Three-toed sloths survive on a nutrient-poor diet consisting almost entirely of tough, fibrous leaves. To adapt, they evolved the lowest metabolic rate of almost any mammal, relying on a complex, multi-chambered stomach full of specialized symbiotic bacteria to slowly ferment their meals. This digestive process can take over a month to complete.
However, because sloths are ectothermic in practice—meaning they depend heavily on external environmental warmth to regulate their body temperature—their gut microbes rely on ambient heat to function. If cold weather persists for an extended period, the sloth's core body temperature drops, causing the fermenting bacteria to go dormant. Digestion grinds to a complete halt.
Even though the animal's stomach remains fully packed with leaves, no nutrients can be absorbed, ultimately leading to starvation on a full stomach.