In Chimp World, Males Find Older Females Sexier
WASHINGTON Chimpanzee males prefer to have sex with older females, U.S. researchers found in a study published Monday that shows one of the biggest behavioral 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, anthropologist Martin Muller and colleagues at Boston University discovered.
"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 a telephone interview.
Writing in the journal Current Biology, Muller and colleagues said they studied chimpanzees living in the Kanyawara community of Kibale National Park in Uganda.
It is easy to observe their mating behavior.
"Chimpanzee copulations are frequently preceded by a series of male courtship signals (e.g., 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 apparently remain fertile their entire lives, although these 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 the 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 guessed.
"The males do end up mating with all the females for the most part," Muller noted. But he said the study challenges common conceptions.
"Normally, I think peoples' default assumption is, 'Well other animals, they must also find young females attractive,"' Muller said. "And people assume that young females are more fertile than older females."
But female chimpanzees do not experience the rapid decline in fertility that is seen in human females after their 20s.
Humans may prefer younger females because of marriage and other "long-term pair-bonds," something that is nonexistent in the promiscuous world of chimps. Human men seeking progeny may need to start with younger prospective mothers, Muller said.
"Chimpanzee males may not find the wrinkled skin, ragged ears, irregular bald patches, and elongated nipples of their aged females as alluring as human men find the full lips and smooth complexions of young women, but they are clearly not reacting negatively to such cues," the researchers concluded.