River dolphins in South Asia are two separate species, as analysis of skulls reveals. Following two decades of research, a landmark study has concluded that the endangered Indus and Ganges river dolphins are actually separate species.
Photo: WWF Pakistan
South Asian river dolphins (Platanista gangetica) are among the most endangered of the world's cetaceans. The two subspecies in the family Platanistidae, Indus, and Ganges river dolphins (P. g. minor and P. g. gangetica), are both threatened by dams and barrages, declining river flows, fisheries bycatch, and pollution.
A new study led by Dr. Gill Braulik of the Sea Mammal Research Unit (SMRU) at the University of St Andrews has classified the river dolphins in South Asia as two separate species, the Indus river dolphin and the Ganges river dolphin.
“Recognising the species-level differences between Indus and Ganges river dolphins is extremely important as only a few thousand individuals of each species remain. They have long been regarded as two of the world’s most threatened mammals and my hope is that our findings will bring much-needed attention to these remarkable animals to help prevent them sliding towards extinction,” says Dr. Braulik.
Both the Indus and Ganges river dolphins are sometimes referred to as blind dolphins. This is because they live in very muddy waters and have lost their eyesight over millions of years of evolution. To navigate and catch their prey, they must rely completely on echolocation. The taxonomy of these dolphins has an interesting history, with the dolphins in the Ganges River named Platanista by Pliny the Elder in his Historia Naturalis published around 77AD. The Ganges river dolphin was formally described in 1801, and the Indus river dolphin in 1859. Since then, they have been deemed as subspecies or as separate species by various scientists over the years.
In recent decades, an Italian scientist had suggested in the early 1970s that they were two species, which was dismissed by other scientists. Then, from the late 1970s to the late 1990s, they were again deemed to be different species, before being downgraded again to subspecies. Now, this study used the largest set of morphology data to date, as well as new analyses of genetic data and comparison of coloration patterns. In total, 80 skulls were measured, with 29 from the Indus, 45 from the Ganges, and six of unknown origin.
Indus dolphin skulls in Stuttgart. © Gill Braulik
Based on their analyses, the scientists concluded that the two dolphins are separate species, and urge that their conservation status be re-examined. “The rapid decline and extinction of the Yangtze River dolphin this century was a very clear warning:
we need to act quickly to protect the remaining species of river dolphins, including the Indus and Ganges, all of which are seriously threatened. The freshwater systems they inhabit must be managed with biodiversity as a top priority,” says Dr. Randall Reeves, chair of the IUCN Cetacean Specialist Group.
The conclusion of this new study brings the river dolphin species up to six in total, all of which are listed as either Endangered or Critically Endangered on the IUCN Red List:
- Irrawaddy dolphins (Critically Endangered) – found in the Mekong River, Cambodia, the Ayeyarwady River, Myanmar, and the Mahakam River, Indonesia.
- Yangtze finless porpoise (Critically Endangered) – in the Yangtze River, China.
- Amazon river dolphin (Endangered) – in the Amazon and Orinoco Rivers in South America.
- Tucuxi (Endangered) – in the Amazon basin in South America.
- Indus river dolphin (Endangered) – in the Indus River system, Pakistan and India.
- Ganges river dolphin (Endangered) – in the Ganges-Brahmaputra River system, India, Bangladesh, and Nepal.
The findings of this study will be put to the Committee on Taxonomy of the Society for Marine Mammalogy, who will make a final decision on the validity of the new species in the next few months. With thanks to Marine Mammal Science and Megan Shersby.