Male moa stayed at home to sit on eggs

The largest moa weighed in at 250kg - which put their fragile eggs in the nest at risk. Photo / Mark Mitchell

Male moa, the weaker sex of the species, looked after their unhatched young while the huge mums went out on the prowl, research has suggested.

Auckland University and Auckland Museum scientists used DNA extracted from eggs to show that the mother birds - up to twice the size of the males - would have smashed their eggs had they sat on them for incubation.

Genetic information from the shells enabled scientists to match eggshells with seven of the 10 known moa species, and compare the thickness of the shells with the estimated weight of adults of each species.

Despite their large sizes, moa eggs were fragile and the researchers were able to show that in most cases the bigger and heavier females would have probably broken the eggs.

The scientists at Auckland University and Auckland Museum worked with Queensland's Griffith University to recover DNA from moa egg shells estimated to be 400 to several thousand years old.

"The smallest species of moa weighed as little as 9kg and the largest up to 250kg, but in most cases the females were considerably heavier than the males - as much as twice the size," said Craig Millar from Auckland University.

"The risk of breakage was also high for males, but they would have been a much safer bet than the females."

One species of giant moa had males weighing around 75kg and fragile eggshells only 1.1mm thick - more typical of birds one third the size.

This meant the eggs were far more breakable than the eggs of any of the other 3434 bird species which have been similarly compared.

DNA also supported the conclusion that males were responsible for incubation. Male DNA found on the surface of egg shells from three of the largest moa species could have been deposited only by direct contact.

The findings were consistent with living members of the ratite family, such as the kiwi and emu in which the male was responsible for incubation.

Dr Millar said there was no evidence to suggest that a tendency to break their eggs was the reason the moa died out.

- NZPA