PS 48-135 - A Goldilocks effect: ranging patterns and activity budgets of Rwenzori Angolan colobus core units in a multi-level society

Background/Question/Methods

Multilevel societies (MLSs) are complex social organizations whereby stable core units fission and fuse non-randomly with one another in a hierarchical manner. MLSs have been reported in a wide range of mammals, including Proboscidea, Perissodactyla, Artiodactyla, and Primates. In 2019, we reported a MLS in Rwenzori Angolan colobus monkeys (Colobus angolensis ruwenzorii) living in a forest fragment (2.8 km²) near Lake Nabugabo, Uganda. Research on other colobine MLSs has revealed large home ranges with bands intensively using one area and then moving long distances to find food resources elsewhere. However, these colobines all inhabit continuous forests. Given the rapid rate at which habitat is being lost and fragmented, it is important to understand how arboreal, nonvolant animals living in MLSs will respond. We investigated the ranging behaviour of the Nabugabo colobines as well as feeding competition among individuals living in core units of different sizes. At Nabugabo, we have identified a band comprised of 139 individuals in 12 core units (size range: 5-25) that cluster into two clans. For this study, we used GPS points collected over two-years to determine ranging and scan samples collected over 10-months to compare sex- and core unit-specific activity budgets and near-neighbor distances.

Results/Conclusions

The 95% home range size estimate of this band was 1.75 km². The colobus rarely utilized the matrix and core units’ home ranges overlapped by 93%. Indicators of scramble competition for food included more time spent feeding in larger core units, near-neighbors at greater distances when feeding relative to resting, and greater feeding for females compared to males. A quadratic relationship was found between core unit size and home range and core area size, where intermediate-sized units showed the smallest ranges. The smallest core units had the longest day ranges and the fastest speed of travel. We thus suggest that while large core units experience more intra-group feeding competition, small units may be displaced more in inter-unit contests for food. These results suggest that within the MLS at Nabugabo, intermediate-sized core units are energetically optimal, indicating a ‘Goldilocks effect’. Despite being restricted to a much smaller range than other colobines living in a MLS due to habitat fragmentation, the Rwenzori Angolan colobus at Nabugabo appear to be faring well; they are able to maintain a high-quality diet and have recently grown to 15 core units.