hear up to 20 kHz in comparison.²⁸ A dolphin’s signature whistle ranges from 7 to 15kHz, echolocation clicks hit the 40 to 130 kHz range, while social communication is strongest at around 40 kHz.²⁹ Dolphin mothers speak to their calves with even higher-pitched tones, as humans do with babies or pets.³⁰

Experiments on bottlenose dolphins suggest that they can communicate anywhere from 2 to more than 20km apart.³¹ But regardless of how sophisticated their sound systems are, dolphins also seem to communicate via body language when they’re close to each other. They’ve been observed flexing their tails, wagging their heads, rolling their eyes and nuzzling  – but researchers are still trying to work out what those actions might actually mean.

The variety of squeaking sounds used by dolphins isn’t just reserved for communicating with each other – they also use the higher-pitched clicks to better understand their surroundings through a system called echolocation.³²

Dolphins emit clicks which bounce off the surface of objects in the water. They then use the echoes of these sounds to determine the dimensions of anything that surrounds them.³³ They use one clicking rhythm for their immediate surroundings, up to 120m, and more frequent clicks for objects as far as 400m away.³⁴ This allows the dolphin to glean information about any possible prey or predators near them, which is very useful when trying to hunt in murky waters.

To amplify their high-pitched clicks, dolphins have a specialised organ called the melon – a ball of adipose tissue in their head which helps to channel the clicks into directional beams.³⁵ Dolphins can pull off this impressive feat because they possess what is deemed some of the best hearing in the animal kingdom: despite having really small ear holes, sound passes through their fatty jaws and heads until it reaches their internal listening system.³⁶

Dolphins can be graceful, playful creatures but they’re also formidable hunters. They are carnivorous and, depending on the species, love to gorge on fish, shrimps, squids, jellyfish, octopuses and more.³⁷ Dolphins swallow their prey whole and only use their tiny teeth to grasp their meals before gulping them down in one bite. The Risso’s dolphin has very few teeth – between four and 14 – whereas bottlenose dolphins have up to 100, and the spinner dolphin can have up to 240.³⁸ But a dolphin’s teeth are never used for chewing. Orcas are the most voracious of dolphins and also hunt seals and penguins.

A bottlenose dolphin eats between 4 and 6% of its body weight in prey per day, with larger dolphin species such as orcas eating between 1 to 3.5% of their heft in daily nutrition, which can still amount to a feast of up to 800kg.³⁹

Dolphins make brilliant hunters as they have plenty of tricks and tips for acquiring the snacks that they want. They often work as a team with their pod-mates to corral entire schools of fish, and then take turns ploughing through them with their mouths open.⁴⁰  Some dolphins cast water nets to pack schools of fish together tightly, as seen in the video below, or push the fish toward the shore where they have fewer escape routes. Other dolphins use a method called fish-whacking, where they slap fish hard with their tail in order to stun and catch their prey.⁴¹

As they have to come up so often for air, dolphins don’t have much time to rest or relax without needing to breach the water’s surface for oxygen. So in order to sleep, they laze just centimetres below the water surface and switch off half of their brain at a time – a process called unihemispheric sleep.⁴² Dolphins spend about a third of their time doing this.⁴³ But they also sleep with one eye open – so they’re not caught off guard by predators – and it’s thought that they can continue to echolocate even when resting.⁴⁴

Dolphins are able to stay close to the water’s surface without sinking because they’re very fatty and their bodies can hold in loads of air.⁴⁵ Even while sleeping, they give themselves small, reflex-like pushes with their tail to keep their blowhole exposed.⁴⁶

Dolphins’ eyes are quite small for their body size, but it doesn’t stop most species from seeing quite well for a mammal.⁴⁷ Marine dolphins have good eyesight both underwater and above, and both in broad daylight and dim light. They can see some colours, but probably not to a great degree of detail. As dolphins have an eye on each side of their heads, they can see to each of their sides and behind them – but they see two separate fields of vision, and do not combine these fields into one in the way that humans do.

Dolphins have unique, different personalities, and they are highly social animals.⁴⁸ They live in pods of around two to 30 dolphins and like to swim together, hunt together, play together, raise each other's calves, and nurse each other if injured. Being part of such tight-knit pods is also helpful for protection against predators.

When scientists closely studied the relationships between some bottlenose dolphins within pods, they discovered that individuals have pod members that they’re closer to than others – rather like having best friends.⁴⁹ These smaller cliques help each other when looking for mates, in what the scientists call “alliances within alliances”.

Dolphins can form such deep bonds thanks to their strong social memory: they can recognise the unique whistles of old friends even after more than 20 years of not seeing them.⁵⁰ Sometimes pods meet up with other pods and come together to make superpods formed from hundreds of dolphins.⁵¹ They also cooperate with other animals of the sea, illustrated by the case of two dolphins that helped to save two whales which had beached.⁵²

Bottlenose dolphins can recognise their reflection in the mirror, suggesting they may have some degree of sense of self, or some self-awareness.⁵³ This is a very sophisticated cognitive ability that very few animals possess, and dolphins can do it at an earlier age than humans.⁵⁴

Dolphins may not have hands but it hasn’t stopped them from using tools  –  another key indicator of an animal having cognitive abilities. They play with rings of air underwater, in a similar way to human children playing with bubbles.⁵⁵ They use sponges as mittens for their beaks to protect themselves from scratches while trawling the seafloor for nibbles, they catch prey by cupping them with empty shells, and they’ve been observed using chemical-oozing corals to rub their skin rashes.⁵⁶ Older generations of dolphins teach tricks to the younger members of their pods too.⁵⁷

Dolphins engage in foreplay with their pod mates with their flippers and mouths, and even masturbate by rubbing or digging their genitals into sand, sponges, and pretty much anything else they can find.⁵⁸ Females have a very large, functional clitoris.⁵⁹

When it’s time to mate, some dolphin species go belly to belly and other species get into T-formation, then the male inserts his penis into a slit on the female’s lower belly.⁶⁰ Intercourse and mating can be a violent and coercive experience for females.⁶¹

The inside of a dolphin’s vagina is made up of many flaps and folds, probably to keep seawater out.⁶²  Some species can be pregnant for 10, 12, 14, or 15 months, while others gestate for up to 18 months before giving birth, such as orcas. Dolphins give birth to one calf at a time – which, surprisingly for mammals, slips out tail first – and then they take care of their young for anywhere from three to six years.⁶³

But dolphins aren’t monogamous and they don’t mate for life: they mate with several different partners throughout the year, even when it isn’t mating season.⁶⁴

Dolphins come across as curious, outgoing, good-humoured animals who love to play and are friendly to humans: they can often be found swimming alongside boats, seemingly interacting with the crafts. Dolphins who have lost their pod and end up roaming the sea solo are often referred to as ‘lone sociable dolphins’ because they seek out human interaction  –  but they’ve also been reported to become very aggressive really quickly.⁶⁵

In fact, dolphins are wild animals, and interacting with them, trying to touch them, or feeding them is heavily advised against. Most species of dolphins do not ever interact with humans throughout their lives. They should not be disturbed in their natural habitat.⁶⁶ If a dolphin is angry, alone, stressed, hungry, confused or annoyed, it may act erratically and violently. Even if a dolphin is just playing, it can bite, headbutt, and drag humans underwater  –  as seen in the famous case of Lisa Costello in 1992.⁶⁷ Swimming close to dolphins and trying to pet them can be exceptionally stressful for the animals.⁶⁸

Whilst dolphins have been friendly with humans over the centuries, humans haven’t always been too friendly back, hunting them for their meat and blubber, and undermining their populations in the wild.⁶⁹ Even in the 21st century, dolphins are often tangled and strangled by far-reaching commercial fishing nets.

Dolphins have acute hearing abilities, and a growing body of research has indicated that the copious amounts of noise that humans have introduced into the seas, including using industrial ships and sonar, heavily disturbs them and undermines their ability to communicate and move around.⁷⁰

Habitat destruction and chemical pollution from industrialisation have also heavily impacted dolphins’ environments. Climate change and an increase in water temperature have also made it harder for dolphins to mate and reproduce.

According to the IUCN, bottlenose dolphins are not threatened, but the Ganges river dolphin, the Indus river dolphin, and the Atlantic humpbacked dolphin are endangered. There are about 54 dolphins left from the Māui dolphin subspecies which live around the coasts of New Zealand according to a recent government survey.⁷¹

Crucially, humans have already contributed to the demise of one species of dolphin: the ‘baiji’ river dolphin of China. This dolphin – also known as the Yangtze River dolphin – is thought to have disappeared after heavy industrialisation, habitat destruction, and aggressive commercial fishing, and is now informally described as extinct.⁷² When researchers spent months at sea in 2006 surveying for any sightings of the animal, they came back empty-handed, declaring the species as ‘functionally extinct’.⁷³ But it’ll take a little more time before the baiji can be officially recognised as extinct by authorities because IUCN regulations require there be “no reasonable doubt” that the last individual member has died, wanting to avoid precocious extinction declarations.

Featured image © Nathaniel J Barrie | Unsplash

Fun fact image © Pablo Heimplatz | Unsplash

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