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.

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