Adding fur to the monkeys takes careful work. The pen strokes should follow the contours of the real fur. The guy hanging from the tree is a challenge because hit coat is relatively light in tone and there are stripes of bright sunlight and shadow that need to be captured. I don't want him to look striped, but shadowed. I will continue to build up this region and will show a comparison in the next post.
Another challenge with fur is that you need to plan where it will be. For instance, the fur on the backside of the hanging monkey's legs overlaps a shadowed region. So, since this is pen and ink, I must build up the darker areas behind the fur while leaving lighter regions of the paper to represent the fur as shown in the closeup below.