Animals end up with tired limbs just as humans do. This is especially true for those who have to carry a baby around constantly. Apes are smart creatures and have found ways to ease their own pain.
Orangutans have developed a method to ease the strain of carrying a baby through the jungle. The mothers (and occasionally males) chew the leaves of a medicinal plant into a paste, rubbing it onto their aching arms like a pain-easing balm. Any tired human mother would understand the motivation behind this behavior.