Fossils reveal that an extinct species of flightless bird returned from the dead by recolonizing its former island home and evolving back into existence twice in less than 20,000 years through an unusual process known as ‘iterative evolution’
Fossils reveal that an extinct species of flightless bird returned from the dead by re-colonizing its former island home and evolving back into existence twice in less than 20,000 years. The white-throated rail from Madagascar in the south-western Indian Ocean colonized the tiny Aldabra Atoll in the Indian Ocean and became flightless before the island and everything on it was swallowed by rising seas 136,000 years ago.
“Aldabra went under the sea and everything was gone,” Natural History Museum avian paleontologist and lead author of the study, Julian Hume, explained in email. “There was an almost complete turn over in the fauna. Everything including an endemic crocodile and a duck, as well as 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.”
The Ice Age is what happened: it caused sea levels to drop so Aldabra resurfaced some 118,000 years ago. Thus, the white-throated rail recolonised the island and once again lost its ability to fly, giving rise to the flightless bird that we know today.
The Aldabra rail, Dryolimnas cuvieri aldabranus, is a handsome chicken-sized bird that is the last surviving flightless bird in the Indian Ocean. It is a close relative to the white-throated rail, Dryolimnas cuvieri, also known as Cuvier’s rail.
“The flightless Aldabra rail hardly differs in colouration and shape from its ancestor, the white-throated rail of Madagascar,” said Dr. Hume in email. But the two species can be easily differentiated because unlike the Aldabra rail, the white-throated rail actually can fly.
But why did the ancestral white-throated rails leave Madagascar in the first place?
“Something sets them off and they fly in all directions,” Dr. Hume said. “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.”
We know that white-throated rails experience frequent population explosions on Madagascar, and this can motivate flocks of the birds to leave, and repeatedly colonize isolated islands in the Indian Ocean (Figure 1). However, most of the wandering birds do not survive: those that flew north or south drowned in the ocean (and probably were eaten by juvenile sharks; ref); those that flew west landed in Africa and were eaten by a variety of terrestrial predators; whereas some of those that flew east got lucky and landed on a remote island such as Mauritius or Réunion — islands that hosted other species of flightless birds, such as the dodo, Raphus cucullatus, or the Réunion Island sacred ibis, Threskiornis solitarius. Occasionally, some of the wandering rails managed to find their way safely to the tiny Aldabra atoll.
Aldabra is special because it formed around 400,000 years ago, and it has the oldest palaeontological record of any oceanic island in the Indian Ocean. During its history, it has been inundated at least twice, and possibly as any as four times, by rising sea levels. Thus, its fossil evidence clearly demonstrates the effects of changing sea levels on extinction and recolonisation events.
To do this research, Dr. Hume collaborated with David Martill, a Professor of Palaeobiology who is a pterosaur and theropod dinosaur expert at the University of Portsmouth. Dr. Hume and Professor Martill measured and compared fossils from before the inundation event with fossils from after the inundation event.
They found that the fossil wing bones from the ancient Aldabra rails were smaller than those of the ancestral white-throated rail, indicating an advanced state of flightlessness (Figure 4). The lower leg bone fossils were more robust and heavier than those of its ancestral parent species, revealing that the Aldabra bird was again becoming heavier and thus, was losing the ability to fly.
These two species of flightless birds on the Aldabra atoll arose from the same ancestral species of rail during the span of less than 16,000 years. This is one of the fastest rates documented whereby flightlessness has arisen in birds, and it arose through a process known as ‘iterative evolution’.
Iterative evolution refers to the repeated evolution of similar or parallel structures from the same ancestor but at different times. There are a number of examples of iterative evolution in the fossil record, spanning a wide range of groups, but some examples that might be most familiar are seen in the extinct ammonites (i.e.; ref), and also the dugong or seacow, which shows iterative evolution occurred during the last 26 million years (ref).
“These unique fossils provide irrefutable evidence that a member of the rail family colonised the atoll, most likely from Madagascar, and became flightless independently on each occasion,” Dr. Hume said. “Fossil evidence presented here is unique for rails, and epitomises the ability of these birds to successfully colonise isolated islands and evolve flightlessness on multiple occasions.”
This is the first time that iterative evolution has been seen in rails and is one of the most obvious examples in birds unearthed to date.
“There is no other case that I can find of this happening where you have a record of the same species of bird becoming flightless twice,” Dr. Hume explained. “It wasn’t as if it were two different species colonising and becoming flightless. This was the very same ancestral bird.”
Iterative evolution is a form of evolutionary conservatism and is probably due to the overriding morphogenetic control exerted by certain regulatory genes.
But how do rails lose the ability to fly so quickly?
“In rail juveniles, the last thing to develop is the wing and flight muscles, so the birds are flightless until they almost reach adulthood,” Dr. Hume explained in email. “If rails land on a predator-free island, they can evolve flightlessness remarkably quickly, even within a few thousand years.”
This ease of evolving flightlessness is because developing and maintaining large wing muscles demands energy that can be used for other purposes, such as reproduction.
Before humans invaded the islands throughout the Indian Ocean, it was home to a plethora of flightless birds, the most famous of which was the dodo, a giant flightless pigeon that lived on Mauritius (more here). But the incursion of humans and their many companion-pest species, such as rats, cats and goats, marked the end of these distinct birds. Yet the Aldabra rail managed to survive some of this onslaught.
“Surprisingly, Aldabra rails can survive alongside rats, even killing them, but they cannot exist with feral cats. The rails have disappeared from the Aldabra islands where cats are feral,” Dr. Hume said in email.
But now that sea levels are rapidly rising, thanks to the effects of uncontrolled climate change, things are looking gloomy for the Aldabra rails.
“Sadly, this bird will become extinct for the third time,” Dr. Hume agreed. “Aldabra is a low-lying Atoll and it will disappear if the present trends of global warming are not controlled. Also, it will add to the long list of human-caused extinctions, rather than a natural extinction event.”
Julian P Hume and David Martill (2019). Repeated evolution of flightlessness in Dryolimnas rails (Aves: Rallidae) after extinction and recolonization on Aldabra, Zoological Journal of the Linnean Society, zlz018 | doi:10.1093/zoolinnean/zlz018