Sometime between 136,000 and 240,000 years ago, a flock of awkward, leggy birds took off from Madagascar and arrived at a pristine island in the Aldabra atoll 400km away. “This is nice,” they may have thought — there were no predators, and the birds colonised the island.
Without the threat of predation, they eventually lost their ability to fly. But 136,000 years ago, a flood washed over the island, wiping out this unique flightless species.
Soon after the island reemerged, another flock took off from Madagascar and arrived at Aldabra — and evolution played out in almost the same way.
Rails are a family of flying birds that tend to skulk around marshes, and some rail species are known for dispersing quite far from their initial homes.
It’s not so surprising that the white-throated rail species colonised a distant island — but a new paper documents perhaps the first case of the same genus of rail colonising the same island and then evolving along the same trajectory in response.
“In 20,000 years or less, the rails were evolving flightlessness again,” Julian Hume from the Natural History Museum in the United Kingdom told Gizmodo. “Evolution can be incredibly quick if the conditions are right.”
According to the research, “Fossil evidence presented here is unique for [rails] and epitomizes the ability of birds from this clade to successfully colonize isolated islands and evolve flightlessness on multiple occasions,” Hume and author David Martill at the University of Portsmouth, both in the United Kingdom, write in the study published recently in the Zoological Journal of the Linnean Society.
The researchers analysed individual wing and leg fossils taken from the island and held at the Smithsonian Institution National Museum of Natural History and the Natural History Museum, London. This particular island has the oldest palaeontological record of any Indian Ocean island, according to the paper.
They compared the fossilised bones to white-throated rail specimens from the Natural History Museum at Tring, some originating from Malagasy birds that could fly and some from Aldabran birds that had evolved such that they lost the ability to fly.
The fossilised bones had almost the same measurements as the bones from the flightless Aldabran birds, according to the paper. Essentially, the wing bones were stunted and the leg bones were thicker, implying that the old rail, too, was a flightless bird.
There’s pretty solid evidence that the Aldabran islands were completely submerged 136,000 years ago, which would have wiped out the older rail population. But it seems another population of rails returned and evolved the same way — figuratively and perhaps literally following the same path as their predecessors.
Let me take a moment to describe rails to you. They are generally goofy, big-legged birds that spend most of their time walking. They do not look like they should be able to fly, and they look hilarious when they do. They’re also notorious for dispersing far and wide — they just stop flying when they find someplace nice.
The flightless rails are “among the most species-rich examples of parallel evolution in vertebrates” according to one paper. It’s unclear why they occasionally seek out new environs, said Hume. But the same genus, colonising the same island, and evolving the same way? That seems to be a first.
Other researchers were impressed with the work. “It’s a very elegant analysis,” Martin Stervander, a University of Oregon postdoctoral researcher who was not involved in the study, told Gizmodo. “Both fossils look very much like the rails today. We can clearly see that they are rail fossils, and that they were flightless.”
But there’s a darker side to the story. The Aldabra rail is the last surviving flightless bird in the Indian Ocean. “They’re more vulnerable to extinction when they become flightless,” Julia Heinen, PhD student at the University of Copenhagen, told Gizmodo. “They’re not used to having predators. Then, when people arrive with rats and cats and whatever else likes to eat birds, they can’t fly away and go extinct much faster.”
According to one estimate, between 440 and 1580 flightless rail species went extinct following human colonisation of Oceania.
As long as cats and rats are still on these islands, it’s unlikely that these re-evolution events will occur again.