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Dr. Sarah Chen
Stanford · NLP & interpretability
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I am a Principal Research Scientist & Affiliate Curator at the University of Washington and an Assistant Project Scientist at the University of California, Berkeley. My research examines the processes that contribute to the phenotypic variation found across macroevolutionary time scales as well as between individuals of a population.
bioRxiv (Cold Spring Harbor Laboratory) · 2026-05-12
articleOpen accessSenior authorCorresponding
Abstract The diversity of body shapes is one of the most prominent features of phenotypic variation in mammals. Yet, mammalian body shapes are poorly quantified and the underlying components contributing to its diversity as well as its relationship to other components of the skeleton are rarely tested. Here, we use lagomorphs (hares, rabbits and pikas) as a model system to (1) investigate which components of the skeleton contributed the most to body shape diversity, (2) examine the relationships between body shape and relative limb lengths, and (3) test how body size, ecotype, burrowing behavior, and locomotor mode influenced variation in lagomorph body shape and appendicular morphology. We quantified the body shape and functional proxies of the appendicular skeleton in 40 lagomorph species from osteological specimens held at museum collections. Using phylogenetic comparative methods, we found the relative length of the ribs and elongation or shortening of the thoracic and lumbar regions contributed the most to body shape evolution across lagomorphs. Second, we found that only leporids (hares and rabbits) exhibited a significant relationship between limb length and body shape, where more elongate species exhibit relatively shorter forelimbs and hindlimbs. Lastly, we found that models incorporating body size were the best predictors of lagomorph body shape and the majority of the appendicular traits, whereas models incorporating burrowing behavior and locomotor mode were largely poor fits. Broadly, these results indicate that larger lagomorphs tend to exhibit more robust body shapes with longer, more gracile forelimbs, whereas smaller lagomorphs tend to exhibit more elongate body shapes with shorter, more robust forelimbs. Overall, this work contributes to the growing understanding of mammalian body shape evolution and demonstrates the importance of not omitting body size in ecomorphological analyses.
Abstract Weasels are among the most widespread small carnivores, playing a key role in extant Holarctic ecosystems by preying on other small vertebrates. Despite their substantial diversification during the Pleistocene, the evolutionary origins of these species before this period remain poorly understood. The most widely accepted view is that the oldest known true weasel, Mustela , dates from the Pliocene (4–3.5 Ma) of central Europe. A newly discovered species of a weasel‐like mustelid, Galanthis baskini gen. et. sp. nov. from Las Casiones (Spain), demonstrates a Late Miocene (6.56–6.26 Ma) origin for Mustelinae, which is consistent with molecular divergence estimates for Mustelinae. Galanthis baskini emerges as the most basal member of the subfamily, confirming a small size (134.55 g) from the beginning of its evolutionary history. These findings shed light on the early evolution of Mustelinae and show that small forms already existed in the Late Miocene. In addition, Proputorius minimum , a related mustelid from China, is reclassified as Zdanskyictis gen. nov., which is closely related to the subfamilies Lutrinae and Ictonychinae. Galanthis and Zdanskyictis highlighting the key role of Eurasia in the evolution of the Mustelidae during the Miocene.
American Journal of Biological Anthropology · 2026-03-31
article
OBJECTIVES: A challenge for linking evolution and development is to find proxies for developmental processes that can be studied from the types of material preserved in the fossil record (i.e., teeth). The inhibitory cascade, an activator-inhibitor developmental model said to explain variation in mammalian molar proportions, provides insight to such processes. However, past studies have generally failed to make a connection between the inhibitory cascade and diet, particularly among primates. MATERIALS AND METHODS: We examined this model and molar proportions in a sample of living (n = 52) and extinct (n = 6) lorisoid primates that vary in the proportions of exudates (e.g., gums, nectar) they consume. We use the reduced major axis regression to test for the ICM. RESULTS: Taxa that consume a large proportion of exudates are characterized by a distinctive pattern of molar proportions (molars decreasing in size moving distally) from those that do not (molars increasing in size moving distally). Among fossil taxa, galagids are reconstructed as intensive or moderate exudativores; the fossil lorisid, Nycticeboides simpsoni, is inferred to have been non-exudativorous. The oldest toothcomb-bearing primate, Karanisia clarki, had molar proportions of a non-exudativore. DISCUSSION: Our results demonstrate that we can explain patterns of phenotypic variation, as they relate to diet, using a developmental framework, thereby providing a link between genotype and phenotype. Furthermore, our results weaken the case for a connection between the evolution of the toothcomb and exudativory as it is unlikely that the earliest strepsirrhines (e.g., Karanisia) were exudativorous.
The transition from milk to solid food requires drastic changes in the morphology of the feeding apparatus and its performance. As durophagous mammals, southern sea otters exhibit significant ontogenetic changes in cranial and mandibular morphology to presumably enable them to feed on a variety of hard-shelled invertebrate prey. Juvenile sea otters begin feeding independently by 6-8 months of age, but how quickly they reach sufficient maturity in biting performance remains unknown. Here, I found that the theoretical bite force of southern sea otters does not reach full maturation until during the adult stage at 3.6 and 5.0 years of age in females and males, respectively. The slow maturation of biting performance can be directly attributed to the slow growth and development of the cranium and the primary jaw adductor muscle (i.e., the temporalis) and may ultimately impact the survival of newly weaned juveniles by limiting their ability to process certain hard-shelled prey. Alternative foraging behaviors such as tool use, however, may mitigate the disadvantages of delayed maturation of biting performance. In analyses of sexual dimorphism, I found that female otters reached bite force maturation earlier, whereas male otters exhibit initial rapid growth in bite force-to quickly reach sufficient biting performance needed to process prey early in life-followed by a slower growth phase toward bite force maturation that coincides with sexual maturity. This biphasic growth in bite force suggests that male-to-male competition for resources and mates exhibits strong selection in the growth and development of skull form and function in male otters. Overall, this study demonstrates how the analysis of anatomical data can provide insight on the foraging ecologies and life histories of sea otters across ontogeny.
Analyses of form-function relationships are widely used to understand links between morphology, ecology, and adaptation across macroevolutionary scales. However, few have investigated functional trade-offs and covariance within and between the skull, limbs, and vertebral column simultaneously. In this study, we investigated the adaptive landscape of skeletal form and function in carnivorans to test how functional trade-offs among these skeletal regions contribute to ecological adaptations and the topology of the landscape. We found that morphological proxies of function derived from carnivoran skeletal regions exhibit trade-offs and covariation across their performance surfaces, particularly in the appendicular and axial skeletons. These functional trade-offs and covariation correspond as adaptations to different adaptive landscapes when optimized by various factors including phylogeny, dietary ecology, and, in particular, locomotor mode. Lastly, we found that the topologies of the optimized adaptive landscapes and underlying performance surfaces are largely characterized as a single gradual gradient rather than as rugged, multipeak landscapes with distinct zones. Our results suggest that carnivorans may already occupy a broad adaptive zone as part of a larger mammalian adaptive landscape that masks the form and function relationships of skeletal traits.
Proceedings of the Royal Society B Biological Sciences · 2025-12-17
article1st authorCorresponding
Ecological opportunities arising from climatic change are hypothesized to promote phenotypic diversification. While neontological analyses are often used to test this hypothesis, extant data only capture time-averaged signals of surviving lineages. More nuanced tests require paired and longitudinal climatic and organismal data. Here, we developed the most comprehensive phenomic dataset to date of pan-carnivorans to test hypotheses that Cenozoic climatic change influenced the evolution of the cranial, appendicular and axial skeleton. We found support for the hypothesis that a hierarchical progression of ecological diversification across the Cenozoic significantly influenced the establishment of modern carnivorans. Specifically, extinctions during the Eocene-Oligocene Transition released crown carnivorans from a constrained adaptive zone to interfamilial skeletal diversification. Intrafamilial skeletal diversification did not occur for another 20 million years until after the Mid-Miocene Climate Transition. Our work demonstrates the essential role of macroevolutionary data from the fossil record for revealing how major global climatic events steered the evolutionary trajectories of modern skeletal phenomes.
Annual Review of Ecology Evolution and Systematics · 2025-11-05 · 3 citations
articleOpen accessSenior author
Bats are one of the most widespread, species-rich, ecologically and morphologically diverse mammal lineages, and the only mammals capable of powered flight. Due to their evolution within the constraints of a mammalian template, bats represent an important and unique system in which to investigate the factors and mechanisms associated with mammalian phenotypic evolution and diversification. Here we review key traits and functions associated with bat diversification: powered flight, specialized senses, diverse diets, and exceptional longevity and immunity. We also highlight the roles of bats in driving the evolution of other species and their importance to global ecosystems. We integrate information across the fields of paleontology, developmental biology, functional morphology, biomechanics, genomics, physiology, and ecology to piece together the complex processes underlying bat evolution and diversification.
The diversity of vertebrate skeletons is often attributed to adaptations to distinct ecological factors such as diet, locomotion, and sensory environment. Although the adaptive evolution of skull, appendicular skeleton, and vertebral column is well studied in vertebrates, comprehensive investigations of all skeletal components simultaneously are rarely performed. Consequently, we know little of how modes of evolution differ among skeletal components. Here, we tested if ecological and phylogenetic effects led to distinct modes of evolution among the cranial, appendicular and vertebral regions in extant carnivoran skeletons. Using multivariate evolutionary models, we found mosaic evolution in which only the mandible, hindlimb and posterior (i.e. last thoracic and lumbar) vertebrae showed evidence of adaptation towards ecological regimes whereas the remaining skeletal components reflect clade-specific evolutionary shifts. We hypothesize that the decoupled evolution of individual skeletal components may have led to the origination of distinct adaptive zones and morphologies among extant carnivoran families that reflect phylogenetic hierarchies. Overall, our work highlights the importance of examining multiple skeletal components simultaneously in ecomorphological analyses. Ongoing work integrating the fossil and paleoenvironmental record will further clarify deep-time drivers that govern carnivoran diversity we see today and reveal the complexity of evolutionary processes in multicomponent systems.
Abstract The timing and nature of evolutionary shifts in the relative brain size of Primates have been extensively studied. Less is known, however, about the scaling of the brain-to-body size in their closest living relatives, i.e., among other members of Euarchontoglires (Dermoptera, Scandentia, Lagomorpha, Rodentia). Ordinary least squares (OLS), reduced major axis (RMA), and phylogenetic generalized least squares (PGLS) regressions were fitted to the largest euarchontogliran data set of brain and body mass, comprising 715 species. Contrary to previous inferences, lagomorph brain sizes (PGLS slope = 0.465; OLS slope = 0.593) scale relative to body mass similarly to rodents (PGLS = 0.526; OLS = 0.638), and differently than primates (PGLS = 0.607; OLS = 0.794). There is a shift in the pattern of the scaling of the brain in Primates, with Strepsirrhini occupying an intermediate stage similar to Scandentia but different from Rodentia and Lagomorpha, while Haplorhini differ from all other groups in the OLS and RMA analyses. The unique brain–body scaling relationship of Primates among Euarchontoglires illustrates the need for clade-specific metrics for relative brain size (i.e., encephalization quotients; EQs) for more restricted taxonomic entities than Mammalia. We created clade-specific regular and phylogenetically adjusted EQ equations at superordinal, ordinal, and subordinal levels. When using fossils as test cases, our results show that generalized mammalian equations underestimate the encephalization of the stem lagomorph Megalagus turgidus in the context of lagomorphs, overestimate the encephalization of the stem primate Microsyops annectens and the early euprimate Necrolemur antiquus, but provide similar EQ values as our new strepsirrhine-specific EQ when applied to the early euprimate Adapis parisiensis.
