Why sloths do not sleep upside down

Several mammal species other than ruminants and camels have a multi-chambered forestomach – kangaroos, hippos, colobus monkeys, peccaries, sloths – but they do not ruminate. As studies on the digestive physiology of these species are largely missing, it is generally assumed that their forestomach functions in the same way as that of ruminants, the most prominent characteristic of which is the selective retention of larger particles.

However, retaining larger particles (which are more difficult to digest due to their unfavourable surface:volume-ratio) only makes sense if you can chew on them again, i.e. ruminate, and thus reduce their size. In rodents and other small hindgut-fermenting herbivores, it is well-known that large particles are selectively expelled from the hindgut, as they are difficult to digest and represent bulk that limits further intake.

Theoretically, therefore, forestomachs of non-ruminating animals should also rather selectively expel, not retain, larger particles. In an article to be published in the July 2004 issue of Mammal Review, Dr. Marcus Clauss collates literature data on sloths which indicates that the interplay of resting posture, digestive anatomy and ingesta characteristics could, indeed, affect a faster expulsion of large particles from the forestomach of these animals.

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