Climbing ungulates as modifiers of tree morphology: a case from the Formosan serow

Tree climbing is rare among ungulates because of morphological constraints, yet its ecological significance remains poorly understood. Here, we investigated the characteristics of trees climbed by Formosan serows ( Capricornis swinhoei ) and the potential consequences of this behavior for tree morphology. We surveyed trees with bite marks and compared their structural traits and surrounding environments with those of non-climbed trees. Serows preferentially climbed trees with horizontally oriented trunks and numerous branches that can serve as footholds. Climbing also occurred more frequently in open environments with high light availability. The bite marks reached heights of up to 680 cm, indicating that serows can access a vertical range of foliage beyond that available to most terrestrial ungulates. In addition, climbed trees frequently exhibited bending structures in which trunks transitioned from vertical to horizontal orientations, suggesting that repeated climbing and browsing may contribute to modifications of tree morphology. These findings indicate that serows selectively use structurally accessible trees to exploit high-quality resources and may modify tree morphology through their climbing and foraging behavior. Our study highlights a previously underappreciated interaction between ungulates and forest structure and suggests that serows may function as ecosystem engineers despite their relatively small body size.