Insectivory leads to functional convergence in a group of Neotropical rodents

Mandibular characteristics of early Glires (Mammalia) reveal mixed rodent and lagomorph morphotypes

Glires (rodents, lagomorphs and their fossil kin) is the most speciose and arguably most diversified clade of living placentals. Different lineages within the Glires evolved basically opposite chewing movements: a mostly transversal power stroke in lagomorphs, and a mostly proal power stroke in rodents, but the ancestral condition for Glires is still unclear. To address this knowledge gap, we studied the mandibles of Chinese Palaeocene Glires representing the duplicidentate (lagomorph-like; Mimotona) and simplicidentate (rodent-like; Eomylus and Heomys) lineages. To assess the mechanical resistance of mandibles to bending and torsion, we calculated the section modulus. The dentaries differ greatly in morphology and the region where the maximum grinding force was likely applied. The early Palaeocene Mimotona lii and the middle Palaeocene Mimotona robusta and Heomys orientalis all show a pattern of increasing strength moving posteriorly along the mandible, similar to sciurids and the mountain beaver. By contrast, the late Palaeocene Eomylus sp. mandible was strongest in the m1 region, a pattern seen in lagomorphs and the stem placental Zofialestes. Our results indicate the early diversification of mandible structure of Glires, demonstrate a mixture of duplicidentate and simplicidentate characters among the basal Glires and suggest an early occurrence of a lagomorph-like morphotype. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

An ecomorphological approach to the relationship between craniomandibular morphology and diet in sigmodontine rodents from central-eastern Argentina

The key role of the skull in food intake and processing implicates its morphology should be to some extent adapted to the functional demands present in different diets, while also showing similarities between those which are closely related. Sigmodontine rodents, with a generalist body plan and broad dietary habits, are an interesting case study to explore these relationships. We used linear morphometrics to assess craniomandibular morphology, and explored its relationship with dietary composition and phylogeny in a sample of sigmodontines from central-eastern Argentina, representative of this subfamily's morphological and ecological diversity. We took 26 measurements performed on 558 specimens belonging to 22 species, and resorted to bibliographic information for proportion of food items in their diets, dietary categories, and phylogeny. Multivariate statistical analyses revealed a strong evolutionary integration between morphological traits of crania and mandibles, and a conspicuous relationship between them and dietary composition in our study group, independent of phylogeny. Species of larger sizes exhibited more robust skulls and a tendency towards folivorous diets, whereas smaller species had more gracile craniomandibular apparatuses and diets richer in seeds and invertebrates. Additionally, we used the observed patterns to made predictions of dietary categories for the three species of this region with unknown diets, completing the map of feeding ecology of one of the most researched group of sigmodontines and enabling future studies to further explore this topic. The present work contributes to understanding the link between morphology, ecology and phylogeny in small mammals.

Functional constraints channel mandible shape ontogenies in rodents

In mammals, postnatal growth plays an essential role in the acquisition of the adult shape. During this period, the mandible undergoes many changing functional constraints, leading to spatialization of bone formation and remodelling to accommodate various dietary and behavioural changes. The interactions between the bone, muscles and teeth drive this developmental plasticity, which, in turn, could lead to convergences in the developmental processes constraining the directionality of ontogenies, their evolution and thus the adult shape variation. To test the importance of the interactions between tissues in shaping the ontogenetic trajectories, we compared the mandible shape at five postnatal stages on three rodents: the house mouse, the Mongolian gerbil and the golden hamster, using geometric morphometrics. After an early shape differentiation, by both longer gestation and allometric scaling in gerbils or early divergence of postnatal ontogeny in hamsters in comparison with the mouse, the ontogenetic trajectories appear more similar around weaning. The changes in muscle load associated with new food processing and new behaviours at weaning seem to impose similar physical constraints on the mandible, driving the convergences of the ontogeny at that stage despite an early anatomical differentiation. Nonetheless, mice present a rather different timing compared with gerbils or hamsters.

Functional Trade-offs Asymmetrically Promote Phenotypic Evolution

Trade-offs are thought to bias evolution and are core features of many anatomical systems. Therefore, trade-offs may have far-reaching macroevolutionary consequences, including patterns of morphological, functional, and ecological diversity. Jaws, like many complex anatomical systems, are comprised of elements involved in biomechanical trade-offs. We test the impact of a core mechanical trade-off, transmission of velocity versus force (i.e., mechanical advantage), on rates of jaw evolution in Neotropical cichlids. Across 130 species representing a wide array of feeding ecologies, we find that the velocity-force trade-off impacts evolution of the surrounding jaw system. Specifically, rates of jaw evolution are faster at functional extremes than in more functionally intermediate or unspecialized jaws. Yet, surprisingly, the effect on jaw evolution is uneven across the extremes of the velocity-force continuum. Rates of jaw evolution are 4 to 10-fold faster in velocity-modified jaws, whereas force-modified jaws are 7 to 18-fold faster, compared to unspecialized jaws, depending on the extent of specialization. Further, we find that a more extreme mechanical trade-off resulted in faster rates of jaw evolution. The velocity-force trade-off reflects a gradient from specialization on capture-intensive (e.g., evasive or buried) to processing-intensive prey (e.g., attached or shelled), respectively. The velocity extreme of the trade-off is characterized by large magnitudes of trait change leading to functionally divergent specialists and ecological stasis. By contrast, the force extreme of the trade-off is characterized by enhanced ecological lability made possible by phenotypes more readily co-opted for different feeding ecologies. This asymmetry of macroevolutionary outcomes along each extreme is likely the result of an enhanced utility of the pharyngeal jaw system as force-modified oral jaws are adapted for prey that require intensive processing (e.g., algae, detritus, and molluscs). The velocity-force trade-off, a fundamental feature of many anatomical systems, promotes rapid phenotypic evolution of the surrounding jaw system in a canonical continental adaptive radiation. Considering that the velocity-force trade-off is an inherent feature of all jaw systems that involve a lower element that rotates at a joint, spanning the vast majority of vertebrates, our results may be widely applicable across the tree of life. [adaptive radiation; constraint; decoupling; jaws; macroevolution; specialization].

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.

Niche partitioning in small mammals: interspecific and biome-level analyses using stable isotopes

Small mammal assemblages from South America provide a unique opportunity to measure coexistence and niche partitioning between marsupials and placentals. We tested how these two major clades partition environmental resources by comparing stable isotopic ratios of similar sized Didelphidae and Sigmodontinae in four Brazilian biomes: Pampas grassland, Pantanal wetland, Cerrado woodland savanna, and Atlantic Forest. Generally, didelphid isotopic niche follows a scaling law, because we found an association between δ15N enrichment and body mass. Sigmodontines that primarily partition the environment via forest strata showed a greater intake of C4 or/and crassulacean acid metabolism (CAM) plants than didelphids, as reflected by their wider trophic niche. Values of δ13C were highest in savannas and grasslands (Cerrado and Pampas biomes), and values of δ15N were highest in the Atlantic Forest (in sigmodontines) and Pampas (in didelphids). While assessing patterns between the two major Brazilian biomes (Atlantic Forest and Cerrado), we found evidence of a broader trophic niche for both clades in the Cerrado. In the Atlantic Forest, niche occupation by Didelphidae was completely enclosed within the Sigmodontinae trophic niche. Both clades showed less overlap in the Cerrado, a less productive environment. Our results highlight the importance of a comparative framework and the use of stable isotopes for testing ecological questions related to how small mammalian communities partition their niche.