Extinct Giant Bird Doomed by Slow Growth, Study Says

John Roach
National Geographic News
June 15, 2005
The large, flightless moa bird that roamed New Zealand in ancient times grew much more slowly than modern birds, according to a new study of their bones. The finding suggests that slow growth doomed the moa to extinction when humans arrived about 700 years ago.

Unlike the bones of all modern birds, several moa bones show growth marks similar to the rings found on tree stumps, said Samuel Turvey, an ecologist at the Zoological Society of London in the U.K.

According to Turvey, the rings indicate that moas took several years to grow to their full size. All modern birds, by contrast, are fully grown within a year of hatching.

"In environments where mammal predators are present, there's strong pressure to mature quickly," Turvey said. "When you're young, you're really vulnerable. If you can get to adult body size relatively fast, it's harder to be picked off by animals that want to eat you."

But moas lived on the islands of New Zealand, which are isolated from the rest of the world and were home to no land mammals. This lack of predators meant no selective pressures for fast-growing moa. The birds matured slowly, Turvey said.

The moa lifestyle, however, failed to take humans into account. When the Maori people colonized New Zealand about 700 years ago, they hunted—and ate—the moa to extinction.

"Population replacement of the juveniles into the new adult cohort would've been a slow process," Turvey said. "Essentially, the species' populations would have collapsed, given even mild hunting pressure."

Turvey and colleagues report their finding in tomorrow's issue of the journal Nature.

Joel Cracraft, a vertebrate zoologist at the American Museum of Natural History in New York, said the authors make the case that the moa grew slowly. He added, though, that the birds were likely doomed to extinction no matter what after the arrival of humans.

"It might have sped things up, but those birds would have been doomed anyway," he said. "The Polynesians who got there were pretty good at trashing the environment and eating everything in their path."

Slow Growth

Turvey and his colleagues examined several bones from nine of the ten recognized moa species—the two species of giant moa Dinornis and seven members of the smaller emeid moa family—and found that nearly all of them contained growth marks.

In particular, almost all of the emeids and one Dinornis specimen had growth rings similar to those found on a tree. Many living animals, such as polar bears, have these markings today. The rings result from the different growth rates over the course of the seasons, Turvey said.

"In the spring and summer there's more food being eaten and faster bone growth. If [the weather] becomes harsher, then the growth slows down," he said. The changes in growth are reflected by changes in bone density. Growth can even temporarily stop altogether, leaving a distinct line running through the bone.

In addition to the growth rings, the Dinornis specimens all contained numerous blood vessels characteristic of fast growth. Bone analysis suggests that, though larger than emeids, the Dinornis species were fully grown in three years. Some of the emeids grew for almost ten years.

Bird species today lack growth rings, because in most cases their growth phases are limited to one year and many birds' bodies "remodel" their bones throughout life, the researchers report.

Moa Ecology

Moas were ratites, flightless birds considered the sister group of all other birds. Living ratites include ostriches, emus, cassowaries, and kiwis.

The biggest moa was the largest bird that ever lived. It could have seen eye-to-eye with a professional basketball player—about six feet (two meters) tall. It weighed as much as 500 pounds (225 kilograms). Smaller species were comparable to turkeys. All species were flightless and lived in New Zealand's forests, grasslands, and mountains, Turvey said.

A DNA analysis published in the June 7 issue of Proceedings of the National Academy of Sciences suggests the first moa evolved about 18.5 million years ago and diversified into at least ten species.

The exact number of moa species and relationships among them is controversial, Cracraft said. Recent DNA research suggests there were at least ten species, nearly all of which Turvey and colleagues studied.

Turvey and his colleagues' study, according to Cracraft, builds on this DNA work to examine the ecology of moas, providing insight to their lifestyle.

"The DNA results serve as an impetus for all this collaborative work," he said. "There's much more to be done, but piecing together biology like this on an extinct group is pretty remarkable."

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