WASHINGTON, Monday- Male chimpanzees prefer to have sex with older females, US researchers have established.

A study published on Monday shows one of the biggest behavioural differences between humans and our closest biological relatives.

Male chimps will chase down and fight over the oldest females, while the youngest female chimps are forced to beg for masculine attention, an anthropologist, Martin Muller, and colleagues at Boston University discovered.

Writing in the journal Current Biology, Muller and colleagues said they studied chimpanzees living in the Kanyawara community of Kibaale National Park in Uganda.

“It’s really dramatic because it’s not just that the old chimps are avoiding the youngest adult females. They actually have a strong preference for the older mothers,” Muller said in an interview. It is easy to observe their mating behaviour.

“Chimpanzee copulations are frequently preceded by a series of male courtship signals (for example glancing with erect penis and branch shaking), after which either the male or the female approaches the other to mate,” the researchers wrote.

They also collected the chimps’ urine to test for various hormones that demonstrate fertility.

They were checking to see if chimpanzees behave like humans, their closest living relatives, who form long-term mating bonds and who value younger females.
This is most definitely not the case with chimps.

The very oldest adult females were the most sought-after.
“The males fight over them more,” Muller said.

“They don’t have to do anything to get the males interested. The males find them. They follow them around. If you look at the very youngest females, the males will mate with them but it does take more work on the female’s part.”

Also unlike humans, female chimpanzees actively advertise when they are fertile, with bright red swellings around the genital area.

And unlike human females, chimpanzees remain fertile their entire lives, although wild Ugandan chimpanzees rarely lived beyond the age of 40. Older female chimpanzees are more dominant socially and have access to better food.

Muller said, “The females that have access to most food are the most fecund-the most likely to conceive in any cycle.”

He said males may know that. Older females may also be better mothers, the researchers said.
“The males do end up mating with all the females for the most part,” Muller noted.