麻豆传媒

WHEN Polynesians first arrived in New Zealand nine centuries ago, they found forests full of moas, huge flightless birds larger than ostriches. The birds must have made easy meals, because a century later they were extinct.

This week two teams have dug a surprise out of the DNA left in the birds鈥� bones. At first the researchers thought one set of skeletons that came in three very different size ranges belonged to three separate species of Dinornis moa. Now they have found that all three belong to just one species, with females up to three times as big as males. This is the first time an extinct species has been sexed using ancient DNA.

Scientists had suspected that disparities in size among some other groups of moa bones was caused by differences between males and females, as with some living bird species. Among kiwis, which are related to moas, females can be 1.8 times as heavy as males. But no one expected a size difference this dramatic between male and female moas. Yet it has now been independently confirmed by Alan Cooper and his colleagues at the Ancient Biomolecules Centre at Oxford University (Nature, vol 425, p 172) and a team led by David Lambert of Massey University in Auckland, New Zealand.

Both teams used PCR to amplify mitochondrial DNA, which showed that differently sized Dinornis bones belonged to the same species. But to sex the bones they had to amplify nuclear DNA, as that is where the sex chromosomes are. This is harder, as each cell has only two copies of each chromosome compared to thousands of mitochondria. The teams used DNA from the closely related rhea, ostrich and kiwi to isolate sequences unique to the moa鈥檚 female 鈥淲鈥� chromosome. Lambert鈥檚 team validated the technique by correctly predicting the sex of two bones known by their shape to have come from females (Nature, vol 425, p 175).

Now that the three 鈥渟pecies鈥� have been recognised as one, bones at cave sites in New Zealand show the ratio of female to males was 1.4 to 1 鈥� like modern kiwis. Using a comprehensive database of bone sizes, the Oxford team calculates that Dinornis females, who could stand three metres tall and weigh 250 kilograms, were up to 2.8 times the weight and 1.5 times the height of males in the same region 鈥� the biggest such difference ever seen among birds or land mammals.

鈥淚n most living birds in this family the males incubate eggs and look after chicks, and the females are larger,鈥� says Cooper. 鈥淪o it is possible that the females compete for males, and the larger size reflects this. It鈥檚 a reversal of the more usual situation, where males compete for females, and the sizes flip as well.鈥� But never have they flipped quite as far as among the doomed moas of New Zealand.