Creature Feature: River dolphins

Oh, dolphins. 

As an avid pinniped (i.e. seals, sea lions, & walruses) nerd and scientist, I hold a certain *eyeroll* disdain for cetaceans (the phylogenetic group that includes dolphins and whales). There is this childish conflict between marine mammal researchers where we all attempt to place the animals we study into a hierarchy above our colleagues’ animals (don’t deny it marine mammal friends, we all know it’s true), and cetaceans always seem to end up on top. This superficial level of immaturity is not completely unwarranted; the typical association with the term “marine mammal” is a slippery, gray torpedo that squeaks at you with a permanently toothy grin. Whales and dolphins dominate the presentations and discussions at marine mammal conferences, and for other types of marine mammal researchers, the amount of funding for cetacean research seems to be disproportionate. But, dolphins can’t help their cartoonish level of charisma. They can’t help their wickedly awesome aquatic adaptations of echolocation, hydrodynamic body evolution, and intricate social behaviors that fascinate and baffle scientists. They can’t help how intelligent they are and how their ability to communicate and innovate may convince us humans that we aren’t the only species with complex cognitive thinking. While I am a permanent lover and studier of the under (the sea) dogs in the marine mammal world, in honor of National Dolphin Day (April 14, 2021), I wanted to highlight the underdogs in the dolphin community—river dolphins! 

A screenshot of a video game

Description automatically generated with medium confidence

Beyond Flipper and friends 

Most marine mammal paraphernalia is modeled after the Atlantic bottlenose dolphin (Tursiops truncatus). Yet, within the family Delphinidae (i.e. oceanic dolphins), there are 30 species that are found across the planet. However, most species reside in tropical waters (I mean, who wouldn’t want that?). Delphinids range from the petite 110 lb (50 kg) Maui’s dolphin (Cephalorhynchus hectori maui) to the daunting 12,000 lb (5,400 kg) killer whale (Orcinus orca) [1]. While bottlenose dolphins may be the MVP, oceanic dolphins all get to sit at the popular cetaceans table: full of charisma, salty smiles, and Hollywood attention. River dolphins, on the other hand, are much less mainstream (get it—stream… river… anyways!). 

A grey dolphin jumping out of the water vertically, with the yellow text below it reading "flipper". The water is a blue-yellow color.
“Flipper” was a movie, turned TV show in the 1960s that was later remade in 1996. The character “Flipper” was played by 5 bottlenose dolphins (Tursiops truncatus) [Source].

River dolphins thrive in fresh or brackish (i.e. a mix of ocean and fresh) water. With warm temperatures in shallow riverbeds and delta habitats, river dolphins don’t need much blubber (i.e. a specialized fat that stores energy, insulates heat, and increases buoyancy) unlike their ocean-dwelling relatives [1]. Similar to oceanic dolphins, river dolphins also rely on echolocation to locate food, navigate, and determine the size and shape of environmental structures around them [2]. Echolocation is a special adaptation similar to SONAR. When echolocating, dolphins send out high-frequency clicks through their melon (the fat-filled organ on the front of their head). As the clicks bounce off the objects in the dolphin’s environment, they are “collected” through another fatty-structure in their lower jaw (mandibular fat pad) and then perceived through vibrations in the inner ear [2]. Their echolocation is so efficient that many dolphins can survive using echolocation, even if they are blind—which brings us to river dolphins’ beady little eyes! These murky waters with strong river currents means they are unlikely to see through muddy turbidity, so river dolphins have very small eyes for their size [1]! The combination visual pinholes, an extra-long rostrum (i.e. the nose/beak area of a dolphin’s face), and protruding melon give them a goofy look that may have led to them being shunned from the cool, charismatic cetacean table?

Dolphins use echolocation to communicate. Outgoing clicks come from the melon at the front of the dolphin’s head. [Source]

But, as we all know, looks aren’t everything! River dolphins are fascinatingly elusive and  prominent characters in different cultural mythology and folklore. They are also vital parts of their ecosystems; however, because of their limited ranges in freshwater areas, they are far more susceptible to anthropogenic (i.e. human-caused) disturbances. Their population size has been decimated by damming rivers, entanglement in fishing gear, illegal hunting for oil and meat, overexposure to run-off pollution from industrial and agricultural chemicals, and reduced river depths due to water diversion. These issues are often additive, and it becomes no surprise that all seven species of river dolphins are on the IUCN Red List of Threatened Species from vulnerable to critically endangered.

Map of the world in grey with the ranges of river dolphins highlighted in teal. River dolphins are found in the Amazon River basin in northern South America, and throughout river basins in China and northern India.
Marine Mammal Commission’s global map of river dolphin geographic ranges shows that they are quite small [Source].

Amazon (Inia geoffrensis geoffrensis) & Bolivian (Inia geoffrensis boliviensis) River Dolphins

A pink Amazon River dolphin. The dolphin has a pointy nose, and is slightly grey on its back. It is swimming in a River, with green plants in the background on shore.
A rare sighting of an Amazon River dolphin [Source]

Where are they:  Amazon and Bolivian river dolphins are found throughout the entire Amazon River basin with a range of 7 million square kilometers that span through six countries in South America, as well as the Orinoco River basin in Venezuela. Bolivian river dolphin populations are separated from Amazon river dolphins by a series of waterfalls and intense rapids, making them more isolated to the southern Amazon basin in (you guessed it) Bolivia. Due to the seasonal fluctuations of the tributaries, the distribution and abundance of these dolphins varies. River dolphins in the genus Inia are considered to be the most abundant [3].

Map of the geographic range of the Amazon River (yellow) and Bolivian (purple) River dolphins in South America [Source]

Why they should be at the “cool kids” table: According to Amazonian folklore and culture, boto (the local name of river dolphins) have the supernatural power of shapeshifting by transforming into attractive people at night who come ashore and seduce humans while retaining some of their dolphin-ly features (such as their blowhole they cover with a hat) [4]. Other tales state Amazon River dolphins are the guardians of the Amazonian manatee and anyone who wishes to locate a manatee must first make peace with their pink-skinned guardian. Unlike other dolphin species, boto are mostly solitary and less gregarious than their oceanic cousins. This, however, does not mean these aquatic “introverts” are not playful; Amazon River dolphins have been documented pulling on canoe paddles, pulling and flinging grass, sticks, and clay, and interacting playfully with turtles, snakes, and fish. Their solitary nature also doesn’t mean they don’t interact with other dolphins at all. In fact, the largest of the species of river dolphin, the Amazon River dolphin’s iconic pink body color is a reflection of scarring across their bodies with males being pinker because there is more intermale aggression [3]!


Baiji or Yangtze River Dolphin (Lipotes vexillifer

A Baiji swimming [Source]

Where are they: Baiji are endemic (found only in one particular area) to the 1,100 miles (1,700 kilometers) of the Yangtze River near Shanghai, China. However, in more recent years, their range was reduced to the main channel of the river which is just under 200 miles (~300 km) [5].

A map of reserves on the Yangtze River for the Baiji [Source]

Why they should be at the “cool kids” table: Baiji are the only living representative of the phylogenetic family of mammals (Lipotes). Scientists regard these types of animals as “living fossils,” because without these living species, we would only have their fossil remains, geological features, and geographical location to speculate about their life history and ecological role.  Ancient folklore reveres the Baiji as the protector of local fishers and mariners. The “Goddess of the Yangtze” is said to be the reincarnation of a girl who freed herself from her evil stepfather by diving overboard during a storm that sank the stepfather’s boat on the Yangtze River [6]. Tragically, despite local and international conservation efforts and undocumented sightings sporadically reported as recently as 2016, Baiji have been declared as functionally extinct. The Yangtze River plain is home to 12% of the world’s population, so human activities, including commercial fishing, intense vessel activity, and the damming of different parts of the river, have affected every part of the Baijis’ existence. The Baiji is the first cetacean species driven to extinction by non-hunting human activity [6,7].


Ganges (Platanista gangetica) & Indus (Platanista minor) River Dolphins

A Ganges River dolphin showing its characteristic long rostrum [Source]

Where do they live: Both Ganges and Indus River dolphins live in southern Asia.  Ganges River dolphins live in the Ganges and Brahmaputra rivers through India, Nepal, and Bangladesh, and Indus River dolphins live primarily in the main channel of the Indus River Pakistan into Punjab, India. Local names for these dolphins (susu for Ganges dolphins and bhulan for Indus dolphins) usually replicate the sound they make when they pop up to breathe [8].

Map of the geographic ranges of the Indus River dolphin (teal) and Ganges River dolphin (blue) [Source]

Why they should be at the “cool kids” table: Ganges and Indus River dolphins were thought to be a single species known as the South Asian river dolphin (Platanista gangetica) in the 1990s. However, a brand-new study found that dolphins found in the Indus river and those in the Ganges rivers are in fact their own unique species [9]! After 20 years of studying dolphin skulls around South Asia, an international collaboration of researchers found that aside from genetic differences, these river dolphin species have different morphological features, including skull shapes and number of teeth. Similar to the Baiji, Ganges and Indus River dolphins are also “living fossils” as they are the only living species left in the Platanistae phylogenetic family. Another fascinating fact about these dolphins is they lack a lens in their eye (which is no use to them in muddy rivers), and thus rely on their echolocation and their long rostrum root through the river bottoms for prey [8]. Finally, these guys are side-swimming cetaceans, which is adaptive for swimming in shallow water, but as they bounce their heads up and down, sweeping the silty bottom for food, their break-dancing-like behavior matches their goofy faces [10].

An Indus River dolphin showing off it’s characteristic swimming pattern [Art & Animation by: Sissy Emmons Hobizal, WWF]

While pinnipeds will remain at the top of my school-kid hierarchy of favorite animals, river dolphins now hold a special place in my heart. We still know so little about these animals’ behavior, ecological roles, and communication abilities. Yet with each passing day, we are losing the opportunity to learn about and from them as their population numbers dwindle to zero, like the Baiji. Science is not just about discovery or cutting-edge research; it is about learning as much as we can about the world and the other beings with whom we share this planet. Conservation efforts are intersectional, thus, valuing local community knowledge and engagement are necessary to make impactful differences. If you want to learn more about the conservation efforts of these species, check out the resources below. If you are fascinated after diving into the world of these delightful, derpy river dolphin species, be sure to learn more about the Araguaian River Dolphin (Inia araguaiaensis) and the La Plata Dolphin (Pontoporia blainvillei) and research ways you can support these animals from near or far! Happy International Dolphin Day!

Conservation Programs and Efforts


Karli Chudeau is a graduate student in the Animal Behavior Graduate Group and a part of the UC Davis Coastal Marine Sciences Institute. She is interested in conservation management and assessing animal welfare in wildlife rehabilitation settings. Her current research examines how we can use behavioral management interventions, such as environmental enrichment, to improve reintroduction success with pinnipeds. She is also an avid ocean nerd.


References:

[1] Würsig, B., Perrin, W. F., & Thewissen, J. G. M. (Eds.). (2009). Encyclopedia of marine mammals. Academic Press.

[2] Nummela, S. (2009). “Hearing”. In Perrin, William R.; Wirsig, Bernd; Thewissen, J.G.M. (eds.). Encyclopedia of Marine Mammals 2nd edition. Academic Press. pp. 553-562. 

[3] da Silva, V.M.F. (2009). “Amazon River Dolphin”. In Perrin, William R.; Wirsig, Bernd; Thewissen, J.G.M. (eds.). Encyclopedia of Marine Mammals 2nd edition. Academic Press. pp. 26-28.

[4] Cravalho, M.A. (1999). Shameless creatures: An Ethnozoology of the Amazon river dolphin. Ethnology, 38(1), 47-58. 

[5] Braulik, G. T., Reeves, R. R., Wang, D., Ellis, S., Wells, R. S., & Dudgeon, D. (2005). Report of the workshop on conservation of the baiji and Yangtze finless porpoise. Gland, Switzerland: World Conservation Union.

[6] Maderspacher, F. (2007) Bye Baiji? Current Biology, 17(18), PR783-784.

[7] Zhou, K. (2009). “Baiji”. In Perrin, William R.; Wirsig, Bernd; Thewissen, J.G.M. (eds.). Encyclopedia of Marine Mammals 2nd edition. Academic Press. pp. 71-75.

[8] Smith, B.D. & Braulik, G.T. (2009). “Susu and Bhulan”. In Perrin, William R.; Wirsig, Bernd; Thewissen, J.G.M. (eds.). Encyclopedia of Marine Mammals 2nd edition. Academic Press. pp. 1135-1139. 

[9] Braulik, G. T., I. Archer, F., Khan, U., Imran, M., Sinha, R. K., Jefferson, T. A., … & Graves, J. A. (2021). Taxonomic revision of the South Asian River dolphins (Platanista): Indus and Ganges River dolphins are separate species. Marine Mammal Science. DOI: 10.1111/mms.12801

[10] Herald, E. S., Brownell, R. L., Frye, F. L., Morris, E. J., Evans, W. E., & Scott, A. B. (1969). Blind river dolphin: first side-swimming cetacean. Science166(3911), 1408-1410.

Main cover image painted by Karli Chudeau

[Edited by Meredith Lutz]

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