Functional morphology and ventilatory implications of the intracoelomic organization of three-toed sloths

Abstract

It recently has been claimed that sloths (Mammalia: Pilosa: Folivora) possess unique adhesions of their visceral organs that were considered to represent intracoelomic adaptations related to maximizing ventilatory efficiency in animals that hang vertically suspended (i.e. with their heads facing downwards) in trees. We examined the visceral anatomy in the brown-throated three-toed sloth (Bradypus variegatus) and compared our results to a number of historical anatomical references, almost all of which were published in languages other than English. In agreement with the majority of these earlier studies, we come to the conclusion that the reported adhesions represent sloth-specific modifications of ordinary mesenteric structures present in all mammals, and that sloths in fact possess no unique visceral suspension structures. However, we agree in principal with the hypothesized influence of these structures on ventilatory mechanics in inverted animals. In addition, we hypothesize that the specific arrangement of these intracoelomic interorgan connections may furthermore serve a role in passively assisting the inspiratory process in animals maintaining their body cavity vertically oriented (i.e. with their heads facing upwards). Taken together, these data suggest that (at least three-toed) sloths may indeed have evolved remarkable respiration-related adaptations of their visceral anatomy, corresponding to their overall energy-conserving lifestyle.