moabirdA study led by Faculty PhD student Charlotte Brassey has shown that the giant moa bird Dinornis robustus, which literally means ‘robust strange bird,’ may not have had robust bones after all. The leg bones of one of the tallest birds in history were actually more like its modern relatives the ostrich, emu, and rhea. In collaboration with Professor Richard Holdaway at The University of Canterbury, New Zealand, Brassey has shown that it was actually a much smaller species of moa that possessed the robust skeleton.

To determine whether the leg bones were overly thick and strong, the researchers had to define how heavy the birds were. Previously, scientists have done this by measuring the thickness of the leg bone and scaling up according to the size of living birds. This becomes a problem when the leg bones have unusual proportions. Ms Brassey explained:

“If we wanted to estimate the weight of a saber-toothed cat, no-one would suggest measuring canine tooth length and then scaling up the tooth size of your standard tabby. You’d end up with a ludicrously high estimate of the body weight of the saber-toothed cat. The same is true for moa. We knew that moa had disproportionately wide leg bones, yet previous estimates of their body mass had been based on those same bones. This probably resulted in overestimates.”

To avoid this, the researchers scanned whole skeletons. As predicted, the new estimates were considerably lower. Nonetheless, the largest moa still weighed in at 200kg; the equivalent of 30 Christmas turkeys.

The researchers then applied an engineering technique known as Finite Element Analysis (FEA) to estimate how robust the moa really were. FEA crash-tests objects using computer simulations, and is usually used for tasks such as testing the strength of bridges or modelling the behaviour of Formula One cars. The FEA techniques and the new estimates suggest that different groups of moa solved the problems of supporting their huge bodies in different ways. Such fundamental differences suggest that the nine species of moa had long histories of independent evolution.

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