The white-throated rail, a flightless bird, went extinct 136,000 years ago. However, the bird later reappeared on the same island in the Indian Ocean through iterative evolution. How did an extinct animal bring itself back from the dead?
What Is a White-Throated Rail?
The white-throated rail (Dryolimnas cuvieri) is about the size of a chicken. This bird has reddish-brown feathers and a long neck. In the Indian Ocean, it's indigenous to Madagascar and has a history of colonizing small islands. Thousands of years ago, the rail actually used its wings and landed in Aldabra, which is a coral atoll (ring-shaped coral reef) in the Indian Ocean. Some consider the Aldabra white-throated rail (Dryolimnas cuvieri aldabranus) a subspecies.
Scientists believe that the original white-throated rail colonizers used their wings on Aldabra. However, a lack of predators on the atoll meant that the wings weren't necessary for survival, so the birds became flightless through evolution. During extreme flooding that covered Aldabra 136,000 years ago, the white-throated rail went extinct along with the other animals because it could not fly.
What Is Iterative Evolution?
To understand the return of the white-throated rail, it's important to look at iterative evolution. The University of Portsmouth explains that iterative evolution is "the repeated evolution of similar or parallel structures from the same ancestor but at different times." This means that the same ancestor can give rise to similar offspring at different times.
After the flooding that happened 136,000 years ago, the fossil record at Aldabra shows that sea levels went down 100,000 years ago. This allowed the white-throated rail to colonize the island again by flying to it from Madagascar. Over time, the birds evolved to be flightless again because they didn't have predators. Scientists consider this the return of the Aldabra white-throated rail.
On Aldabra, the same ancestor (the white-throated rail from Madagascar) has evolved twice at different times to be a flightless subspecies. This is a clear example of iterative evolution in action.
Vestigial Structures and Birds
Vestigial structures are features from a past ancestor that no longer seem to serve a purpose in the offspring. These structures appear to have no current function. For example, the snake's pelvic bone is a vestigial structure. Another example is wisdom teeth, which used to help people grind plants, but they're not necessary for modern humans, so they're vestigial.
When people think of vestigial structures, they usually don't consider wings as an example, since birds depend on them. However, for the Aldabra white-throated rail, they're vestigial because there aren't predators on the island that make it necessary for the birds to fly.
Scientists use vestigial structures as evidence for evolution over time. In the case of the Aldabra white-throated rail, it's easy to trace the modern bird to a past ancestor who used wings. It's possible that the rail will continue to evolve, and its wings may disappear completely. Since organisms spend energy to develop and maintain vestigial structures, it makes sense for them to eventually lose these structures altogether if possible.
The White-Throated Rail Today
Today, the white-throated rail isn't endangered and is labeled as of "least concern" on the IUCN Red List of Threatened Species. The species has a large range, and the population is stable. It's estimated that there are 3,400 to 5,000 adult white-throated rails in their natural habitats. The IUCN Red List notes that its only threat is the accidental introduction of feral domestic cats.
On Aldabra, the rails breed during the rainy season and have one to four eggs per nest. Their nests consist of twigs and leaves, which they build in either dense vegetation or rock depressions. Researchers point out that the white-throated rail is capable of surviving in different habitats, such as sand and pebble beaches, subtropical forests, wetlands and other areas. The rails eat insects, small mollusks and small ghost crabs. They may also eat the eggs and hatchlings of green turtles.
The Threat of Feral Cats
Although the Aldabra white-throated rail doesn't have any predators or serious threats on the island, the same isn't true for rails on other islands. On Grande-Terre and Picard, settlers introduced feral cats that threatened the birds. This wiped out the flightless rail on the two islands. Scientists later successfully reintroduced the white-throated rail to the island of Picard after the feral cats were removed.
Feral cats are an enormous problem for flightless birds. Without being able to use their wings, the birds are easy prey and can't escape the predators. This explains why the cats were able to destroy the entire population of rails on Picard. Cats are indiscriminate predators, so they're not selective and will kill and eat whatever is available. However, birds are often a large part of their diet. Native island species, like the rail, lack defense mechanisms against invasive predators.
The Aldabra Atoll
One of the reasons why scientists were able to find an example of iterative evolution on Aldabra was because it's an isolated area that's perfect for research. The atoll is difficult for people to access, so its isolation has preserved fossils and saved many species for centuries. It's considered one of the biggest atolls in the world, so it supports many habitats.
From tortoises to rails, different species make Aldabra their home. Aldabra is a welcoming home for many birds due to the limited number of natural predators. The lack of human interactions and activities also makes it easier for them to survive. The white-throated rail is the last flightless bird in the Indian Ocean.
In 1982, Aldabra was added to the World Heritage List, and the Seychelles Islands Foundation manages the conservation of Aldabra. In 2018, the World Heritage Centre expressed concern about the creation of an Indian naval base on Assumption Island, which is 27 km from Aldabra. After the Seychelles parliament blocked the plan initially, India and the Seychelles agreed to work together to build the base. The World Heritage Centre is monitoring the establishment of the base and its impact on rails and other species.
- CNN: Extinct Species of Bird Came Back from the Dead, Scientists Find
- How Evolution Brought a Flightless Bird Back From Extinction
- IUCN Red List of Threatened Species: White-throated Rail
- HBW Alive: White-throated Rail
- UoP News: The Bird That Came Back from the Dead
- Conservation Evidence: Individual Study
- World Heritage Fund: Aldabra Atoll
- World Heritage Fund: State of Conservation of Aldabra Atoll