When common marmoset monkeys hear sound recordings of conspecifics, they can clearly distinguish whether these are individual sounds or connected conversations. In addition, they apparently rate the content of the conversation and adjust their behavior according to how cooperatively they assess the eavesdropped – similar to us humans. This was shown by researchers with a study in which they concluded the emotional state of the monkeys based on temperature measurements and observed their behavior.
Common marmosets live in cooperative groups in which they raise their offspring together. So social interactions are crucial for the little primates. Which signals do they use to assess their conspecifics? Can you understand and interpret overheard interactions of other monkeys? For us humans this is part of everyday life, but whether the primates related to us can also do this has so far been difficult to answer. This is due, among other things, to the fact that classic behavioral studies provide little information about the emotional states of the animals and their behavior only reveals to a limited extent what exactly they are reacting to.
Excitement makes the nose cold
Researchers working with Rahel Brügger from the University of Zurich have now chosen a new approach: while they played recorded calls from conspecifics to common marmosets, they measured the temperature of the animals’ faces in order to gain information about their emotional reactions. As in humans, the blood circulation in the skin changes in the monkeys, for example when they are excited. These changes are particularly clear in exposed areas such as the nose and can be recorded with the help of thermography in an infrared image. As a stimulus, the researchers played the monkeys on the one hand interactions between a young animal begging for food and an adult who either complies with the request of the young animal or makes negative noises. On the other hand, they played the sounds individually, without context.
“It turned out that the reaction to the call interactions was significantly different than the reaction to the sum of the corresponding individual calls,” says Brügger. “Common marmosets can distinguish a dialogue between conspecifics from a mere monologue.” The individual reactions to the stimuli differed depending on whether the monkey listening had offspring itself or whether it was just helping others raise it. The gender of the listener and the overheard also played a role.
Females without offspring of their own showed a particularly strong emotional reaction – signaled by a drop in temperature at the nose – when they overheard interactions between a male individual and a baby monkey. If, on the other hand, males without offspring of their own heard isolated sounds from females offering food to young animals, their nasal temperature rose – a sign of decreasing excitement. Monkeys with their own offspring showed the least reaction to the various stimuli.
Cooperative conspecifics preferred
In the next step, the researchers examined whether the common marmoset monkeys could draw conclusions about the character of their conspecifics from the overheard calls and whether they behaved accordingly. To do this, they gave the monkeys the opportunity to get closer to the source of the noise after they had played the sound recordings. As expected, the animals preferred those supposed conspecifics who had previously reacted positively to the begging offspring. “This indicates that the marmosets not only processed the vocal interactions holistically, but also used this information to evaluate the interactions,” the researchers said.
So that the results were not distorted by personal relationships between the individuals, the researchers played the monkeys in the study with sounds made by conspecifics they did not know. “Overall, the study contributes to the increasing evidence that many animals are not only passive observers of interactions between their conspecifics, but also interpret them for themselves,” says Brügger’s colleague Judith Burkart. “In addition, we show in our study what thermography can contribute as a method to uncover how the social exchange of non-verbal subjects is perceived.”
Source: Rahel Brügger (University of Zurich) et al., Science Advances, doi: 10.1126 / sciadv.abc8790