Moa or less: extinct ‘robust’ birds of New Zealand might not have been so robust after all

The giant moa birds (Dinornis robustus, literally meaning ‘Terrible robust bird’) may not have actually had robust bones, according to new research conducted by the University of Manchester. Researchers have shown that one of the tallest birds ever to exist actually had similar or even less robust leg bones than its modern relatives, such as ostrich, emu and rhea.
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A study led by biomechanics researcher Charlotte Brassey, in collaboration with palaeontologist Professor Richard Holdaway at the University of Canterbury in New Zealand, has found that the largest of the moa species possessed leg bones similar to those of modern flightless birds that are capable of high-speed locomotion. In contrast, a much smaller species of moa was discovered to have an extremely robust skeleton.

Charlotte Brassey says: “Our research suggests that this group of birds came up with several different solutions to deal with the problem of being isolated on New Zealand. 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 living side-by-side, but perhaps engaging in different behaviours. We could speculate that they were moving around differently, or maybe we’re seeing evidence of kicking or digging in some species.”

The project was funded by the Natural Environment Research Council. It involved academics from Earth Sciences and Life Sciences at Manchester, together with Biological Sciences at Canterbury.

The researchers began by predicting the body mass of the extinct fossil birds. In the past, scientists have done this by taking information on leg bone width or circumference, and then scaling these values up compared to living birds. The problem comes when the leg bones are unusually proportioned.

“If we’d wanted to estimate the weight of a saber-toothed cat, no-one would have suggested measuring canine tooth length and then scaling up the tooth size of your standard tabby” says Charlotte Brassey. “That’s because we know that the saber-toothed cat had unusually oversized canines compared to house cats. It wouldn’t be a fair comparison, and you’d end up with a ludicrously high estimate of the body weight of the saber-toothed cat.”
The same was true of the moa. Previous studies had already found the moa birds to have disproportionately wide leg bones, yet all body mass estimates had been based on those same bones. Because of this, the authors suspected that earlier predictions of body mass could have been overestimates.

Instead the authors based their mass estimates on the whole skeleton of the moa birds, rather than relying on a single bone. As predicted, the new estimates were considerably lower than those previously published. Nonetheless, the largest moa species still weighed in at a hefty 200kg, or 30 family-sized Christmas turkeys. And if you wished to serve up a roast moa on Christmas day, you could expect a cooking time of almost 3 days.

Dr William Sellers, co-author on the study comments: “If you don’t get the body mass right, the rest of your analysis will just spit out the wrong numbers. By using the whole skeleton rather than just a single bone we get much better mass estimates, and we can even calculate how good this estimate actually is.”

Charlotte Brassey and her colleagues then applied an engineering technique know as Finite Element Analysis (FEA) to estimate how robust the moa really were. FEA is a way of ‘virtually crash-testing’ an object using computer simulations, and is commonly used in civil engineering to estimate the strength of bridges, or in Formula One racing to model the behaviour of cars. By using the FEA technique and incorporating their new estimates for body mass, the authors found considerable variation in the robustness of the leg bones between moa species.