In all species, the predominant diet is shoots and leaves of forest trees. Sloths feed on Cecropia, the most abundant tree of the Amazonia forest. Regenerating agricultural land river margins and natural gaps may sometimes be important (despite the biting ants that swarm in the tree's hollow stems), but it is never the sole food source. The idea that Bradypus sloths feed only on Cecropia probably arises because an open growth form makes a sloth in a Cecropia easier to see than in almost any other kind of rainforest tree. When feeding, the forelimbs are used to pull leaves slowly towards the mouth. Sloths are highly specialized for an existence that centers around squeezing as much energy as possible out of a low-intake rate of highly indigestible food. This is because leaves eaten by Bradypus sloths are, though energy-rich, also rich in tannins and fiber. Digestion must therefore be a simultaneous process of detoxification and energy extraction. The gut is extensive, making up 30% of the total body weight. Digestion, by bacterial fermentation in a complex multi-chambered stomach, occurs over an extended period to permit maximum absorption of scarce resources. Passage of food through the intestine is also very slow, providing maximum opportunity for the uptake of nutrients and energy. This
means that sloths have little energy to spare and hence move slowly. Their metabolic rates are about half of what would be expected for an animal of their size (sloth adaptations provide plenty of opportunities to contemplate the chicken-and-egg nature of complex physiological and behavioral adaptations). The three-toed sloth has struck a compromise between being large enough to move about efficiently between the canopies of tall trees, the weight of the long gut and big stomach needed for food processing and the need to be light enough to avoid breaking the limbs from which it feeds. In minimizing weight, while maximizing mass, the sloth has compromised on muscles and has the lowest ratio of muscles to skeleton of any comparably sized ground-dwelling mammal. Its muscles are thin and ribbon-like. Much of the sloth's apparent volume comes from its long hair. The compromise works: sloths are the most abundant large mammal in neotropical rainforests (up to 70% of the arboreal mammalian biomass), and cropping some 2% of the forest's annual leaf production. It has been suggested that the blue-green algae may also provide some nutrition, being licked directly from the hair or having nutrients absorbed via the skin. Despite great similarities, two species of Bradypus sloth coexist in certain parts of their range. Bradypus and Choloepus sloths can also coexist.
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