What is a defining characteristic of brachiating species among nonhuman primates?

A defining characteristic of brachiating species among nonhuman primates is their ability to swing by their arms. This mode of locomotion allows these primates to navigate through their arboreal habitats with agility and efficiency. Specifically, brachiation involves using the arms to grasp branches and propel the body forward from one branch to another, which is particularly well-suited for moving in the tree canopy where stability and the ability to reach food sources are crucial.

This physical adaptation is generally associated with specific anatomical features, such as long arms relative to body size, flexible shoulder joints, and strong grasping hands, which collectively enable these primates to efficiently travel between tree limbs. This behavior is primarily observed in gibbons and some species of spider monkeys, showcasing their specialized evolutionary adaptations to life in the trees.

In contrast, other options describe behaviors or characteristics that do not apply to brachiating species: walking upright describes terrestrial locomotion; flying is a completely different form of movement that does not relate to nonhuman primates; and ground-dwelling behavior indicates a lifestyle that involves more time spent on the ground, which is not representative of the primarily arboreal lifestyle of brachiating species.