Social system and spatial behavior drive interspecific differences in foraging intensity and plant-level damage in sympatric ungulates

Foraging behavior in ungulates affects not only their own fitness but also the fitness and morphology of the plants they consume. While intraspecific variation in ungulate foraging behavior has been extensively studied, interspecific differences—particularly those associated with social systems and spatial behavior—remain poorly understood. Here, I conducted direct observations of the foraging behavior of two sympatric ungulates with contrasting social and spatial behavior: the gregarious, non-territorial, and migratory sika deer ( Cervus nippon ) and the solitary, territorial, and sedentary Japanese serow ( Capricornis crispus ), which feed on the same plant species during the warm season (2017–2022) in the alpine zone of Mount Fuji, Japan. Serow bite rates (bites/min) were significantly lower than those of deer, whereas serow step rates (steps/min) were significantly higher, indicating that deer fed more intensively while serows moved more frequently while foraging. The number of bites per plant individual was also significantly lower in serows than in deer, and individual plants experienced substantially greater tissue removal when browsed by deer than by serow. These results suggest that serows forage in a manner consistent with reducing damage to individual plants, whereas deer forage more intensively, resulting in greater plant damage. Differences in gregariousness and territoriality likely influence the intensity of intraspecific competition for food resources, whereas differences in spatial behavior may shape the degree to which individuals rely on long-term resource persistence within a given area.