"A rolling stone gathers no moss  ," but there is one living animal that does gather moss -- the three-toed sloth of South America. This slowest-moving member of the animal kingdom is so inactive that moss actually gathers on its body and turns it green -- strange as it seems! Most of the sloth's life is spent motionless, hanging upside down from a limb . And that is the way its hair grows. Long and coarse, the strands from receptacles for the damp jungle algae turn the brown fur a mossy green. Actually this moss helps the animal survive because it serves as a perfect camouflage against the leafy trees and hides the sloth from the jungle's swift-moving hunters. The sloth would have little chance of survival on the ground. With long, curved claws hooded over the limb of a leafy tree, it spends the long hot hours during the day drowsing and eating. Inch by inch, it strips the leafy limbs bare and crawls slowly down the trunk to find a new dining spot, but only at night. In addition to looking rather like a vegetable, the sloth is a strict vegetarian.  Running out of its favorite leaf is about the only thing that will make a sloth move. Then its appetite may even force the animal into swimming a stream to reach a succulent . It will also force it into fighting to keep the tree all to itself. The sloth is lazy and prefers to be alone, but it will tolerate its own  s! According to the passage, the sloth will move about when  _  .
It senses a coming danger. it is forced to swim in a hot day. it is short of its favorite food. it wants to keep the vegetable to itself.