Female Moa Bird Liked the Little Guys, Studies Suggest

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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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