What kind of animal is the river dolphin? An updated explanation of its characteristics, ecology, and habitat

What kind of animal is the River Dolphin? We will explain its characteristics, ecology, and habitat. It is a Southeast Asian dolphin, and some can be seen in northern Australia, so its habitat is very wide, but it is actually designated as an endangered species. Let’s take a closer look at the details.

What is a River Dolphin? Basic Stats

The Irrawaddy Dolphin is a marine mammal that belongs to the genus Orcaella, family Delphinidae, suborder Odontoceti, order Cetacea. Its English name is 河巨頭, and its scientific name is Orcaella brevirostris. The list of information is as follows. Its total length is over 2m and its weight exceeds 100kg.

Japanese（和名）	カワゴンドウ
English（英名）	Irrawaddy Dolphin
scientific name(学名)	Orcaella brevirostris
classification（分類）	Mammalia、Odontoceti、 Delphinidae、Orcaella 哺乳綱、ハクジラ亜目、マイルカ科、カワゴンドウ属
IUCN Status(保全状況)	ENDANGERED
Length（体長）	2m
Weight（体重）	130kg

What are the roots and origins of the Irrawaddy dolphin?

The roots of the Irrawaddy dolphin (Orcaella brevirostris) differ significantly from those of the beluga whale; they evolved from a lineage of oceanic toothed whales that expanded into coastal and riverine environments. Although they bear a slight physical resemblance to belugas, they are not closely related in evolutionary terms. Under current classification, they belong to the family Delphinidae (oceanic dolphins).

The evolutionary path of the Irrawaddy dolphin

The general evolutionary progression is as follows:

Land mammals (approx. 50 million years ago)
↓
Primitive cetaceans (moved into the ocean)
↓
Toothed whales (approx. 30 million years ago or later)
↓
Ancestors of the Delphinidae family
↓
Coastal dolphins
↓
Genus Orcaella (Irrawaddy dolphins)

In other words, the Irrawaddy dolphin is:

Not a dolphin that originated in rivers, but rather an oceanic dolphin that moved back into river environments.

What kind of dolphin was its ancestor?

The ancestor of the Irrawaddy dolphin was likely a primitive member of the Delphinidae family that inhabited the oceans during the Miocene epoch (approximately 23 million to 5.3 million years ago).

In the oceans of that era, dolphins were diversifying into various forms, such as:

High-speed dolphins that pursued fish

Small dolphins that lived in coastal areas

Dolphins adapted to shallow waters

Among these groups, certain lineages expanded into:

Estuaries

Bays

Mangrove areas

The lower reaches of large rivers

Why did they move into rivers?

The primary reason the Irrawaddy dolphin moved into rivers was environmental adaptation.

① Abundant food sources

Estuaries and large rivers teemed with:

Small fish

Shrimp

Crabs

Shellfish

In particular, the murky waters likely offered fewer large marine predators and less competition from other species.

② Adaptation to coastal shallows

The Irrawaddy dolphin is not a strictly freshwater dolphin. Habitat:

Coasts

River mouths

Brackish water areas

Some large rivers

Representative habitats:

Myanmar: Irrawaddy River

Cambodia/Laos: Mekong River

Indonesia: Mahakam River

etc.

Classification

River whales were recognized in 1866 when they were reported as a new species by Richard Owen. Genetically, they are closer to killer whales than to dolphins. The Australian river whale is found in Australia and is becoming recognized as a separate species.

Taxonomy of River Dolphins

• 界（Kingdom）: 動物界 (Animalia)
• 門（Phylum）: 脊索動物門 (Chordata)
• 綱（Class）: 哺乳綱 (Mammalia)
• 目（Order）: 鯨偶蹄目 / 鯨目 (Cetacea)
• 亜目（Suborder）: 歯鯨亜目 (Odontoceti)
• 科（Family）: ハクジラ科 (Delphinidae)
• 属（Genus）: ゴンドウクジラ属 (Orcaella)
• 種（Species）:
  • カワゴンドウ（淡水系個体群）: Orcaella brevirostris
  • 近縁種: 海洋性のゴンドウクジラ (Orcaella brevirostris) とは同種だが、生息域が川や沿岸域に限定される。

Habitat

They are found in Southeast Asia (India, Bangladesh, Myanmar, Thailand, Indonesia, Cambodia, Philippines) and northern Australia. River whales can also be found in the Mekong River and Ayeyarwady River.

Main Habitats

Rivers of Asia

Mekong River (Laos, Cambodia, Thailand, Vietnam)

The world’s best-known freshwater population. It is listed as endangered, with an estimated population of only a few hundred individuals.

Indus River (Pakistan)

Small populations exist, but their numbers are extremely small.

Rivers of India

Lower Ganges River Basin

Although reported at one time, their population may have declined dramatically in recent years.

Coastal and brackish waters of Southeast Asia

Estuarine estuaries and lagoons in Malaysia and Indonesia

Populations adapted to brackish waters where freshwater and seawater mix.

China

Yangtze River

There once existed a population known as the “Yangtze River dolphin,” but the remaining individuals are considered nearly extinct.

Habitat Characteristics

Freshwater: Mid- to lower reaches of rivers, with relatively slow currents.

Brackish water: Places where seawater and freshwater mix, such as river estuaries and lagoons.

Depth: They feed on fish in relatively shallow rivers where the bottom is visible.

Impact of human activities: Their habitat is shrinking due to dam construction, shipping traffic, and water pollution.

What are its characteristics? What is it like?

The river whale has no beak and looks similar to a finless porpoise or a beluga whale. Its entire body is brown with a lighter color underside. Its lips are flexible and it can blow water out of its mouth. It inhabits the warm coastal waters and rivers from the Bay of Bengal to northern Australia. It swims by rotating as it rises and only raises its tail above the water when it dives.

1. Physical Characteristics

Size

Adults measure approximately 2-2.5 m in length and weigh around 90-150 kg.

Body Shape

Their dorsal fin is small and round (not nearly triangular, but turtle-shell shaped).

They have a rounded head, a dolphin-like face, and a short snout.

Their body color is gray to pale blue-gray, with a whitish underside.

Unique Features

Because they belong to the Pilot Whale genus, they have a rounder shape than typical dolphins.

They are similar to oceanic pilot whales, but the freshwater species is slightly smaller and slender.

2. Behavior and Ecology

Primaries

They often live in small groups (5-20 individuals).

Diet

They feed on fish and small crustaceans.

They are adept at catching fish even in shallow rivers.

Swimming

They live in rivers with a leisurely pace, rather than fast swimming.

Calls

They communicate with high-pitched clicks and calls.

Reproduction

Mothers care for their young for long periods of time.

They give birth to one calf at a time.

3. Ecological Characteristics

Because they are adapted to freshwater and brackish waters, their body shape is suited to river currents, something not seen in marine dolphins.

Their dorsal fins are small, allowing them to swim in shallow waters, reducing resistance from currents.

Because they live in rivers close to human habitation, they are heavily affected by boats and fishing.

What is its personality like?

The river whale, like other dolphins, has a very gentle and calm personality.

River Whales’ Personality and Behavior

Gentle and calm

They often swim leisurely in rivers and rarely become suddenly aggressive.

They are relatively docile toward humans and may approach them out of curiosity (however, contact with wild individuals is dangerous).

Highly curious

They may approach boats or anglers and are sensitive to changes in their surroundings.

They may also play with floating objects or bubbles on the water’s surface.

Social

They live in small family or parent-child groups.

They actively communicate with each other (by calling and clicking).

They exhibit cooperative behavior within the group to protect and care for their young.

They can also be timid.

They are sensitive to strong currents, boat noise, and large-scale fishing activity.

They will suddenly flee as a group if they sense danger.

Playful

They have been observed jumping on the water surface and splashing with their tails and bodies.

In river environments, they also play games with fish in shallow waters, chasing each other.

What is its ecology like?

The river whale mainly eats squid and fish. It lives by eating creatures that live on the bottom of the water. Its lifespan is about 30 years.

1. Habitat

Freshwater rivers: Live in shallow areas and winding channels in the mid- to lower reaches.

Brackish waters: Live in places where freshwater and saltwater mix, such as estuaries and lagoons.

Water depth: Can catch fish in shallow rivers.

Impacts of human activity: Boat traffic, fishing, water pollution, and dam construction limit their habitat.

2. Diet (What do they eat?)

Mainly fish (carp, catfish, freshwater fish, etc.)

Also prey on small crustaceans.

Preying method

Work together in schools to drive and catch fish.

Because rivers are shallow, they hunt by jumping and circling.

3. Behavior

School Life: Usually small groups of 5-20 individuals.

Swimming Pattern: Often swims slowly in rivers, rarely making sudden movements.

Calls/Communication:

Communicates with peers using high-frequency clicks and whistling calls.

May alter sounds to suit the river environment.

4. Reproduction

Gestation Period: Approximately 10-12 months.

Birth: One baby at a time.

Rearing: Mothers take the lead in caring for the baby for an extended period.

Sexual Maturity: Females reach sexual maturity at approximately 5-10 years, males slightly later.

5. Adaptations and Characteristics

Small dorsal fin → Easier to swim in shallow, flowing rivers.

Small, rounded body → Adapted to river navigation and prey hunting.

Timid and cautious → Sensitive to boats and humans.

Do they have any natural enemies?

Sharks are the biggest threat to the river whale.

About Diving

The diving ability of the Irrawaddy dolphin is not of the “deep-diving” type seen in belugas or bottlenose dolphins; instead, it is characterized by capabilities evolved for hunting prey in shallow rivers and coastal waters.

Dive Duration

Typical: Approximately 1–3 minutes

Long dives: Around 5–6 minutes

Rather than diving for extended periods, Irrawaddy dolphins tend to:

Thoroughly explore the underwater environment

Repeatedly perform short dives

Rivers and Coastal Areas

Typical:

Around a few meters to 20 meters

Deep areas:

Can dive up to

Several tens of meters

Since their habitat consists primarily of shallow waters—such as

The Mekong River

The Irrawaddy River

Estuaries

Mangrove areas

—they do not require the ability to dive to depths of hundreds of meters.

Why don’t they dive deep?

Their hunting grounds are not in the deep ocean, but rather:

Near the riverbed

In shallow coastal waters

Over muddy bottoms

Around rocky areas

Their prey consists of

Small fish

Shrimp

Crabs

Mollusks

so there is no need to pursue them into deep water.

About Their Vocalizations

The vocalizations of the Irrawaddy dolphin differ somewhat from the “varied, song-like sounds” of the beluga whale; instead, they have evolved into a high-performance acoustic system suited for life in murky rivers and coastal waters.

① Click Sounds (Pulsed Sounds)

These are the most important sounds.

Uses:

Locating fish

Scanning the riverbed or rocky areas

Gauging distance from other dolphins

Detecting obstacles

Mechanism:

Emit sound
↓
Reflects off an object
↓
Analyze the returning sound
↓
Understand the surrounding environment

In murky rivers, sound is more important than vision.

② High-Frequency Pulsed Sounds

Irrawaddy dolphin clicks contain very high frequencies.

Characteristics:

Rapid, continuous emission of short sounds

Precise scanning of narrow areas

Capable of detecting even small fish

Particularly effective in:

Muddy bottoms

Mangrove areas

Areas with strong currents

③ Whistles

Sounds commonly observed in dolphins.

Uses:

Communicating with others

Maintaining the pod

Social interaction

Irrawaddy dolphins are highly social and travel in pods ranging from a few to several dozen individuals, making acoustic coordination essential.

How do they produce sound?

It differs from the human vocal cord mechanism.

Irrawaddy dolphins use:

“Phonic lips” (sound-producing organs)

located deep within the nasal passage.

Process:

Move air from the nasal passage

Vibrate the sound-producing organs

Focus the sound using the “melon” (fatty organ) on the head

Radiate the sound into the water

Seasonal Activities

The seasonal activities of the Irrawaddy dolphin are not characterized by large-scale “Arctic-style seasonal migrations” (like those of beluga whales); instead, they are strongly influenced by changes in the river environment caused by the alternation between the rainy and dry seasons.

🌱 Spring (March–May): End of the Dry Season / Foraging Season

Varies by region

Key Activities

Gathering in deep areas

Increased time spent foraging

Moving in pods

As river water levels drop during the late dry season,

Shallow areas increase

Fish migration routes shift

…causing Irrawaddy dolphins to utilize deeper waters and river pools (deep sections).

Behavioral Characteristics

Frequent use of echolocation

As water levels recede,

Rocks

Driftwood

Riverbed

…obstacles become more prevalent, making exploration via click sounds crucial.

☔ Summer (June–September): Rainy Season / Period of High Activity

This period sees the most significant environmental changes in tropical regions.

Rising river levels

During the rainy season,

River width expands

Water depth increases

Variety of prey increases

…leading to an expanded activity range.

Moving in pursuit of prey

During the rainy season, fish move to:

Tributaries

Shallow areas

Coastal zones

Irrawaddy dolphins follow them, sometimes:

Entering tributaries

Moving toward river mouths

Utilizing new feeding grounds

Calf-rearing period

In many regions, the following are observed:

Birth

Calf-rearing

Calves learn the following from their mothers:

Swimming techniques

Echolocation

Foraging methods

🍂 Autumn (October–November): Falling Water Levels / Season of Movement

As the rainy season ends,

River water levels drop

Fish distribution shifts

…causing changes in their activity range.

Key Activities

Shifting feeding grounds

Moving to deeper waters

Reorganizing pods

Efficiently catching fish

As falling water levels make it easier for fish to congregate in limited areas,

Herding fish in pods

Foraging in shallow areas

…become more common behaviors.

❄️ Winter (December–February): Dry Season & Energy-Saving Period

Although located in the tropics, this period marks the dry season.

Key activities include:

Utilizing specific deep-water areas

Minimizing travel distances

Concentrating foraging efforts in specific areas

These behaviors are particularly critical for river-dwelling populations.

In rivers like the Mekong, water levels drop significantly during the dry season, leading to issues such as:

Shrinking habitats

Concentration of individuals

Increased vulnerability to human activity

Swimming Speed

The Irrawaddy dolphin is not a high-speed swimmer like the bottlenose dolphin; instead, it is a slow-moving but highly maneuverable swimmer capable of making tight turns in narrow, complex river environments.

Normal Speed

Approximately 3–8 km/h

This is the speed at which they typically swim.

Uses:

Foraging for food

Traveling with the pod

Patrolling the river

Irrawaddy dolphins excel at hunting by using sound to locate and approach prey, rather than chasing it down.

Cruising Speed ​​(During Travel)

Approximately 5–10 km/h

They swim at a steady pace to conserve energy when traveling through rivers or along the coast.

Their range of movement varies depending on environmental conditions, such as:

Deep sections of the river

Estuaries

Shallow coastal waters

Maximum Speed

Approximately 20–25 km/h

They are capable of short bursts of speed.

Situations:

Chasing fish

Escaping danger

Rapidly approaching other dolphins

However, they are not adapted for sustained high-speed swimming.

About Baby River Whales

Baby River Whales (juveniles) have the following characteristics. Because they are adapted to life in rivers and estuaries, they exhibit behaviors and ecology slightly different from adults.

1. Physical Characteristics

Body Length: Approximately 80-100 cm at birth

Weight: Approximately 10-15 kg

Body Color: Light gray, slightly darker on the back, and whitish on the ventral side

Dorsal Fin: Small, rounded, the same shape as adults

2. Behavior and Personality

Dependent on Mother: For about a year after birth, they swim close to their mother, protecting them from food and danger.

Playful: They jump on the water surface and chase each other with their mother and siblings.

Learning Behavior:

They learn how to hunt and catch fish from their mother.

They also practice adjusting to the currents of rivers.

Sociability: They learn to communicate with their group members early on.

3. Biological Features

Diet: Breastfed for the first few months of life.

Weaning: Around 6-12 months of age, babies gradually begin eating fish.

Growth: Over the course of 1-2 years, they nearly double their body length and become capable of moving like an adult.

Dangers: Young babies are at high risk of accidents in shallow rivers, due to boat traffic, and becoming entangled in fishing gear.

Are River Dolphins Endangered?

River dolphins are designated as an endangered species. Their population is declining due to humans, with the biggest threat being capture for the purpose of catching oil. Decreasing population and shrinking habitat are major issues, and habitat fragmentation has also occurred. In the early 2000s, the number of individuals dropped dramatically to just around 50, and complete extinction was feared. However, conservation efforts are progressing, and the population is beginning to increase.

1. International Union for Conservation of Nature (IUCN) Assessment

Freshwater river populations (e.g., Mekong River, Yangtze River)

Some populations are classified as “Endangered” (EN) or “Critically Endangered” (CR) on the IUCN Red List.

For example, the Mekong River river dolphin is Endangered (EN).

The Yangtze River river dolphin is considered nearly extinct.

Marine populations

Coastal-dwelling pilot whales are sometimes classified as Vulnerable (VU).

2. Reasons for Endangerment

Habitat Destruction

Dam Construction and Channel Improvements Divide River Environments

Changes in Flow Speed ​​and Depth, Reducing Feeding Grounds

Conflicts with Fisheries

Frequent Entanglement in Nets and Fishing Gear

Dwindling Fish Stocks Lead to Less Food

Water Pollution

Deteriorating River Water Quality Due to Industrial Wastewater and Pesticides

Vessel Traffic

Collisions Caused by Vessel Traffic on Rivers

Low Reproductive Rate

Small Herds and Low Number of Offspring

Can River Dolphins be kept as pets?

River dolphins are an endangered species and cannot be kept by the general public. We recommend viewing them at an aquarium or other venue and being guided through events. You can see them at dolphin shows.

1. Legal and Conservation Issues

Designated as an endangered species, capture and sale are strictly regulated internationally (listed under CITES Appendix I or II).

Capturing and keeping wild river dolphins is illegal in Japan.

Overseas, captivity is generally permitted only in special research facilities and aquariums.

2. Ecologically Difficult to Keep

Requires a vast freshwater or brackish water environment.

Recreating the flow of a river or estuary is nearly impossible.

Special dietary habits.

Eat large amounts of raw fish and must learn predatory behavior.

Highly social.

Living in small groups, keeping them alone is stressful.

Health management is difficult.

Wild individuals are sensitive to disease and stress, and there are few examples of them surviving long lives in artificial environments.

3. Aquarium Breeding

Marine pilot whales and freshwater river dolphins are only kept in limited numbers in specialized facilities.

They are extremely rare in Japanese aquariums and are rarely observed.

Breeding them requires large pools, specialized staff, and the constant presence of veterinarians.