Female Moa Bird Liked the Little Guys, Studies Suggest

By John Roach
for National Geographic News
September 11, 2003
Female moa birds had a sweet spot for the little guys, according to two papers appearing in the September 11 issue of Nature.

The research teams, led by scientists in New Zealand and England, applied a pioneering technique in genetic analysis that allowed them to determine the sex of extinct moa by analyzing nuclear DNA extracted from fossils.

The results from both teams show that moa were characterized by reverse sexual dimorphism—the females were bigger than males. In one case, females of the same species of moa were at least twice the size as their male counterparts.

Joel Cracraft, an ornithologist at the American Museum of Natural History in New York, said these papers represent the first example of sexing moa based on fossils and "are really nice data to confirm this is sexual dimorphism."

The New Zealand team, led by David Lambert at the Allan Wilson Centre for Molecular Ecology and Evolution at Massey University in Auckland, was the first to attempt the technique on moa. The England team, led by Alan Cooper at the Ancient Biomolecules Centre at the University of Oxford, designed their own test based on the published results of the New Zealand team.

The research sheds light on the problem of accurately classifying the number of recognized moa species. The number has gone from 38 to 11 in the past 20 years due to bone analysis work by Cracraft and Trevor Worthy, a paleontologist from Masterton, New Zealand, and member of the research team led by Cooper.

The new DNA analysis by the two teams shows that some species currently recognized as distinct are probably different sized male and females of the same species. The Lambert paper also raises the possibility that other distinct species may have existed.


Until about 500 years ago, New Zealand was home to several species of moa, a flightless bird ranging in height from less than two feet to more than six feet (less than half a meter to more than two meters) tall and weighing from 45 to 550 pounds (20 to 250 kilograms).

Living relatives of moa include the emus, ostrich, and kiwi, which are members of a bird group called ratites. The moa thrived in New Zealand's lush forest habitat until the A.D. 1100 arrival of humans and rats, which drove the moa and nearly half the island nation's bird life to extinction, said Michael Bunce, a member of the Oxford team.

Thousands of moa bones have been recovered from swamps and caves and, due to the wide range of shapes and sizes found, over 60 different species names have been assigned in the 160 years since their discovery.

"It is a particularly difficult group to classify due to the large variation in the size of bones at different sites across the country," said Worthy, whose bone analysis work in recent years helped whittle down the number of currently scientifically recognized species of moa to 11.

The species are divided into two families—eight species of short and robust Emeidae and three species of tall and thin Dinornithidae. The two families are distinguishable by differences in skull shape and the structure of their spines.

Genetic Study

Geneticists study two kinds of DNA, mitochondrial and nuclear. Sex can only be determined from nuclear DNA. Lambert said that the ability to determine sex from fossilized nuclear DNA is a breakthrough in genetics.

"The amplification of nuclear genes from ancient DNA material is much more difficult than mitochondrial genes," said Lambert. "The reason for this is that mitochondrial DNA is found typically in thousands of copies per cell. However, in the case of single locus nuclear genes, there are two copies—one derived from the male parent and one from the female parent."

Lambert said it is thanks to the skill of researchers, such as his colleague Leon Huynen, that better techniques to extract nuclear DNA from fossils and make thousands of copies of it have been designed. This skill allows the researchers to sex fossils.

Based on independent sexing of Dinornis fossils, both Lambert and Cooper's teams concluded that a single Dinornis species lived on the North Island and a single Dinornis species lived on the South Island. In both species, the females were at least twice as big as the males.

In addition Lambert and his colleagues sexed fossils of the Emeidae species of moa and found that the "extent of sexual dimorphism is much less in other species apart from Dinornis." The females were about half again larger than the males.

Reversed Sexual Dimorphism

Lambert said that his team's data indicates that an excess of the female fossils came from swamps. "This suggests that females may have foraged more widely than males, and it seems likely that the large size of females is related to this," he said.

Cooper said that one reason the females may be larger than males is that the females must compete for access to males. "The reason we think this is in ratites the males generally incubate the eggs and raise the young and quite often females are little bigger than males," he said.

Bunce added that the size difference between male and female moa raises questions about how the species mated. "In ostrich, following prolonged courtship, the female sits before the male jumps on. Presumably, with some maneuvering, such a system may have also functioned in Dinornis," he said.

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