From Fairies to Giants: Untangling the Effect of Body Size, Phylogeny, and Ecology on Vertebral Bone Microstructure of Xenarthran Mammals

Trabecular bone is a spongy bone tissue that serves as a scaffolding-like support inside many skeletal elements. Previous research found allometric variation in some aspects of trabecular bone architecture (TBA) and bone microstructure, whereas others scale isometrically. However, most of these studies examined very wide size and phylogenetic ranges or focused exclusively on primates or lab mice. We examined the impact of body size on TBA across a smaller size range in the mammalian clade Xenarthra (sloths, armadillos, and anteaters). We µCT-scanned the last six presacral vertebrae of 23 xenarthran specimens (body mass 120 g-35 kg). We collected ten gross-morphology measurements and seven TBA metrics and analyzed them using phylogenetic and nonphylogenetic methods. Most metrics had similar allometries to previous work. However, because ecology and phylogeny align closely in Xenarthra, the phylogenetic methods likely removed some covariance due to ecology; clarifying the impact of ecology on TBA in xenarthrans requires further work. Regressions for Folivora had high P-values and low R-squared values, indicating that the extant sloth sample either is too limited to determine patterns or that the unique way sloths load their vertebral columns causes unusually high TBA variation. The southern three-banded armadillo sits far below the regression lines, which may be related to its ability to roll into a ball. Body size, phylogeny, and ecology impact xenarthran TBA, but parsing these effects is highly complex.