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Moa Males Hatched Eggs

Moa egg shell

Moa egg shells were remarkably thin for their size, providing indications that males incubated eggs. Photo: S. Brookbanks

By Stephen Luntz

Detective work on egg shells from moa have provided insights into the behaviour of these extinct species, including evidence that males kept eggs warm prior to hatching.

Along with colleagues at Griffith University, Dr Craig Miller of the University of Auckland’s School of Biological Sciences collected DNA from the outside of remnant moa egg shells. Miller was able to match the DNA with samples from bones to identify which species the eggs came from, including some of the largest of the ten known moa species.

Moa ranged in size from 9–250 kg, with females up to double the weight of males. After studying the thickness of the shells Miller says: “In the larger species our calculations showed that females would almost certainly break their eggs if they used the standard incubation method of sitting directly on top of them. The risk of breakage was also high for males, but they would have been a much safer bet than the females.”

Shells 1.1 mm thick from one species whose males weighed 75 kg easily represent the most breakable avian eggs relative to parental bodyweight ever observed.

Some bird species keep their eggs warm using the heat of composting materials, while others build elaborate nests that bear some of their weight. However, evidence suggests that moa nests were little more than rough scrapes in the ground with perhaps small amounts of foliage for cushioning.

The idea that males kept the eggs warm is supported by the fact that some of the DNA on the outside was male in origin. The finding is not entirely surprising. Moa were ratites, and the males of living ratites such as emus, ostriches and kiwi contribute to incubation.

Ratites generally have thin egg shells, but Miller is unable to explain why natural selection would allow such a risk. “It’s possible the thickness stayed the same as they got bigger,” Miller says. “Maybe they evolved too fast.”

A shortage of calcium in the moa diet has largely been ruled out. Shell thinness did not affect moa survival, with the birds dominating the New Zealand landscape until they were swiftly wiped out due to the arrival of Maori.

This is only the second time that ancient DNA has been usefully recovered from egg shells, an achievement Miller attributed to a colleague with the patience to combine many overlapping short chains of DNA so that the species could be identified.