The phenomenon of repeated evolution has allowed an ancient bird species to reappear in the Indian Ocean and survive to this day.
Aldabra Atoll in the Indian Ocean was once home to hundreds of species, including the Aldabra giant tortoise. However, 136,000 years ago, a massive flood submerged the entire island, wiping out all life, including the Aldabra rail (Dryolimnas cuvieri aldabranus). Subsequently, they re-emerged as the last remaining flightless bird species in the Indian Ocean today.
The Aldabra rail has a body length of approximately 25 cm.
The Aldabra rail (scientific name: Dryolimnas cuvieri aldabranus) belongs to the Rallidae family and Gruiformes order.
The Aldabra rail has a body length of about 25 cm. Males and females differ significantly in appearance. The male has a brown back, reddish-brown head and chest, while the female has a light gray upper body and chest. Both sexes feature bold black and white stripes on the sides, belly, and under the tail, with long toes and a short tail.
Its body is flattened to allow easy maneuvering through the underbrush. It has long toes and a short tail. The coloration includes a brown back, chestnut-colored head and breast, and black and white stripes on the sides, belly, and under the tail. The throat is white, with a light yellow color, and the legs are greenish. Juveniles are dark brown on the top and bottom but have a white belly and throat.
This bird species is territorial but quite secretive, hiding in dense foliage when disturbed. They forage in mud or shallow waters with their long beaks and locate food with their eyes. They feed on grass, berries, and insects on the ground or wander through thickets and underbrush. They build nests in dry places on the ground or in low shrubs, laying 4-8 eggs. A study conducted in Nilambur, Kerala, in southern India, found that the eggs hatch after about 20 days of incubation.
The Aldabra rail is the last flightless bird species in the Indian Ocean. (Photo: IFL Science).
The cause of this strange phenomenon is a rare natural process known as repeated evolution. Thousands of years ago, the Madagascar rail (Dryolimnas cuvieri) migrated to Mauritius, Réunion, and Aldabra Atoll. There, due to the absence of predators, they lost their ability to fly, forming a new subspecies known as the Aldabra rail (Dryolimnas cuvieri aldabranus).
Most species in the rail family can fly, similar to the native species in Madagascar. However, the habitat on Aldabra had almost no predatory animals, making it unnecessary to fly to find food or shelter. The wings of this species gradually diminished.
Then, about 136,000 years ago, a massive flood wiped out the rails from the atoll. Without functional wings, they could not escape. However, 100,000 years ago, the Ice Age caused sea levels to drop, making Aldabra habitable once again. Therefore, the rails migrated from Madagascar back to the atoll. Due to the lack of predators, they continued to lose their ability to fly.
The Madagascar rail evolved into two different flightless subspecies over a few thousand years. This is quite unusual. Scientists from the University of Portsmouth and the Natural History Museum in the UK reached this conclusion after analyzing rail fossils before and after the flooding event. They discovered that both species were flightless.
Thousands of years ago, after the sea levels dropped, Aldabra emerged again, and this bird species once again “re-emerged.” On the same island, they evolved independently, twice separated by hundreds of thousands of years. This double evolution is referred to as repeated evolution.
David Martill, a paleontologist at the University of Portsmouth, explained that some native birds in Madagascar, the ancestors of the Aldabra rail, migrated again to the atoll and evolved into a different species within the rail family.
Fossil samples demonstrate the two species evolved similarly. (Photo: CNN).
Researchers have proven that the new bird species is completely identical to the extinct Aldabra rail by comparing fossil samples. They both originated from the Madagascar rail and evolved to lose their wings and flight capability.
This is an extremely rare case of repeated evolution in nature. And it is also the only documented case involving bird species.
“This situation seems surprising, but rails are resilient settlers on isolated islands and can evolve rapidly towards losing flight if conditions are favorable. Therefore, it is certainly true that the dispersal of Dryolimnas from Madagascar to the distant Aldabra occurred multiple times,” the research team reported in the journal of the Linnean Society of London.
Julian Hume, another researcher from the Natural History Museum, stated that the evolution process occurring in just a few thousand years is a very short timeframe.
When the atoll re-emerged, the flight-capable white-throated rail re-invaded the island and began evolving to become flightless once again. Researchers found that the fossilized legs from the rails dated to 100,000 years ago were heavier and sturdier than those of the white-throated rail. According to the research team, this indicates that the rails on the atoll became increasingly heavier and lost their ability to fly.
Being flightless seems to be an advantageous trait in this environment. Birds that lay eggs on the ground, thus possessing strong legs to run around immediately after hatching, may aid their survival. According to Hume, when they grow up, the last thing to develop in rails is the breast and wing muscles. In the process of losing their ability to fly again, the Aldabra rail essentially evolved twice, reviving from extinction through a process known as “repeated evolution,” where one species goes extinct, and another emerges and evolves similar traits, becoming nearly identical to the vanished species.
