Fossil DNA analysis yields surprise.

Analyses of genetic material from the fossils of large flightless birds called moas suggest that three types of the extinct birds may not be separate species after all.

Moas weren't merely flightless; they're the only known birds with no wings whatsoever. The creatures, whose weight ranged from that of a large turkey to that of a small cow, lived exclusively in New Zealand and were hunted to extinction soon after people permanently settled on the islands around 1310. Although the last moas probably died less than 2 centuries later, plenty of their bones remain, many in heaps left over from ancient feasts.

Scientists currently recognize 11 species of moa. Other than slight differences in some skull features, the three presumed Dinornis species have been distinguished only by their size, says Alan Cooper, a paleontologist at the University of Oxford in England. Dinornis struthoides stood around 1 meter tall and weighed up to 115 kilograms. Dinornis novaezealandiae measured about twice that height and weight. Dinornis giganteus, the largest of all moas, stood as much as 3 m tall and weighed up to 270 kg. Members of all three types were found on both of New Zealand's major islands, Cooper notes.

Now, genetic analyses by Cooper and his colleagues hint that these species designations should be revamped. The team's studies of 30 adult Dinornis specimens suggest that all of those from New Zealand's North Island are genetically identical. So, they're a single species. Similarly, the Dinornis specimens from South Island are genetically identical to one another, although their DNA differs substantially from that extracted from the North Island fossils.

What's behind the dramatic size differences that led investigators to suggest several Dinornis species? In part, it's a matter of gender, says Cooper. Detailed analyses of the moa fossils' DNA show that all the remains from D. struthoides, the smallest Dinornis moa, came from males and that the other remains came from females.

The disparity between small and large females probably stemmed from diet. The specimens now designated D. novaezealandiae were recovered from ecosystems that weren't as biologically productive--and thus didn't provide as much nutrition--as those that yielded remains of D. giganteus.

Overall, Dinornis moas exhibit a difference between the sexes in average size and weight that outranks that of all other birds and probably that of any terrestrial animal, says Cooper. He presented his group's data last week in Reno, Nev., at a meeting of the International Union for Quaternary Research. The Quaternary period encompasses the last 2 million years.

The new findings indicate that size alone isn't a good way to distinguish species, especially in moas, notes Richard N. Holdaway, a paleontologist with Palaecol Research in Christchurch, New Zealand. Members of one moa species, Pachyornis elephantopus, weighed up to 170 kg about 20,000 years ago, at the height of the last ice age, but only 90 kg or so 1,000 years ago. Holdaway speculates that much of that variation derives from responses to temperature differences between the climates of those two periods, but some variation results from declines in the ecosystem's biological productivity.