A multi-level society comprised of one-male and multi-male core units in an African colobine (Colobus angolensis ruwenzorii)

Several mammalian species exhibit complex, nested social organizations, termed multi-level or modular societies. Multi-level societies comprise stable core units that fission and fuse with one another in a hierarchical manner, forming groups that vary in size over time. Among nonhuman primates, these social systems have been confirmed in several African papionin and Asian colobine species. We use data from August 2017 to July 2018 on individually-recognized Rwenzori Angolan colobus living near Lake Nabugabo, Uganda to document the first multi-level society in an African colobine. The study band comprised up to 135 individuals organized into 12 socially and spatially distinct core units that ranged in size from 4 to 23 individuals. These core units showed a strong affinity to one another, spending roughly 75% of their time together. Core units fissioned and fused non-randomly with one another throughout the day, leading to different combinations of core units being observed. Using association indices between core units, we employed hierarchical cluster analyses and permutation tests to show that some core units associated preferentially into clans. Thus, we confirm three tiers of social organization for Rwenzori Angolan colobus: core unit, clan, and band. The social organization of this subspecies is unlike any reported previously in a nonhuman primate, with about half the core units containing a single adult male and the others containing multiple reproductive adult males. The discovery of a unique primate multi-level society in a novel lineage could allow for a better understanding of the evolution of these complex social systems across the Animal Kingdom. Preliminary data show males transfer within the band and females transfer outside of the band, which is proposed for hominin multi-level societies. This subspecies could thus also provide insight into the selective pressures underlying multi-level societies in our own lineage.