If one species is socially dominant to another, the subordinate usually narrows its niche when they occur together. When one species is dominant in some circumstances and a second in others, both narrow their niches when together. Subordinates usually have a larger fundamental niche than their dominants. The presence of dominants should result in selection for enlarged (or changed) fundamental niches by the subordinate species. Linear hierarchies of species should result in guilds whose members have different-sized fundamental and realized niches. Though large mobile species often have greater fundamental niches than small similar species, an inverse relationship between size and niche breadth usually occurs where clear dominance hierarchies exist, suggesting that social dominance has more than counteracted the effect of body size. The presence of dominants is a factor making conditions uncertain for subordinates. Limits to the niche breadth of dominants are in some cases directly set by physical factors ...