A genetic island
In the murky waters of the Ayeyarwady River lives a critically endangered subpopulation of the Irrawaddy dolphin, Orcaella brevirostris. While the species exists in other freshwater rivers and coastal areas of Southeast Asia, the Ayeyarwady dolphins are a demographic island. Genetic studies focusing on mitochondrial DNA reveal a significant level of differentiation between riverine and coastal populations. This shows a long-standing isolation from their marine relatives, with no evidence of interbreeding. This genetic separation makes them particularly vulnerable. With a population estimated at fewer than 80 individuals, their low genetic diversity increases the risk of inbreeding and reduces their capacity to adapt to environmental changes.
The physical appearance of the Irrawaddy dolphin is distinctive. It has a blunt, rounded head and lacks the prominent beak common to most other dolphins, giving it a look sometimes compared to the beluga whale. Adults can reach up to 2.75 meters in length and weigh between 90 and 200 kilograms. Their coloration is typically a uniform slate-gray, slightly paler on the underside. Unlike many other dolphins, they possess a flexible neck, allowing them to turn their head to scan their surroundings, a useful adaptation for navigating the complex river environment. They navigate and hunt using echolocation, producing a series of clicks and buzzes to interpret their environment.