Abstract
Convergent evolution is a major topic in evolutionary biology. Low bone cortical compactness (CC, a measure of porosity of cortical bone) in the extant genera of "tree sloths," has been linked to their convergent slow arboreal ecology. This proposed relationship of low CC with a slow arboreal lifestyle suggests potential convergent evolution of this trait in other slow arboreal mammals. Femoral and humeral CC were analyzed in "tree sloths," lorisids, koala, and extinct palaeopropithecids and Megaladapis, in comparison to closely related but ecologically distinct taxa, in a phylogenetic framework. Low CC in "tree sloths" is unparalleled by any analyzed clade and the high CC in extinct sloths suggests the recent convergence of low CC in "tree sloths." A tendency for low CC was found in Palaeopropithecus and Megaladapis. However, lorisids and the koala yielded unexpected CC patterns, preventing the recognition of a straightforward convergence of low CC in slow arboreal mammals. This study uncovers a complex relationship between CC and convergent evolution of slow arboreality, highlighting the multifactorial specificity of bone microstructure.
Highlights
From an ecomorphological perspective, adaptively convergent morphotypes are expected in phylogenetically distant groups occupying similar niches (Wainwright and Reilly 1994; Muschick et al 2012)
As they are often employed in microstructural analyses (e.g., Laurin 2004; Straehl et al 2013), we investigated CC in the humerus and the femur
Stochastic Character Mapping (SCM) revealed a high probability of convergent acquisition of slow arboreality in “Lorisidae,” Megaladapis, Palaeopropithecidae, Choloepus, Bradypus, and P. cinereus, referred to as slow arboreal clades hereafter
Summary
Adaptively convergent morphotypes are expected in phylogenetically distant groups occupying similar niches (Wainwright and Reilly 1994; Muschick et al 2012). In addition to “tree sloths,” we analyzed the koala (Phascolarctos cinereus, Marsupialia), “Lorisidae” (arguably representing a paraphyletic group of primates, see Methods), and subfossil Palaeopropithecidae and Megaladapis (Godfrey et al 2006, Godfrey et al 2016), with the latter three members of the primate clade Strepshirrhini They share suspensory/vertical climbing and low metabolic rates (references in Table 1) and are “slow arboreal.”. Despite the outlined potential for our understanding of convergent evolution, no comparative CC analysis was performed in a wide mammalian sample, representing slow arboreal clades and close relatives with diverging lifestyles, yet As they are often employed in microstructural analyses (e.g., Laurin 2004; Straehl et al 2013), we investigated CC in the humerus and the femur. We expected to find the convergent slow arboreal groups consistently showing lower CC in respect to close relatives
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