Moa mystery solved: why females were giants

By Mischa Vickas 12 April 2013
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The reason giant female moa birds towered over diminutive males may be simpler than experts thought.

THE MYSTERY OF WHY female giant moa were more than twice the size of their male counterparts may have finally been solved, say researchers.

The moas of New Zealand were made up of around nine species of giant flightless birds (Dinornis), which became extinct when the first Polynesians arrived 700 or so years ago.

Giant moas are thought to have been the tallest birds that ever lived. Females of some species reached over 2m in height and weighed in at up to 240kg. Males, on the other hand, weighed just 34-85kg.

The enormous size that moas grew to was possible because were no other large herbivores with which they had to compete, and few predators.

But the size difference between males and females had always been puzzling to experts.

Tallest bird ever to have lived

New research now suggests that existing size differences between males and females were simply scaled up as the moas evolved to great sizes, which led to seemingly enormous size differences.

“Our study is the first to explicitly investigate the evolutionary mechanism responsible for making male and female moa so wildly different in size,” says co-author Dr Sam Turvey, a palaeontologist at the Zoological Society of London.

“Beyond documenting this remarkable situation, there has been little consideration of the evolutionary reason as to exactly why these birds had evolved such an unusual and striking level of difference,” Sam says.

To understand these differences, Sam and co-worker Dr Valérie Olson, compared the body mass of giant moa to their relatives, both living and extinct. These included the emu, ostrich and cassowary.

Their findings, published this week in the journal Proceedings of the Royal Society B, reveal that a similar size difference occurs in related species, though not to the same extent.

As a result, the researchers argue that the giant moa scaled up its existing size difference, following a pattern of body size difference between genders begun by its predecessors.

“In moa, greater female body size may be associated with the lack of mammalian predators in the prehistoric New Zealand ecosystem,” Sam says.

He adds that some of the strongest selective forces acting on moa populations could have been competition between individuals for resources. This could have driven females to increase in size, says Sam, in association with providing for their offspring.

Studying biological oddities in extinct animals

Dr Wayne Longmore, a bird expert at Museum Victoria, in Melbourne, says the study is significant, and raises new questions about the breeding biology of giant moas, and whether or not males and females used the same habitat resources.

Working with the skeletal remains of moas, and drawing comparisons with other birds, may also shed light on the breeding biology of these animals, he says.

This study of moas is just one of many that Sam and his co-workers are carrying out looking at biological oddities.

“We are continuing to investigate the evolution of other bizarre morphological structures shown by recently extinct species, such as the elongated teeth of sabretooth cats,” he says.