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By Stephen Luntz

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Kiwi Gender Defies Temperature Effect
Gender ratios among New Zealand children are not dependent on temperature, a study has found.

The result at first seems barely worth noting; while many reptiles use temperature to determine the sex of their offspring (AS, Jan/Feb 2011, p.7), mammals generally do not. However, studies have found subtle relationships between temperature and the ratio of male to female births around the world.

According to Dr Barnaby Dixson of Victoria University’s School of Biological Sciences, studies in northern Europe have reported fewer males born after particularly cold winters. Male babies are also less likely to survive in cold years.

This fits with widespread evidence that male human foetuses are more prone to stress than females. On the other hand, aside from regions where sex-selective abortions are common, a lower proportion of live births are male in tropical regions, suggesting that heat may have an even more potent effect.

Dixson set out to test how New Zealand fits into this, comparing average temperature records with birth data. Only 51.3% of New Zealand live births over the past 135 years have been male compared with 51.7% worldwide, but Dixson says that “colder years or seasons had no influence on this percentage”.

Thinking that the areas most New Zealanders inhabit may not get...

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