Moa driven to extinction by slow growth

One of the largest birds ever to have lived became extinct because it grew too slowly to sustain a viable population after the arrival of Polynesian hunters in New Zealand 700 years ago.

The giant moa, which grew more than 2m tall, took at least 10 years to grow to its full size, making it the slowest-growing bird on record.

This would have made it incapable of replacing its breeding population quickly enough to withstand the arrival of Maori hunters on New Zealand about 1300AD.

Moa are known to have become extinct within a couple of centuries of the Maori's arrival on the remote islands and now scientists believe they have found out why - moa just took too long to grow up.

All birds alive today reach full size within a year or so, with sexual maturity following soon after.

But the moa was an exception to this avian rule, said Samuel Turvey of the Zoological Society of London's Institute of Zoology.

An analysis of the bones of the flightless giant moa, published in the journal Nature, shows concentric rings that represent the annual growth spurts during the Antipodean summers, Dr Turvey said.

"Previously we thought that all birds grew very quickly, reaching maturity in about a year. Moa lived on an island without any mammalian predators and this made them unique," he said.

"It allowed them to go in for delayed maturity, enabling them to grow slowly over a long period of time. But this also made them vulnerable to humans because they replenished themselves so slowly."

Growth rings are seen in many slow-growing vertebrate animals but not in birds, other than the moa, because they grow so quickly over the course of a single year.

Moa belong to the group of flightless birds that includes the ostrich, emu and kiwi. Moa lost all remnants of their forelimbs and consisted of about 10 different species, with the two largest growing as big as buffalo.

Archaeologists have found sites where Maori hunters butchered large numbers of moa.

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* Female giant moa were about three times the size of males - the greatest female-male difference in the animal kingdom.

* Scientists say it may have been because it was the males who were left to look after the chicks.