Ancient DNA may reveal the sex lives of moa

Friday, 13 November 2020

Moa females were much larger than the boys and may have been the dominate sex.

Whether female moa birds fought each other to secure mates or even kept harems of males will be tested by examining their DNA.

Dr Kieran Mitchell of Otago and Adelaide universities may also learn if female moa competed for territory with other females.

Casting light on behaviour of the extinct moa from DNA isn’t easy and Mitchell was awarded a $300,000 Marsden Fast-Start grant last week for the project. It will take at least three years.

The research starts with two observations. The first is that in some species of moa, the females were markedly larger and heavier than the adult males.

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With giant moa, for example, the females were up to 150 per cent taller and 280 per cent heavier than the males, according to NZ Birds Online.

This is a reversal of the usual situation among mammals and birds and is called “reverse sexual dimorphism”.

“In living animal species, we know that [reverse] sexual dimorphism is very frequently linked to … competition for access to mates”, Mitchell said in an email.

In theory at least, female giant moa may have actively searched for passive males, fought other females, maintained territories or kept harems. Many in the field already think male moa sat on eggs.

The second observation comes from other animals with large differences between the sexes.

Peacocks have brilliantly coloured feathers compared to peahens. Male southern elephant seals are up to four times the size of females.

Mitchell collects samples of bone from ice age mammals for ancient DNA analysis in Wyoming.

“These intriguing differences are thought to result from competition between males for mating rights with females”, Mitchell wrote in his Marsden citation. Brighter and heavier males procreated more and evolution kept selecting those traits.

So, there are two large hints about moa mating behaviour, but how does behaviour show up in DNA?

In species that using “mating systems” such as harems, one male mates with many females and fathers many offspring. One result is that many males produce no – or few – offspring.

The result is a “skew” of diversity towards relatively few males and that will likely show up in the DNA. In other words, a few males “disproportionately” contribute to the next generation, Mitchell said.

Meanwhile, where animals generally maintain monogamous relationships, such as humans, such a diversity skew is not seen as much.

Among birds – which have ZW chromosomes as opposed to XY chromosomes – Mitchell and colleagues will be looking hardest at the Z lines.

It’s been established for a decade that DNA can be obtained from moa bones. The bird was driven to extinction several generations after the arrival of Polynesians on Aotearoa but there are enough well-preserved specimens for this research. Mitchell hopes to test 40 individual birds.

And helpfully from a science point of view, not all moa exhibited such large differences between the sexes as the giant moa. Eastern moa, for example, were thought to be up to 1.8 metres tall but the females only about 20 per cent larger than the males.

So it's possible that moa mating behaviour was different among the nine different species and those traces of differences could be found in their DNA.

Mitchell also hopes to zero in on moa DNA from the time of the last glacial maximum (ice age), which ended about 14,000 years ago.

An abundance of ice would have compressed moa into less territory and increased competition for resources and mates. It could have been a time of rapid change in moa DNA.