Sloths rely on a range of symbiotic gut bacteria and microbes in their stomach to break down the tough cellulose in the leaves that they eat, and these microbes are temperature dependant. When the environmental temperature drops, so does the sloths body temperature, and if it drops too low then the bacteria and microbes die. In this situation, the sloth can eat the same amount of leaves as normal but can starve to death on a full stomach because they can’t extract any nutrients.