A monkey descending a tree trunk often keeps its head up, moving almost like a cautious climber backing down a ladder. Squirrels and many other mammals, by contrast, tend to go headfirst.

Far up a giant Amazon jungle tree, a large howler monkey walked among the branches. As it stepped from branch to branch, its long tail would wrap around a convenient trunk providing extra stability.

A young monkey takes his first wobbly steps into the treetops, and the results are as adorable as they are suspenseful. Each branch is a new test of balance and bravery. His family watches closely, ...