New York, Nov 25 (IANS): In the male-dominated society of chimpanzees, mothers go the extra mile to make sure their male offspring get early social exposure. Researchers have found that mothers of sons are about 25 percent more social than the mothers of daughters.

The study spanning nearly four decades of observations of Tanzanian chimpanzees found that mothers of boys spend about two hours more per day with other chimpanzees than mothers of girls.

"It is really intriguing that the sex of her infant influences the mother's behaviour right from birth and that the same female is more social when she has a son than when she has a daughter," said one of the study authors, Anne Pusey from Duke University in the US.

"Mothers obviously increase social exposure for their young male infants," added Carson Murray, lead author and assistant professor at the George Washington University.

Chimpanzees have a male-dominated society in which rank is a constant struggle and females with infants face physical violence and even infanticide.

Mothers with infant daughters were likely to be avoiding competitive and stressful situations.

"...mothers with sons seem willing to incur those costs for the benefit of having their sons socialised," Murray said.

During the first six months of an infant's life, mothers with sons spend significantly more time in mixed-sex parties than mothers with daughters, the researchers observed.

But as their sons become older, boy moms spend more time in female-only, nursery groups, probably because the young males are attracted to the offspring of other females as playmates.

Based on an analysis of 37 years of daily observations of East African chimpanzees from the Gombe National Park in Tanzania, the findings appeared in the journal Proceedings of the National Academy of Sciences.