Giant moa not so robust after all

Giant moa not so robust after all, but their small cousins were

December 22, 2013

The South Island giant moa, dinornis robustus, (the name means robust strange bird) may not have actually had robust bones at all, according to new research conducted by the University of Manchester in association with a researcher at the University of Canterbury.

The leg bones of one of the tallest birds that ever existed were actually rather like those of its modern but distant relatives, such as ostrich, emu and rhea.

A study led by Manchester biomechanics student Charlotte Brassey, in collaboration with palaeobiologist Professor Richard Holdaway at the University of Canterbury (UC), has found that the largest of the moa species had leg bones similar to those of modern flightless birds that can run fast, whereas a much smaller species of moa - from a different family - had an extremely robust skeleton.

Professor Holdaway says the research suggests that different groups of moa came up with different solutions to deal with the problem of supporting the large body necessary to process a diet of coarse vegetation.

``We know that these species of moa were living together in the same locations, at the same time. So we don’t think the differences we’re seeing in leg robustness are adaptations to a particular habitat type. Instead, it seems they were perhaps engaging in different behaviours, although both could deal with extremely rough terrain.”

The research project was funded by the United Kingdom Natural Environment Research Council. It involved academics from Earth Sciences and Life Sciences at Manchester, together with the School of Biological Sciences at UC.

To work out whether the leg bones were overly thick and strong, the researchers had to work out how heavy the birds were in life.

Scientists have done this in the past by working from how thick or round the leg bones themselves are, then scaling up according to the size of bones of living birds. The problem comes when the leg bones have unusual proportions. This was true for moa.

Brassey says they already knew moa had disproportionately wide leg bones, yet previous estimates of their body mass had been based on those same bones, which probably resulted in overestimates.

To get around this, they scanned whole skeletons and, as predicted, the new estimates were considerably lower.

The largest moa still weighed in at a hefty 200kg, or 30 family-sized Christmas turkeys. Brassey suggested that if people wanted to roast moa on Christmas day, they would have to start cooking on December 23.

Using the new weight estimates, the researchers then applied Finite Element Analysis (FEA), an engineering technique, to estimate how robust the legs of different kinds of moa really were.

FEA is like crash-testing Formula One cars using computer simulations. The results revealed that the different families of moa had solved the engineering problems of supporting and moving their huge bodies in very different ways.

Such fundamental differences in structure suggest that the three taxonomic families of moa had long histories of independent evolution.

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