I THINK Dumont and Rifkind1 are mistaken in explaining their interesting observations on the rudimentary condition of the thoracic duct in bats, New World monkeys and sloths. They suggest that “the unique functional characteristic shared by all three species is nothing more than the habit of hanging upside down …”. Their interpretation might apply to bats, which spend most of their lives hanging head downwards, but not to New World monkeys and sloths, which do not. Some New World monkeys can hang head downwards suspended by their prehensile tails, but surely they do not spend even half their lives in that position; the attitude is adopted occasionally and momentarily. Sloths do not hang head downwards, but back downwards; they are inverted pronogrades, and the effect of gravity on the contents of the thoracic duct can be little different from that in all four-footed pronograde mammals. When sloths do not hang downwards, their position is usually head upwards clinging to the trunk or sitting in the fork of a tree.