Taxon of the Week: Leg or Breast?

This week's highlight taxon is one that is very familiar to me as a New Zealander, except it's not really. I've heard of these creatures since I was a little lad, and representations of them have been almost everywhere I've gone. I've never actually seen one. I don't know anyone who's every seen one. Probably no-one has seen one for hundreds of years, in fact. The Dinornithiformes are but a memory these days, long since converted into quarter-pack meals for Polynesian settlers. Ka ngaro i te ngaro a te moa - lost as the moa is lost.

The taxonomy of moa is complicated, but at present there are eleven species recognised as valid*. The order was unique to New Zealand - the "Australian" Dinornis queenslandiae De Vis, 1884, was based on a partial femur in the Queensland Museum, but this bone is now believed to have come from New Zealand and is assigned to Pachyornis elephantopus. In the past, Dinornithiformes has been divided into two families - the lightly built, more cursorial genus Dinornis in its own family and the other smaller and/or more heavily built genera in the Emeidae, but phylogenetic analysis has shown that Dinornis is nested within the Emeidae (Worthy & Holdaway, 2002).

*Eleven species were recognised in Worthy & Holdaway (2002), the most recent major review of Dinornithiformes. Bunce et al. (2003) reduced the number of species of Dinornis from three to two (see below), but Baker et al. (2005) increased the number of species of Megalapteryx from one to two.

The photo at the top of the page (from Wikipedia) shows the reconstructed Dinornis in the Auckland Museum. This specimen has been around for some time - it was built in 1913, though when the Natural History section of the museum was rebuilt it lost the tussock-land diorama it had previously inhabited (if I recall correctly) and moved into a glass case. The Auckland Museum moa stands about three metres tall, and modern interpretations would, unfortunately, label this a severely inaccurate reconstruction. It has been mounted in an unnaturally elevated stance, and should have been much more low-slung. A more realistic reconstruction was shown on a recent stamp issue, shown below (from New Zealand Birds):



Such a lowered reconstruction significantly lowers the height, but we're still looking at about two metres for the tallest Dinornis specimens. A moa of this size may have wighed over 150 kg (Worthy & Holdaway, 2002). Other species were smaller - the smallest was Megalapteryx didinus at probably about 40 kg. The super-heavy Pachyornis elephantopus was considerably shorter than large Dinornis, but may have weighed about the same amount. The image below comes from Nature, and shows three moa species against a 1.8 metre tall human. From left to right, the moas are a female Dinornis novaezealandiae, Megalapteryx didinus and Pachyornis elephantopus.



Despite their extinction prior to European settlement in New Zealand, a surprising amount of molecular data has been gleaned from ancient DNA studies of moa. Among other things, said molecular analyses have demonstrated that Dinornis displayed the highest degree of size dimorphism known from any bird (Bunce et al., 2003). Previously Dinornis had been divided into three species on the basis of size. Bunce et al. tested DNA from Dinornis remains for female-specific markers (birds differ from mammals in that it is the female that possesses different sex chromosomes [ZW], while the male has identical sex chromosomes [ZZ]). They found that all specimens that had been assigned to the smaller 'species' Dinornis struthoides were male, while all specimens of the larger 'species' D. novaezealandiae and D. giganteus were female. Molecular phylogenetic analysis also showed that these specimens were intermingled, with the major divide in the genus being not by size but by geography - the North Island and South Island populations (both containing representatives from all three 'species') were distinct, and were recognised as the separate (but morphologically indistinguishable) species Dinornis novaezealandiae (North Island) and D. robustus (South Island). While females showed a great deal of variation in size, the largest females would have been about 280% of the weight and 150% of the height of the largest males.

There is very little reliable information on the life habits of moa. By the time of European settlement, it had been long enough since the extinction of the moa that records of it in Maori oral tradition were seemingly few and far between, and those that were present had become significantly mythologised. Early researchers such as Owen and Haast interpreted moa as birds of open country, comparing them to modern ostriches and emus. However, the extensive New Zealand grasslands and fernlands these authors pointed to hadn't existed prior to human settlement, so moa were undoubtedly forest birds, foragers rather than grazers as also shown by preserved gizzard contents. Dinornis and Pachyornis seem to have had more fibrous diets with gizzards containing twigs and fibrous plants such as Phormium (the New Zealand flax) while Emeus and Euryapteryx with less robust bills had more selective diets of fruit and leaves. Interestingly, a well-preserved specimen of Euryapteryx geranoides showed a massive intrathoracic loop in the trachea, 1.2 metres long. In other birds such loops are associated with the ability to make loud, far-carrying calls, so moa (or at least Euryapteryx) would have been quite vocal birds in life.

REFERENCES

Baker, A. J., L. J. Huynen, O. Haddrath, C. D. Millar & D. M. Lambert. 2005. Reconstructing the tempo and mode of evolution in an extinct clade of birds with ancient DNA: The giant moas of New Zealand. Proceedings of the National Academy of Sciences of the USA 102(23): 8257-8262.

Bunce, M., T. H. Worthy, T. Ford, W. Hoppitt, E. Willerslev, A. Drummond & A. Cooper. 2003. Extreme reversed sexual size dimorphism in the extinct New Zealand moa Dinornis. Nature 425: 172-175.

Worthy, T. H., & R. N. Holdaway. 2001. The Lost World of the Moa: Prehistoric life of New Zealand. Indiana University Press: Bloomington (Indiana).