A three-toed sloth spends nearly its whole life in the canopy, and about once a week it does the most dangerous thing it ever does: it climbs all the way down to the ground, digs a little hollow at the foot of its tree, and goes to the toilet there. It could simply let go from the branches. Instead it makes the slow trip into the place where the jaguars are, and that trip is when a good share of sloths are caught. Nobody is sure why. One theory is that moths living in its fur lay their eggs in the dung, their young fly back up to find a sloth, and the nutrients they carry feed the green algae growing on its coat, which the sloth may eat. If that is right, the riskiest walk of the week is a gardener going down to tend the compost.