Professor finds that nonhuman primates have evolutionary reason to bond with their offspring

Dario Maestripieri, Assistant Professor in the Committee on Human Development and the Committee on Evolutionary Biology, has studied the behaviors of nonhuman primates in his research. His most recent study found that these primates bond with their offspring and demonstrate a strong motivation to look after their young.

Monkeys and other nonhuman primates recognize their young shortly after birth and bond with them during a postpartum sensitive period, a University researcher has found in a study that points to similar implications for humans, who, like other primates, demonstrate a strong motivation to look after offspring.

In the article, “Is There Mother-Infant Bonding in Primates?” Dario Maestripieri, Assistant Professor on the Committee on Human Development and the Committee on Evolutionary Biology, challenges an accepted view of other scholars that among mammals, only cattle, sheep and goats have an evolutionary reason to bond with their young during a postpartum sensitive period. Those researchers have contended that the bonding period in such animals is necessary because the young are able to walk shortly after birth and therefore could be confused with the offspring of other individuals. In contrast, because baby primates, including humans, are unable to move at birth, there would be no evolutionary reason for a bonding period in these species.

By studying several hundred reported cases of primate infants being separated from their mothers and adopted by other females shortly after birth, Maestripieri found evidence that primate mothers also recognize their young very early in life and undergo a sensitive period for bonding with them.

“Even though olfaction is underdeveloped in primates and humans relative to other mammals, monkey and human mothers are probably capable of recognizing their infant’s odor within hours after birth and learn their visual and vocal characteristics shortly afterward,” wrote Maestripieri in an article in the current issue of Developmental Review.

The sensitive period for bonding in primates is characterized by a heightened motivation for care giving on the part of the mothers, he pointed out. “Just because primate infants are so dependent on their mothers, it is crucial not so much that their mothers recognize them, as that they are motivated to take care of them,” he wrote. Heightened care giving motivation during the sensitive period may lead to the adoption of unrelated infants when mothers’ own offspring die.

For his study, Maestripieri examined the records of his own research at the Yerkes Regional Primate Research Center in Georgia as well as data gathered by other scholars who have observed primates and studied infant adoption. In addition to mother primates adopting infants after their own offspring die, researchers also have found that lactating mothers will adopt an additional baby if that animal has been abandoned. In some cases, they even kidnap infants.

Maestripieri found that adoption is most common during the first two to three weeks after an adopting mother gives birth. If adoptions are attempted when the offspring are several months old, they often are unsuccessful.

Some data gathered by other scholars showed recognition beginning shortly after birth. After studying the records of a group of 122 long-tail macaques, Maestripieri found that mothers’ recognition for their infants apparently develops gradually. The mothers were able to recognize their young readily by the time the offspring were 2 to 3 weeks old, he found. The recognition becomes complete by the time the young are 2 months old. The monkey behaviors were similar to those reported among cattle, sheep and goats. Maestripieri found that mother monkeys, like the other mammals, reject their own offspring if they have been separated after birth and reunited with them after the end of the sensitive period for bonding. Therefore, mother-offspring bonding processes in primates are similar to those reported among cattle, sheep and goats.

The research on non-human primates provides a window to understanding human behavior, Maestripieri said. Like monkeys, human mothers experience changes in their hormones at the time of birthing. Other studies have shown that those hormones influence a human mother’s ability to recognize her new child’s odor and her care giving motivation. Heightened care giving motivation during the postpartum period, in turn, can facilitate bonding with the baby. Hormonal changes, however, are not necessary for bonding because fathers as well as adoptive mothers also become attached to their infants.

Many scholars have disputed the idea of a bonding period in humans because they have not detected any effects of different amounts of mother-infant contact after birth on subsequent parenting behavior or child development.

“The assessment of mother-infant bonding in humans depends on how this phenomenon is defined and which of its features are believed to be relevant enough to human parenting and child development to warrant investigation,” Maestripieri said.

“Primate research in this area can stimulate and complement human research and enhance our understanding of some adaptive and maladaptive processes underlying parenting behavior and child development.”