Final answer:
Gibbons and siamangs are characterized by their brachiation locomotion, low sexual dimorphism, and highly arboreal nature.
Step-by-step explanation:
Gibbons and siamangs are notable for their mode of locomotion known as brachiation, where they swing from tree to tree using their long arms. Their anatomical characteristics include low sexual dimorphism, which means that there is not a marked difference in size between males and females, although some species may show color differences between sexes. Another characteristic of gibbons and siamangs is that they are highly arboreal, spending most of their time in trees, which is reflected in their long arms and is an adaptation to their canopy-dwelling lifestyle.