Scientists have discovered one of the biggest behavioral differences between chimpanzee and man: chimpanzee males prefer sex with older females, opposite to human preference.
Male chimps compete intensively and even fight over the oldest females, while the youngest female chimps have to work harder to get masculine attention. "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," said anthropologist Martin Muller at Boston University.
Compared to younger females, older females were more likely to be approached for copulation, associated with males more often during estrous periods, copulated more frequently with high-ranking males, and gave rise to higher rates of male-on-male aggression during mating period. "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."
The team studied mating behavior on chimpanzees living in the Kanyawara community of Kibale National Park in Uganda. "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 wanted to see if chimpanzees behave like humans - their closest living relatives - who form long-term pairs and who value younger females. The results were the opposite. Unlike humans, female chimpanzees can warn around about their fertility, with bright red bottom swellings.
Apparently, chimpanzee females are fertile their entire lives, although they rarely live beyond the age of 40 in the wild and chimpanzees participate in a relatively promiscuous mating system. Humans form long-term mating partnerships, and younger females are more valuable, having a greater reproductive potential as female fertility is limited by menopause. "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.
Older females may also be better mothers as they are more experienced. "The males do end up mating with all the females for the most part," Muller noted. "People assume that young females are more fertile than older females", but as female chimpanzees do not experience menopause, that's without importance for 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.
Thus, the monogamy and menopause deeply influenced the preference of human males for young females. This human trait may have appeared in the human lineage after the split from the common branch with the chimps.