The base unit is one adult male with one to several females and their young, held together by the male's herding behaviour, including neck bites directed at females who stray. Two to four such units whose males are often related make a clan. Clans travel and forage together as a band, and it is the band that functions as the daily social and movement unit.
At dusk, several bands converge on a limited number of safe cliff faces, forming sleeping troops that may exceed two hundred animals. Cliffs are scarce in the arid habitat, and that scarcity is the ordinary explanation for the top tier: safety from predators pulls unrelated bands together at night without merging them socially.
The structure is not fixed. Hamadryas hybridise with olive baboons where their ranges meet in Ethiopia, and hybrid zones show intermediate social behaviour, which complicates any account that treats the multi-level system as a simple species-typical trait.