A study published in the journal Current Biology showed that monkeys tend to become pickier with their acquaintances as they get older, just like humans.
The research team, led by Laura Almeling from the Leibniz Institute for Primate Research, studied how monkeys aged and their changes related to social and cognitive behavior.
The idea behind the study is that the way humans select their goals in life is usually restricted by the loss of resources. Humans are also known to prefer acquaintances whose mutual relationships are meaningful as they become older, this is mainly due to people eventually becoming aware of the constant decrease of their lifetime.
Studying primate behavior
Heterogeneous Barbary macaques from the La Forêt des Singes at Rocamadour were analyzed, and it was found that although monkeys did remain interested in social activity, they often had a preference for individuals they considered important.
Gregarious species must be able to know which of their skin belongs to their same social group. This can be accomplished through analyzing appearance, vocalization, sense of smell and many other mechanisms. Previous studies have shown that monkeys are able to identify an individual’s voice as they establish a link with the place of origin of the vocalization. The ability of mentally labeling different individuals to different groups has been seen in lizards, ants, birds, and of course, monkeys.
The basis of social behavior: Identifying acquaintances
Facial features are able to be perceived by both monkeys and humans, and they appear to employ the same mechanisms to recognize other individuals, and this skill appears to be heavily influenced by experience. Curiously, this ability is also present on pigeons.
Monkeys have been proven to be able to identify members of their social group through the use of pictures. A past study at the same La Forêt des Singes showed how Barbary macaques were able to distinguish photographs that displayed members of their own group from pictures that showed monkeys that belonged to a different group. This trait was more prominent in older monkeys than in younger ones.
The monkeys were able to roam in the park where food and water are freely available. The researchers noted three major groups of monkeys, and these groups were often kept apart by the park staff. The researchers managed to take pictures of the faces of 46 monkeys and they were cropped using Photoshop and printed on matte paper to avoid reflections.
The experimenter then sat on a bench and placed food nearby so the monkeys approached voluntarily. When a monkey jumped on the bench, the experimenter would remove the cover of one of the photographs and the subject’s reaction would be recorded. The experimenter was not aware of which picture was being shown to which monkey and if the picture corresponded to a member of the subject’s group. There was no “food prize” other than the food placed so the monkeys approached the experimentation area. Each monkey was supposed to be tested no more than once to avoid the feeling of “task and reward,” but one monkey was tested seven times while the others were tested five times or less.
Then, researchers recorded how long the monkeys looked at the photographs and their reaction. There were three types of responses. Either the monkey acted upon itself (such as a yawn), made a gesture towards the picture, or interacted with the photo.
How monkeys reacted to their friends’ pictures
Younger subjects did not show any difference in how they interacted with the pictures, but older monkeys did show an increased interest in the pictures of monkeys from their own social group.
The age of the monkey did not seem to affect the interest of the subject regarding overall interest in social activity, but older monkeys were indeed less interested in monkeys that they did not consider important.
It was also noted that subjects were less inclined to groom monkeys that they did not have close relationships with. Grooming is a key social interaction method for primates, as it is calming and allows them to form bonds with each other. In contrast, older monkeys were frequently groomed by a larger amount of younger monkeys.
Pictures of baby macaques were shown to the subjects and sound recordings with vocalizations from their friends and strangers were played. The monkeys showed an increased response to the offspring and the screams of subjects they were related to.
Almeling said that monkeys become more socially selective as they become older, but retained interest for social activity. It was also suggested that older monkeys find excessive social interaction to be stressful, which may indicate a reason to avoid them whenever they don’t feel comfortable with their acquaintance. The theories were conceived while assuming that monkeys are not aware of their limited lifetime.
It is also suggested that a larger number of social interactions means a higher risk for both humans and monkeys, which may indicate a reason why social activity tends to change with age.
Although monkeys and humans are millions of years apart when it comes to biology, studying monkeys often helps to isolate psychological and social behavior that is still present in humans. Studying the behavior of monkeys to understand humans has been often regarded as a magnifying lens to discover the inherent traits that us, humans, share with our closest relatives in the animal kingdom.
Source: Current Biology