The three-toed sloth shows surprising diversity in the structure of its semicircular canals — part of the inner ear responsible for detecting motion. The sloths' lethargy and lack of agility might have reduced the need for sensitivity to motion, leading to relaxed selection pressure on this apparatus.
Semicircular-canal morphology is usually remarkably stable between members of the same species in placental mammals. Guillaume Billet at the University of Bonn, Germany, and his colleagues used high-resolution computed tomography to compare the semicircular canals of the three-toed sloth (Bradypus variegatus) with those of faster-moving mammals. The structures showed greater variation in shape, size and orientation in sloths than in squirrels, moles or armadillos.
Proc. R. Soc. B http://dx.doi.org/10.1098/rspb.2012.1212 (2012)
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Sloth inner-ear diversity. Nature 488, 256–257 (2012). https://doi.org/10.1038/488256d