Sizing up female giant moas
Image: ZSL Living Conservation
Research by the Zoological Society of London (ZSL) has revealed that, while female giant moa were always bigger than males, this was accentuated by evolution.
The species, which belonged to the ratite family of large, flightless birds, developed to be much larger than its ancestors. To this day, it was among the largest birds ever to exist.
In particular, females could reach a weight of more that 200 kilograms (31 stone), and towered above the males.
According to the study, the difference in size between the two sexes has been present throughout history, but became accentuated over time.
The giant moa inhabited the forests, grasslands and mountains of New Zealand, before it was driven to extinction by hunters about 700 years ago.
By comparing patterns of body mass of both extinct and living ratites, researchers were able to see that female giant moa's larger size grew more substantial as both sexes upsized.
"A lack of large mammals, such as elephants, bison and antelope, allowed New Zealand's birds to grow in size and fill these empty large herbivore niches," said ZSL senior research fellow and lead author, Samuel Turvey.
"Moa evolved to become truly huge, and this accentuated the existing size differences between males and females as the whole animal scaled up in size over time."
The research has been published in Proceedings of the Royal Society B. Future studies will look at similar scaling relationships in other animals.
The BSAVA has opened submissions for the BSAVA Clinical Research Abstracts 2026.
