Weighing homoplasy against alternative scenarios with the help of macroevolutionary modeling: A case study on limb bones of fossorial sciuromorph rodents

Abstract

Homoplasy is a strong indicator of a phenotypic trait's adaptive significance when it can be linked to a similar function. We assessed homoplasy in functionally relevant scapular and femoral traits of Marmotini and Xerini, two sciuromorph rodent clades that independently acquired a fossorial lifestyle from an arboreal ancestor. We studied 125 species in the scapular dataset and 123 species in the femoral dataset. Pairwise evolutionary model comparison was used to evaluate whether homoplasy of trait optima is more likely than other plausible scenarios. The most likely trend of trait evolution among all traits was assessed via likelihood scoring of all considered models. The homoplasy hypothesis could never be confirmed as the single most likely model. Regarding likelihood scoring, scapular traits most frequently did not differ among Marmotini, Xerini, and arboreal species. For the majority of femoral traits, results indicate that Marmotini, but not Xerini, evolved away from the ancestral arboreal condition. We conclude on the basis of the scapular results that the forelimbs of fossorial and arboreal sciuromorphs share mostly similar functional demands, whereas the results on the femur indicate that the hind limb morphology is less constrained, perhaps depending on the specific fossorial habitat.

Integrative Approach Uncovers New Patterns of Ecomorphological Convergence in Slow Arboreal Xenarthrans

“…Future studies should substantiate the models of convergence proposed here by, for example, more thoroughly defining the degree of morphospace overlap using quantitative analyses of disparity or model-fitting analyses (Ingram and Mahler 2013;Mahler et al 2013;Mahler and Ingram 2014;Wölfer and Nyakatura 2019;Grossnickle et al 2020). For the latter, it is noteworthy that the pattern-based approach used here ('convevol', Stayton 2015a) is based on past phenotypic distances which, in turn, are estimated through ancient state reconstructions assuming BM.…”

Incipient morphological specializations associated with fossorial life in the skull of ground squirrels (Sciuridae, Rodentia)

“…Four other sciurids were defined as arboreal ( Ratufa , Protoxerus , Sciurus Tamiasciurus ), and one sciurid ( Tamias ) is defined as arboreal/fossorial because of its intermediate lifestyle (see Rocha et al, 2016 ; Steppan et al, 2004 ; and Mielke et al, 2018 ). These different categories of locomotor behavior also follow the nomenclature used in recent ecomorphological studies on Sciuromorpha (Mielke et al, 2018 ; Scheidt et al, 2019 ; Wölfer & Nyakatura, 2019 ; Figure 1 ). Numerous studies that focused on sciurid skull shape, especially on mandibles, have been performed on large comparative databases, which also permitted to reliably test for phylogenetic signal, dietary imprint on shape, as well as the allometric component (e.g., Calede et al, 2019 ; Casanovas‐Vilar & Van Dam, 2013 ; Lu et al, 2014 ; Michaux et al, 2008 ; Velhagen & Roth, 1997 ; Zelditch et al, 2015 , 2017 ).…”

“…It is interesting to note that these characters are only incipient in Spermophilopsis (see Casanovas‐Vilar & Van Dam, 2013 for similar observation on the mandible) showing the most elongated claws, which may improve its scratch digging activity in soft sandy soils (Ognev, 1966; Ružić, 1967), where the use of incisors is not necessary for burrowing. Wölfer and Nyakatura (2019) on femoral traits mentioned that Xerini are much closer to the arboreal condition than most Marmotini. This result might be based on diverse functional demands since that Xerini spend less time underground than most Marmotini, which can build burrows in soft to hard soils (e.g., Hoogland, 1996; Janderková et al, 2011; Ponomarenko, 2007).…”

“…These rodents use mainly their claws for digging and far less their incisors (Agrawal, 1967 ). Focusing on susliks and relatives from the genus Spermophilus , Lagaria and Youlatos ( 2006 ) said that “given that scratch digging is an integral part of their biology that may increase fitness, it is very likely that (these) ground squirrels exhibit associated morphological correlates.” This statement can be generalized to all fossorial ground squirrels since it has been demonstrated that they do have muscular, anatomical and histological adaptations for scratch‐digging revealed by investigations of both forelimbs and hindlimbs (Lagaria & Youlatos, 2006 ; Mielke et al, 2018 ; Scheidt et al, 2019 ; Thorington, Jr. et al, 1997 ; Wölfer & Nyakatura, 2019 ; Wölfer, Amson, et al, 2019 ). But, does the skull of ground squirrels similarly show fossorial specializations?…”

A macroevolutionary common-garden experiment reveals differentially evolvable bone organization levels in slow arboreal mammals

“…Addressing this phenomenon and its underlying mechanisms may be crucial to explain how intrinsic effects drive the distribution of phenotypic diversity. Mosaicism has been detected among regions of the entire skeleton 3 or traits of the same skeletal element 14 , often identifying heterogeneous convergence patterns 16 , 17 . Yet, a dimension of this process remains unexplored: does evolvability vary across different anatomical levels of bone organisation?…”