<p>Jarman’s hypothesis maintains that larger-bodied grazers that feed mainly on grass-like graminoids tend to form large groups, and smaller-bodied browsers that feed mainly on non-graminoids tend to be solitary. Whether this hypothesis held true for forest-dwelling ungulates in subtropical zones had not been investigated. We evaluate Jarman’s hypothesis by investigating grouping behaviour of three sympatric subtropical forest-dwelling ungulates—sambar (a larger-bodied grazer), and Reeves’ muntjac and Formosan serow (smaller-bodied browsers)—in Yushan National Park, central Taiwan. We found no differences in group size among these species, with each being mainly solitary throughout the year. This suggest that grazer-like diets and large body size do not promote increased group size in this forest habitat. Subtropical cloud forests provide many opportunities for animals to conceal themselves, rendering antipredator benefits associated with being in groups of limited value. Female groups—the presumed foundation of gregarious societies—occurred only in sambar, suggesting that a grazer-like diet and large body size provide the basis for group formation. Highly productive and uniformly distributed graminoids—the staple food of sambar, and thought to mitigate intraspecific food competition—possibly explain why female group formation occurred only in sambars. These findings suggest that the direct selective pressure for increased group size in open habitat where places for animals to conceal themselves are few is predation, rather than the need to mitigate intraspecific competition for food resources.</p>