Birds on an island in the Indian Ocean evolved flightlessness twice
On Aldabra, in the Seychelles, lives the last surviving native flightless bird in the Indian Ocean. But fossil evidence has revealed that the Aldabra rail lost the ability to fly not only once, but twice.
The islands scattered throughout the Indian Ocean used to be home to a whole menagerie of flightless birds unique to the outcrops on which they lived.
The most famous of these is the dodo of Mauritius, but they also include the giant elephant bird of Madagascar, the Rodrigues solitaire and potentially the Réunion ibis, among many more. As humans spread out across the ocean and colonised these islands, each of these bird species spiralled into extinction.
All except the Aldabra rail (Dryolimnas cuvieri aldabranus). This bird, which belongs to the same group as coots, moorhens and crakes, is the last surviving species of flightless bird in the Indian Ocean.
Now new research, led by the Museum's Dr Julian Hume, is uncovering the surprisingly complex history of the rail. Published in the Zoological Journal of the Linnean Society, Julian has revealed fossil evidence of the flightless rail on Aldabra dating to at least 136,000 years ago.
'That in itself doesn't mean a lot,' says Julian, 'because you could say that the flightless rail has simply been on the island for 136,000 years.
'But this is where it starts to get really interesting.'
Lost beneath the waves
At the time to which these fossils date, Aldabra was teeming with many endemic species found nowhere else on the planet.
While the rail pottered about on the dappled forest floor, giant tortoises lumbered through the undergrowth, endemic ducks lived on the lagoons and horned crocodiles lurked beneath the surface. But this oceanic idyll hasn't always been so.
The sediments from Aldabra show that in the past, the island had been completely submerged by the ocean on multiple occasions, eradicating all life on its shores. What's more, the record shows that in the years immediately following the 136,000-year-old rail fossils, this happened again.
'Aldabra went under the sea and everything was gone,' explains Julian. 'There was an almost complete turn over in the fauna.'
Everything including the crocodile, the duck, the tortoise and the rail went extinct. Yet as the Aldabra rail still lives on today, something must have happened for it to have returned.
Return of the rail
The flightless rail is descendent from a species of flying bird known as the white-throated rail (Dryolimnas cuvieri).
In the layers of rock immediately after the last inundation event, Julian found some more fossil bones. 'We found the leg bone of a rail in these deposits,' he says. 'But from that one bone we can see that it is already becoming more robust when compared to the flying rail, showing that the bird is getting heavier and so losing its ability to fly.'
This suggests that once the sea levels dropped again and Aldabra reappeared, the white-throated rail once again recolonised the islands and became flightless, giving rise to the modern birds we see today.
'There is no other case that I can find of this happening,' explains Julian, 'where you have a record of the same species of bird becoming flightless twice. It wasn't as if it were two different species colonising and becoming flightless. This was the very same ancestral bird.'
This raises questions about whether the white-throated rail is in some way primed to colonise and then become flightless on remote islands. Julian thinks that this likely comes down to the bird's unusual life history.
Rails are particularly good at getting to remote islands. According to Julian, this in part due to a curious behaviour that they occasionally display.
The white-throated rail has been known to build up large numbers on bigger bodies of land, such as Madagascar. Then suddenly, maybe in response to diminishing food supplies or overcrowding, the rails take flight across the ocean.
'Something sets them off and they fly in all directions,' says Julian. 'It can happen every fifty years or every hundred years. People still don't really understand it, but if the birds are lucky some of them will land on an island.'
Among these islands was the newly restored Aldabra. When the rails got there, it is likely that plants had already re-established themselves and the island was starting anew. While the crocodiles and endemic ducks were no more, giant tortoises were well-placed to float across the ocean from Madagascar and return.
Once back on the island, the breeding strategy of rails means that they were then well-placed to rapidly lose their ability to fly, as Julian explains:
'Rails in particular can become flightless very quickly because of their life history. The birds lay their nests on the ground and when the chicks hatch they are immediately able to run around really fast as they follow the parents about.
'As they grow, the very last thing to develop in the rails is the pectoral muscles and the wing muscles.'
This means it does not take much for evolution to favour flightlessness on an island in which there were no terrestrial predators and plenty of food, so there was no need to develop these wing muscles at all.
The fossils in the deposits show that the white-throated rail became flightless again in under 16,000 years, potentially the fastest recorded timeline of a bird losing its ability to fly.