A big, warm hug works wonders, even in the monkey world. Female spider monkeys without infants, it seems, will hug mother monkeys in exchange for permission to kiss, sniff and touch their babies.
The discovery, which will be outlined in an upcoming issue of Animal Behavior, not only shows how much primates, especially females, value infants, but it also reveals that an embrace conveys good intentions and provides comfort in primate species other than humans.
"An embrace is defined as one monkey approaching another monkey and wrapping their arms around them, in very much the same way as humans do, with one arm wrapped around the neck and the other around the waist," lead author Kathy Slater told Discovery News.
Slater, a researcher in the Department of Psychology at the University of Chester, added that the embrace is often accompanied by a "kiss on the cheek" and a "pectoral sniff," when one monkey moves its head next to the other monkey's chest scent glands to get a whiff.
She said both males and females tend to hug when they haven't seen each other for a while. Males will also sometimes hug each other in front of females "to reduce tension and prevent aggression" in a situation that can foster competition.
But Slater's observation that females without young often embrace new mothers is new. She and her team observed hugs received by 15 such mothers in two communities at Otoch Ma'ax Yetel Kooh reserve in the Yucatan Peninsula, Mexico.
In most instances, a recently embraced mother would grant the hugger permission to handle her infant, which the mother carries for the first year of its life. The mother does not release the baby, but grants the hugger permission to sit in close proximity and handle the infant.
"Embraces in spider monkeys appear to be a method of reassuring the recipient of benign intent and reducing tension," Slater explained. "Embracing is a potentially risky behavior, as it exposes vulnerable parts of the body, such as the shoulders, face and neck, and is therefore an honest signal to the mother that the (hugger) does not intend to harm her or her baby."
Mothers approached by females who did not offer a hug protectively snubbed the curious females by either turning their bodies to create a physical barrier, or by moving — baby in tow — away from the approaching individual.
Louise Barrett, a researcher in the Evolutionary Psychology and Behavioral Ecology Research Group at the University of Liverpool, previously found that female baboons engage in similar behavior, only they exchange grooming for baby handling.
Barrett told Discovery News that at first she wondered how anyone could make a similar finding about spider monkeys, which do not spend much time socially grooming.
"The fact that embraces function in the same way (as grooming) — and perhaps more effectively — is interesting," Barrett said.
Slater believes the "biological market," whereby individuals exchange either grooming or hugs for baby time, may illustrate the origins of human hugging.
She explained that spider monkeys live in the same type of societies as chimpanzees do. Since chimp societies are, in turn, similar to human societies, "it is therefore possible to make direct links between spider monkey social behavior and the evolution of human social behavior."