Friendly but stressed: A novel finding on social stress in wild Japanese macaques

For social animals, maintaining stable social relationships and responding appropriately to other group members is crucial. However, understanding how animals perceive their social environment is challenging. Stress levels, which fluctuate depending on social context, offer a valuable window to examine their internal states. Previous research has shown that primates exhibit lower stress levels when they are with close relatives and important social partners, and higher stress levels in the presence of dominant or unfamiliar individuals.

Our study investigated stress levels in wild female Japanese macaques through self-scratching behavior, as an indicator of stress level. We observed 11 adult females in a wild group on Kinkazan Island, Miyagi Prefecture (total observation: 205 hours). During the observation, we recorded self-scratching, dominance ranks, and proximity of other individuals (defined as within 1 meter). Relationship closeness was assessed based on the proportion of time pairs spent in close proximity. Using data from periods when the focal females were either resting or foraging, we analyzed the relationship between the frequency of self-scratching and the following factors:

1. Whether another individual is present within 1 meter of the focal female.
2. When a female is in proximity, the effects of the proximate female’s kinship, relative rank, and proximity rate.

As a result, two key findings were revealed:

1. During resting, the frequency of self-scratching was higher when no other individuals were nearby compared to when they were present (Figure 2a). However, this pattern was not observed during foraging (Figure 2b).
2. During foraging, the frequency of self-scratching was higher when a related female (mother or sister) was proximate than when a non-related female was present (Figure 3). No effect of kinship was observed during resting.

During resting, the absence of proximate individuals indicates their potential separation from the group, which may induce an elevation of their stress level. In contrast, the increased self-scratching when a related female was present during foraging differs from previous studies and was an unexpected result. The result suggested that co-feeding with a related female led to increased stress, possibly due to foraging competition. In Japanese macaque societies, related females are regarded as valuable partners. This means that the presence of valuable partners does not always link to relief. More broadly, this study provides a novel view that determinants of social stress is more complicated than previously thought.