Social organization in a one-male group of red-tailed monkeys (Cercopithecus ascanius): insights from two proximity thresholds.

Primates exhibit diverse social structures, yet how proximity reflects social relationships may depend on the spatial threshold applied. This study examined within-group social relationships in red-tailed monkeys (Cercopithecus ascanius) in Mahale Mountains National Park, Tanzania, by comparing two proximity thresholds, 5 m and 1 m. Proximity data were collected from one group over 122 days and used to construct separate proximity networks for each threshold. In the 5-m network, the adult male occupied the most central position and was approached more frequently by other group members than he approached them. In contrast, in the 1-m network, the adult male was peripheral, whereas females were relatively more central, with especially strong ties concentrated in specific dyads. These results indicate that different proximity thresholds reveal different aspects of within-group social organization within the same group. In this one-male group, broader spatial associations were centered on the resident male, whereas closer-range associations were more female-centered and selective. By showing that the apparent social structure of the same group varies with the spatial threshold used to define proximity, this study provides a new perspective on social organization in arboreal primates forming one-male groups.