River dolphin species may hold the key to the evolution of communication in marine mammals.

River dolphins

There are six species of freshwater dolphins (order Cetacea). These dolphins are found in rivers of south -central Asia, China, South America and in the coastal waters of Argentina, Uruguay and Brazil. These dolphin's morphology is characterised by having one beak and rounded foreheads, distinguishing them from the more familiar-looking dolphins such as the bottlenose dolphin ( Tursiops truncates ) of the family Delphinidae. River dolphins have evolved from marine dolphins that inhabited freshwater environments during high sea-level periods, which occurred near the end of the Miocene Epoch (23 to 5.3 mya). There are two families of river dolphin - the Plantanistidae e.g. Indian river dolphins ( Platanista), and Iniidae, with species of the the genera Inia , Lipotes and Pontoporia .

River dolphins are thought to be less athletic than other delphinid species and do not breach or have charismatic surface displays. Instead, they spend much of their time at river bottoms, probing with sensitive snouts for mud-dwelling crustaceans and fish. River dolphins have small eyes: for example, Ganges and Indus river dolphins’ eyes even lack a lens. This means that river dolphins depend heavily on sound to navigate muddy waters and so, have developed advanced sonar capabilities—perhaps the best among all the cetaceans.

1. The Amazon River dolphin ( Inia geoffrensis ), also known as the Boto, bufeo, and pink dolphin is the largest of the river dolphin species and is found in the turbid waters of the Amazon and Orinoco river basins. Adult male Amazon river dolphins reach 2.4 meters and can weigh 160 kg. It's colour can vary from dark grey to mottled pink- and gray to bright pink, with calves born gray. They have been seen associating with other species of small dolphin such as the Tucuxi ( Sotalia fluviatilis ), which is found in both freshwater and marine habitats and is not classified as a river dolphin species.

2. The smallest river dolphin species, is the La Plata River dolphin ( Pontoporia blainvillei ), which reaches only 1.2 -1/7 meters and weighs 20-60kg, lives in South America. This species is also known as the franciscana, - it inhabits the coastal waters of Brazil, Uruguay, and Argentina. The La Plata has countershading of gray above and pale below, and females are usually larger than males.

La Plata Dolphin (Pontoporia blainvillei), Tucuxi (Sotalia fluviatilis guianensis). Source Wiki

3. Another grey-is pink river dolphin, a smaller subspecies of the Amazon River dolphin, is the Bolivian river dolphin ( Inia boliviensis ). This species is native to a few remote streams in the Bolivian Amazon. Between Bolivia and Brazil, the Teotônio rapids are thought to have caused speciation of the two species as DNA studies suggest that neither competition nor interbreeding has occurred between the Bolivian river dolphin and the boto for many tens (or possibly hundreds) of thousands of years.

4. The susu or Ganges River dolphin ( Platanista gangetica ), inhabits the rivers and estuaries in India, Bangladesh, Nepal and Bhutan. Adults can reach 3 meters in length and they are a dark coloured dolphin, known for swimming on their side, trailing a pectoral fin along the river benthos to forage for fish, squid and mollusks. The Indus River dolphin ( Platanista minor ) is only found in Pakistan. These river dolphins are endangered because of heavy industrial traffic, hunting for meat and oil, and the construction of dams that restrict migration.

The baiji, or Chinese river dolphin ( Lipotes vexillifer ), was native to China's Yangtze River however the last confirmed sighting of the pale blue-grey dolphin was in 2002. The International Union for the Conservation of Nature and Natural Resources (IUCN) considers the species to be endangered although it is now widely believed that it is functionally extinct.

https://www.youtube.com/watch?v=u3962vtKzo8

5. In addition, the Atlantic humpback dolphin or Cameroon river dolphin ( Sousa teuszii , family Delphinidae), is a marine species that also inhabits rivers, estuaries and near shore waters along the African coast between Morocco and southern Angola. This species became critically endangered on the IUCN Red List in 2017 after studies suggested that fewer than 3,000 remained in the wild.

The Atlantic humpback dolphin or Cameroon river dolphin ( Sousa teuszii , family Delphinidae). Source: Endangered Wildlife Trust.

6. Finally, the Araguain boto ( Inia araguaiaensis ), is physically similar to the Amazon river dolphin, was recently classified as a separate species in 2014 after the discovery of its distinct DNA. This species inhabits the Araguaia- Tocantins river system of Brazil.

The Araguain Boto of Brazil was thought to be a solitary dolphin until recently when a study published in PeerJ carried out by Dr. Laura May Collado at the University of Vermont, and her colleagues had discovered that this species have a very diverse vocal repertoire for social interactions. These shy animals are difficult to study, but with the help of biologist Gabriel Melo-Santos from the University of St. Andrews in Scotland and the leader of the project found a fish market in the Brazilian town, Mocajuba, where the Boto's regularly visit the locals shopping in the local. The clear water there and regular encounters provided a unique opportunity to gain insight into how these animals behave and interact and to monitor the population on an individual basis.

The researchers used a combination of underwater cameras and microphones to record sounds and interactions of the dolphins that visited the market and took genetic samples. From 20 hours of recordings, the scientists identified 237 different kinds of sounds, but don't believe that they have yet described the species entire vocal repertoire. The most common sound type recorded were short, two-part calls that calves made on approach to their mothers. This is thought to be similar in function to the signature whistles that marine dolphins like the bottlenose ( Tursiops truncatus ) use for contact, but different in form. The river dolphins also made longer calls and whistles, but less frequently and the purpose for them is not yet clear. It was also suggested that the boto's signature whistles serves the opposite function than for bottlenose - used to maintain distance rather than for group cohesion.

The acoustic characteristics of the calls were also extremely interesting, remaining somewhere between the low frequency calls used by baleen whales to communicate over long distances, and the high-frequency calls used by marine dolphins to communicate over short distances. Dr. Collado has suggested that this reflects the riverine environment to which the dolphins inhabit as communication for the boto's there have evolved in response to avoiding echo's from vegetation to improve the communication range between mothers and their calves.

These findings are ground breaking in the context of marine mammal communication as river dolphins, represented by such few species, diverged from other cetaceans much earlier than other dolphins and yet, similar calls are also seen in other species such as pilot whales ( Globicephala spp .) and killer whales ( Orcinus orca ), and the similarities and differences between species could help tease out which signals evolved first, and why. This new study leaves many new questions to be explored as the findings will only make clear the evolutionary process of communication in river dolphins once all the sounds produced by the river dolphin species have been analysed and can be related back to these findings.

Reference:

Gabriel Melo-Santos, Angélica Lúcia Figueiredo Rodrigues, Rodrigo Hipólito Tardin, Israel de Sá Maciel, Miriam Marmontel, Maria Luisa Da Silva, Laura Johanna May-Collado. The newly described Araguaian river dolphins, Inia araguaiaensis (Cetartiodactyla, Iniidae), produce a diverse repertoire of acoustic signals . PeerJ , 2019; 7: e6670 DOI: 10.7717/peerj.6670