Ecological and phylogenetic variability in the spinalis muscle of snakes

Abstract

Understanding the origin and maintenance of functionally important subordinate traits is a major goal of evolutionary physiologists and ecomorphologists. Within the confines of a limbless body plan, snakes are diverse in terms of body size and ecology, but we know little about the functional traits that underlie this diversity. We used a phylogenetically diverse group of 131 snake species to examine associations between habitat use, sidewinding locomotion and constriction behaviour with the number of body vertebrae spanned by a single segment of the spinalis muscle, with total numbers of body vertebrae used as a covariate in statistical analyses. We compared models with combinations of these predictors to determine which best fit the data among all species and for the advanced snakes only (N = 114). We used both ordinary least-squares models and phylogenetic models in which the residuals were modelled as evolving by the Ornstein-Uhlenbeck process. Snakes with greater numbers of vertebrae tended to have spinalis muscles that spanned more vertebrae. Habitat effects dominated models for analyses of all species and advanced snakes only, with the spinalis length spanning more vertebrae in arboreal species and fewer vertebrae in aquatic and burrowing species. Sidewinding specialists had shorter muscle lengths than nonspecialists. The relationship between prey constriction and spinalis length was less clear. Differences among clades were also strong when considering all species, but not for advanced snakes alone. Overall, these results suggest that muscle morphology may have played a key role in the adaptive radiation of snakes.

Surprising simplicities and syntheses in limbless self-propulsion in sand

“…Surprisingly, limbless locomotion on granular media provides opportunities to gain insights into these substrates and terrestrial locomotion more broadly; a combination of factors creates a tractable system for examining fundamental concepts in locomotion, such as neuromechanical phase lags, control 'templates' (Full and Koditschek, 1999) and morphological and behavioral adaptations. First, dry granular substrates can pose substantial challenges to such locomotors, and thus may drive behavioral or morphological adaptations (Marvi et al, 2014;Tingle et al, 2017). However, unlike more complex terrestrial environments, the granular natural habitat is often homogeneous and obstacles are sparse, making these environments particularly amenable to the creation of laboratory model systems.…”

Phylogeny, character evolution and tribal classification in Crambinae and Scopariinae (Lepidoptera, Crambidae)

“…Ancestral character states were estimated with the ace function of the r package ape (Paradis et al, 2004) using ML (Pagel, 1994) for discrete characters, and with the fastAnc function of the phytools package in r (Revell, 2012) using ML and tree rerouting (Felsenstein, 1985) for continuous characters (scripts available as Files S1 and S2, respectively). These packages were used in previous publications for reconstructing character evolution in plants (Soltis et al, 2013;Landis et al, 2018;Spriggs et al, 2018) and animals (Tingle et al, 2017;Irisarri et al, 2018). Minor modifications of the source code of the 'ace' function as suggested by Emmanuel Paradis (personal communication) were done to take uncertain character states (coded '?')…”

Moving beyond the surface: Comparative head and neck myology of threadsnakes (Epictinae, Leptotyphlopidae, Serpentes), with comments on the ‘scolecophidian’ muscular system

“…spinalis capitis tends to be shorter in ‘Scolecophidia’ ( Anilios nigrescens , Afrotyphlops schelegeli and Argyrophis muelleri in Typhlopidae and Rena dulcis in Leptotyphlopidae) when compared to Alethinophidia, and such assumption was corroborated herein. Additionally, both burrowing and aquatic snake species tend to have shorter spinalis muscle-tendon portions [36], and, therefore, the strict fossorial habitat of scolecophidian might be associated to their short spinalis muscle-tendon portion.…”

“…Although variation on the extension of the spinalis muscles might be associated with the huge variation on vertebrae number in snakes, as a higher number of vertebrae is directly related to longer areas of insertion [36], species herein analysed with higher number of vertebrae did not exhibit wider areas of insertion (but see Table 5). Finally, due to the adaptive hypothesis that the evolution of habitat usage has strongly influenced the morphology of the spinalis muscle-tendon portion in snakes [36], such variation found in scolecophidians must be addressed in future studies, considering differences in habitat type and excavatorial methods.…”

“…Besides the classical descriptions of the trunk myology by Mosauer [24] and Gasc [25], very few additional studies are available, which are mostly focused on Alethinophidia (e.g., [26–31]). Descriptive and/or comparative studies of the craniovertebral myology in snakes have received even less attention in the past (e.g., [18,32–36]).…”