Social behavior among monkeys may be more nature than nurture

An unusual experiment with monkeys who were switched between mothers shortly after birth has demonstrated the importance of nature over nurture in behavior.

Young monkeys reared by a mother other than their own are more likely to exhibit the aggressive or friendly behavior of their birth mothers rather than the behavior of their foster mothers, a University of Chicago researcher has shown for the first time.

The discovery of inheritability of social behavior traits among non-human primates has important implications for people as it reinforces other research that suggests that such characteristics as sociability and impulsive aggressiveness among humans may have a genetic basis, said Dario Maestripieri, Associate Professor in Human Development at the University. The work with monkeys may help other researchers understand the biological origins of characteristics that promote socialization among humans, he said.

His work on monkeys is reported in the article “Similarities in Affiliation and Aggression Between Cross-Fostered Rhesus Macaque Females and Their Biological Mothers,” published in the current issue of Developmental Psychobiology.

Rhesus macaques provide an important research population because they organize in strong matrilineal structures, and the female offspring often exhibit the same social behavior as their mothers. The experiment was intended to show if some aspects of that behavior were inherited or learned by the female offspring.

“I was surprised by what we found,” Maestripieri said. Scholars have felt that social learning from the mother would play an important role in the development of female social behavior from early infancy. The study shows that inherited behavioral predispositions are probably more important.

For the study, Maestripieri and his colleagues swapped rhesus monkey female babies between mothers who had recently given birth. These adoptions are typically difficult to achieve, but the team was much more successful than other researchers have been by making the matches soon after birth.

To understand the origins of behavior, the team looked at the expression of social contact and aggression among the offspring and their biological and foster mothers. The researchers noted, for example, how many times the offspring had bodily contact and how many times they expressed aggression, such as threats, slaps, bites and chases with other group members.

When Maestripieri looked at the behavior of the monkey offspring and their mothers over the span of three years, he found that while the offspring’s behavior mirrored the behavior of their biological mothers, there was practically no similarity between the offspring and their foster mothers. For instance, offspring who often used threats and slaps to get their way had biological mothers who also displayed that behavior, he found.

Primate expert Joan Silk, Professor of Anthropology at the University of California, Los Angeles, said, “This study adds to a growing body of evidence that temperament and behavioral predispositions vary among individuals and that temperamental differences are stable over the life course. However, it is usually difficult to determine whether such differences are the results of inherited dispositions and/or the effects of environment and experience.

“Using an innovative design to disentangle the effects of ’nature’ and ’nurture,’ Maestripieri demonstrates that heredity has a surprisingly important impact on the behavioral dispositions of infant macaques. These findings have important implications for understanding how evolution shapes behavior and temperament in primates and humans,” she added.