An ecomorphological analysis of forelimb musculotendinous system in sigmodontine rodents (Rodentia, Cricetidae, Sigmodontinae)

From slenderness to robustness: Understanding long bone shape in sigmodontine rodents

The morphological evolution of the appendicular skeleton may reflect the selective pressures specific to different environments, phylogenetic inheritance, or allometry. Covariation in bone shapes enhances morphological integration in response to ecological specializations. In contrast to previous multivariate studies using classical linear morphometry, we use a geometric morphometric approach to explore the morphological diversity of long bones and examine relationships between ecological categories and morphological characters in a species-rich and ecomorphologically diverse group of rodents. We examined the humerus, ulna, femur, and tibiofibula of 19 sigmodontine species with different locomotor types (ambulatory, quadrupedal-saltatorial, natatorial, semifossorial and scansorial) to investigate the influence of locomotor type and phylogeny on limb bone shape and morphological integration of the appendicular skeleton. This study represents the most detailed examination of the morphological diversity of long bones in sigmodontines, employing geometric morphometrics within an ecomorphological framework. Our results indicate that functional demands and evolutionary history jointly influence the shape of forelimb and hindlimb bones. The main variation in bone shape is associated with a slenderness-robustness gradient observed across all ecological categories. Quadrupedal-saltatorial species, with their need for agility, possess slender and elongated limbs, while natatorial and semifossorial species exhibit shorter and more robust bone shapes, suited for their respective environments. This gradient also influences bone covariation within limbs, demonstrating interconnectedness between elements. We found functional covariation between the ulna-tibiofibula and humerus-tibiofibula, likely important for propulsion, and anatomical covariation between the humerus-ulna and femur-tibiofibula, potentially reflecting overall limb structure. This study demonstrates that the versatile morphology of long bones in sigmodontines plays a critical role in their remarkable ecological and phylogenetic diversification.

Bridging macroecology and macroevolution in the radiation of sigmodontine rodents

Investigations of phenotypic disparity across geography often ignore macroevolutionary processes. As a corollary, the random null expectations to which disparity is compared and interpreted may be unrealistic. We tackle this issue by representing, in geographical space, distinct processes of phenotypic evolution underlying ecological disparity. Under divergent natural selection, assemblages in a given region should have empirical disparity higher than expected under an evolutionarily oriented null model, whereas the opposite may indicate constraints on phenotypic evolution. We gathered phylogenies, biogeographic distributions, and data on the skull morphology of sigmodontine rodents to discover which regions of the Neotropics were more influenced by divergent, neutral, or constrained phenotypic evolution. We found that regions with higher disparity than expected by the evolutionary-oriented null model, in terms of both size and shape, were concentrated in the Atlantic Forest, suggesting a larger role for divergent natural selection there. Phenotypic disparity in the rest of South America, mainly the Amazon basin, northeastern Brazil, and Southern Andes, was constrained-lower than predicted by the evolutionary model. We also demonstrated equivalence between the disparity produced by randomization-based null models and constrained-evolution null models. Therefore, including evolutionary simulations into the null modeling framework used in ecophylogenetics can strengthen inferences on the processes underlying phenotypic evolution.

Ecological correlates of the morphology of the auditory bulla in rodents: Application to the fossil record

For rodents, hearing is essential to survival. It enables predator evasion, prey detection, and conspecific recognition; it is also likely to be constrained by the physical environment. The resulting hypothetical link between tympanic bulla morphology and ecology has never been investigated across a broad array of rodent species before. Such link may enable the determination of the ecological affinities of many fossil species only known from partial skulls. In this study, we used geometric morphometrics to quantify the shape of the auditory bulla of 197 specimens representing 91 species from 17 families of extant rodents across four different locomotory modes. We used landmarks and semi-landmarks on the ventral and lateral views of the skull to capture morphological characteristics of the bulla and external auditory meatus (EAM). Our results demonstrate an association between bullar morphology and locomotion in rodents. Bullar shape enables the correct classification of 76% of the species in our training set. Fossorial taxa, in particular, show a characteristic morphology including an asymmetric bulla with a dorsally located and laterally expanded EAM that has a small opening diameter. A phylogenetically informed flexible discriminant analysis shows a weak phylogenetic effect on tympanic morphology. There is no evidence for differences in bullar hypertrophy across locomotory categories. The application of this approach to select fossil rodents from the Oligo-Miocene shows broad agreements with prior studies and yields new locomotory inferences for 14 fossil species, including the first proposed locomotion for members of the family Florentiamyidae. Such results call for the timing of burrowing diversification in rodents to be reevaluated.

Swimming behavior and performance of the marsh rat Holochilus vulpinus (Brants, 1827) (Cricetidae, Sigmodontinae)

Semiaquatic habits evolved in several Sigmodontinae rodent species, but this issue remains poorly studied. The marsh rat Holochilus vulpinus (Brants, 1827) has been associated with palustrine wetland environments and exhibits several morphological traits related to the semiaquatic habit. However, its swimming behavior and the way its morphological specializations contribute to it have not been evaluated so far in this species. This note focuses on describing and quantifying the swimming behavior and swimming performance of H. vulpinus. In order to address these issues, five individuals of H. vulpinus, four adults and one subadult, were filmed while swimming and their performance was quantified. The obtained results indicate that the swimming mode is bipedal paddling, alternating the stroke of the hind feet. The quantitative indicators also qualify H. vulpinus as a good swimmer compared to other terrestrial and semiaquatic mammals.

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

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.

Locomotory adaptations in entoptychine gophers (Rodentia: Geomyidae) and the mosaic evolution of fossoriality

Pocket gophers (family Geomyidae) are the dominant burrowing rodents in North America today. Their fossil record is also incredibly rich; in particular, entoptychine gophers, a diverse extinct subfamily of the Geomyidae, are known from countless teeth and jaws from Oligocene and Miocene-aged deposits of the western United States and Mexico. Their postcranial remains, however, are much rarer and little studied. Yet, they offer the opportunity to investigate the locomotion of fossil gophers, shed light on the evolution of fossoriality, and enable ecomorphological comparisons with contemporaneous rodents. We present herein a quantitative study of the cranial and postcranial remains of eight different species of entoptychine gophers as well as many contemporary rodent species. We find a range of burrowing capabilities within Entoptychinae, including semifossorial scratch-digging animals and fossorial taxa with cranial adaptations to burrowing. Our results suggest the repeated evolution of chisel-tooth digging across genera. Comparisons between entoptychine gophers and contemporaneous rodent taxa show little ecomorphological overlap and suggest that the succession of burrowing rodent taxa on the landscape may have had more to do with habitat partitioning than competition.

Functional Morphology and Behavioral Correlates to Postcranial Musculature

In this the second issue of a two-volume set of the Anatomical Record on the relationship between muscle functional morphology and behavior, the focus is on the postcranial musculature. Traditionally, when talking of the postcranium we think of the skeletal parts that primarily provide the lever system necessary for body movements. However, without the force produced by muscle, the postcranial skeleton could not perform these or most other tasks. In this special issue, our colleagues present ten papers that focus on postcranial muscle morphology and function from different perspectives. They include papers on forelimb and hindlimb muscle functional morphology of vertebrates, including lizards, bats, primates, a carnivoran and a rodent, and involved in different substrate use (arboreal, terrestrial, and flying) and locomotion behavior (quadrupedal, leaper, and suspensory) along with a historical overview to help bookend the contextualization of the issues. The picture that these papers provide is one of great liveliness in the field of muscle functional morphology where both young students and affirmed professors continue to contribute with both traditional approaches and new techniques to further our knowledge of muscle morphology and its relationship with animal behavior. Anat Rec, 301:419-423, 2018. © 2018 Wiley Periodicals, Inc.

Phylogenetic patterns and correlation of key structures for jumping: bone crests and cross-sectional areas of muscles in Leptodactylus (Anura, Leptodactylidae)

Anurans are characterized by their saltatory mode of locomotion, which is associated with a specific morphology. The coordinated action of the muscles and bones of the pelvic girdle is key to the transmission of the force of the hindlimbs to the axial skeleton during jumping. Two features are critical for optimal locomotory performance: the cross-sectional area of muscle and the bone crest attachment sites. The first character is a proxy of the force exerted by the muscle, whereas the crests are muscle attachments sites related to muscle force. The provisory relationship between these features has previously been identified and bone crest size can be used to infer the magnitude and, therefore, muscle force in fossils records. In this work, we explore the correlation between the cross-sectional area of essential muscles to the jumping mechanism (longissimus dorsi, extensor iliotibialis B, tenuissimus, puboischiofemoralis internus B, coccygeo-sacralis and coccygeo-iliacus) and the bone crests where these muscles are inserted (dorsal tubercle, dorsal crest and urostylar crest) in species of the genus Leptodactylus. This genus, along with other leptodactylids, exhibits a diversity of locomotor modes, including jumping, hopping, swimming and burrowing. We therefore analyzed the morphometric variation in the two features, cross-sectional area and bone crest area, expecting a correlation with different locomotor types. Our results showed: (i) a correlation between the urostylar crest and the cross-sectional area of the related muscles; (ii) that the bone crest surface area of urostyle and ilium and the cross-sectional area of the corresponding muscles can be utilized to infer locomotor faculties in leptodactylid frogs; and (iii) that the evolution of both characters demonstrates a general tendency from lower values in leptodactylid ancestors to higher values in the Leptodactylus genus. The results attest to the importance of the comparison of current ecological and phylogenetic analogues as they allow us to infer functionality and behavior in fossil and extant groups based on skeletal evidence. Phylogenetic patterns in character evolution and their correlation with locomotory types could imply that functional restrictions are also inherited in leptodactylid.

Relating appendicular skeletal variation of sigmodontine rodents to locomotion modes in a phylogenetic context

Sigmodontinae rodents constitute the second-largest subfamily among mammals. Alongside the taxonomic diversity, they are also ecologically diverse, exhibiting a wide array of locomotion modes, with semifossorial, terrestrial, semiaquatic, scansorial, arboreal, and saltatorial forms. To understand the ecomorphologic aspects that allow these rodents to display such locomotion diversity, we analyzed 35 qualitative characters of the appendicular skeleton (humerus, ulna, radius, scapula, femur, tibia, ilium, ischium and pubis) in 795 specimens belonging to 64 species, 34 genera and 10 tribes, representing all locomotion modes assigned to this subfamily. We performed a statistical analysis based upon the coefficient of trait differentiation to test the congruence of character states and the different locomotion modes. We also mapped characters states in a molecular phylogeny in order to reconstruct ancestral states and to evaluate how appendicular characters evolved within main lineages of Sigmodontinae radiation under a phylogenetic framework. The statistical analyses revealed six characters related to specific locomotion modes, except terrestrial. The mapping and parsimony ancestral states reconstruction identified two characters with phylogenetical signal and eight characters that are exclusively or more frequently recorded in certain modes of locomotion, four of them also detected by the statistical analysis. Notwithstanding the documented morphological variation, few changes characterize the transition to each of the locomotion modes, at least regarding the appendicular skeleton. This finding corroborates previous results that showed that sigmodontines exhibit an all-purpose appendicular morphology that allows them to use and explore a great variety of habitats.

Tendinous framework of anurans reveals an all-purpose morphology

Tendons are directly associated with movement, amplifying power and reducing muscular work. Taking into account habitat and locomotor challenges faced by anurans, we identify the more conspicuous superficial tendons of a neotropical anuran group and investigate their relation to the former factors. We show that tendons can be visualized as an anatomical framework connected through muscles and/or fascia, and describe the most superficial tendinous layer of the postcranium of Leptodactylus latinasus. To analyze the relation between tendon morphology and ecological characters, we test the relative length ratio of 10 tendon-muscle (t-m) elements in 45 leptodactylid species while taking phylogeny into account. We identify the evolutionary model that best explains our variables. Additionally, we optimize t-m ratio values, and the shape of the longissimus dorsi insertion onto a selected phylogeny of the species. Our data show the existence of an all-purpose morphology that seems to have evolved independently of ecology and functional requirements. This is indicated by no significant relation between morphometric data of the analyzed tendons and habitat use or locomotion, a strong phylogenetic component to most of the analyzed variables, and a generalized pattern of intermediate values for ancestral states. Ornstein-Uhlenbeck is the model that best explains most t-m variables, indicating that stabilizing selection or selective optima might be driving shifts in tendon length within Leptodactylidae. Herein, we show the substantial influence that phylogeny has on tendon morphology, demonstrating that a generalized and stable morphological configuration of tendons is adequate to enable versatile locomotor modes and habitat use. This is an attempt to present the tendinous system as a framework to body support in vertebrates, and can be considered a starting point for further ecomorphological research of this anatomical system in anurans.