New twist in evolution of flightless birds

5/22/2014 | 0

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The skeleton of an adult Brown Kiwi appears beside the egg of a huge elephant bird, the largest bird to ever walk the Earth. A new study indicates that the two dramatically different species are close cousins. Photo by Kyle Davis and Paul Scofield, Canterbury Museum

Many of the world’s flightless bird species, the ratites, are related to each other, despite the fact that they live far apart. The Ostrich is from Africa, the rheas live in South America, the Emu is an Aussie, cassowaries are found in Australia, New Guinea, and nearby islands, and the five species of kiwi occur in New Zealand.

The ratites also include the extinct moas, nine species that were found in New Zealand until about 500 years ago, and the two species of elephant bird, the largest birds to ever walk the planet. They lived in Madagascar and died out about 1,000 years ago.

For a long time, scientists have believed that the ratites evolved into their various species after the supercontinent Gondwana split apart. Gondwana included Africa and Madagascar, the first landmasses to drift. Thus, existing models of ratite speciation suggest the Ostrich and elephant bird are the oldest branches on the ratite family tree.

A study published today in the journal Science, however, throws the old theory out the window. (Read the abstract.) It suggests that the birds dispersed to Earth’s far corners through flight, not because the splitting of landmasses forced their separation. Only after separating, the study says, did most members of the group lose the ability to fly.

An elephant bird wanders through the spiny forest of ancient Madagascar. Painting by Brian Choo

An elephant bird wanders through the spiny forest of ancient Madagascar. Painting by Brian Choo

Kieren J. Mitchell and Alan Cooper of the University of Adelaide in Australia and researchers from other universities in Australia and New Zealand sequenced and analyzed the mitochondrial genomes of the two species of elephant bird, leading them to a surprising result: The large, herbivorous birds were the closest relatives to the small, omnivorous kiwi. And Ostriches are only distantly related to these species.

Thus, say the authors, instead of ratite speciation starting with a common flightless ancestor and being driven by continental separation, it appears it was driven by the divergence of species from ancestors capable of flight, ancestors like the elephant bird that flew long distances to new corners of the world, after which they independently evolved the inability to fly.

Elsewhere, the inability to fly is almost only seen on islands lacking mammalian predators. So, the authors suggest, the initial evolution of ratite birds to the large flightless birds we see today likely began in the period after the mass extinction of dinosaurs, the major predator of the time.

Past research has found that most mammals probably remained relatively small for about 10 million years after the extinction of the dinosaurs, “potentially providing a window of opportunity for the evolution of large flightless herbivores in continental bird lineages,” the authors write.

The discovery also appears to explain why the kiwis, which are about the size of chickens, are small, while most other ratites are giants. Mitchell and Cooper theorizes that the first moas arrived in New Zealand long before the first kiwis, enabling the moas to monopolize “the available niche space for large flightless herbivores and omnivores, forcing subsequent arrivals to adopt an alternative role and remain much smaller.” — Matt Mendenhall, Managing Editor

Read the abstract

Kieren J. Mitchell, Bastien Llamas, Julien Soubrier, Nicolas J. Rawlence, Trevor H. Worthy, Jamie Wood, Michael S. Y. Lee, and Alan Cooper (2014). Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution. Science, 344:6186, May 23, 2014. Abstract.