Two of the wild rhesus macaques that formed part of the study in the province of Henan in central China.
Credit: Jundong Tian
BEIJING: During the drier years, wild monkeys in China have more female babies than male, revealing how climate change might affect primates, said Chinese researchers.
Jiqi Lu from Zhengzhou University has been studying the group of wild rhesus macaques – a short-tailed old world monkey – for seven years and presented his observations on changing sex ratios at a recent Chinese Academy of Sciences meeting on climate change held in Kunming, China.
"We have found a relationship whereby in drier years more females than males are born, and in wetter years the opposite happens," said Lu.
But why? Mechanism unclear
"We are not yet sure of the mechanism behind this phenomenon," Lu said.
"We think that changes in rainfall may affect the growth of plants and food items eaten by macaques. In turn this is possibly influencing population dynamics and the sex ratio of baby macaques," he suggests.
While the total number of births remained the same, the ratio of males to female births has. In primates, sex is determined by chromosomes - a foetus with two X chromosomes develops into a girl and a foetus with one X and one Y chromosome develops into a boy.
Rainfall, not temperature is the key
From 2003 to 2009, the researchers collected temperature and rainfall records for the province of Henan in central China, where the macaques live.
The overall average annual temperature has increased at the study site and the amount of rain falling in the area has declined by 50% since Lu and his colleagues began studying the monkeys. But the researchers found no relationship between temperature and the number of male and female births.
"Temperature has no effect on the sex ratio, but rainfall certainly does," said Lu. The total number of births remained the same.
Sex ratios differ across species
A similar finding was reported in humans - more females are born in the tropics than elsewhere in the world, according to a study in Biology Letters in April 2009.
Wildlife biologist Dr Lifeng Zhu of China's Institute of Zoology says that studies into sex ratios can be difficult because determining exactly what causes a difference in the number of males and females born varies across species.
