About

Robert Cox is a Professor of Biology at the University of Virginia. His research lies at the interface of two topics: the evolutionary dynamics associated with the divergent reproductive roles of males and females, and the ecological factors that shape natural selection on physiology, morphology, and life history. His current projects address causes and consequences of sexual conflict, the evolutionary and endocrine basis of sexual dimorphism, and the physiological and ecological mechanisms that shape life-history trade-offs. His research involves vertebrates, usually reptiles, and combines laboratory and field components, emphasizing experimental approaches and new genetic and genomic techniques. Field studies are conducted in diverse natural habitats, including deserts and mountains of Arizona and coastal islands in the Bahamas and Florida. His overarching goal is to understand how particular phenomena are regulated through physiology and genetics, and why they have evolved through ecological and evolutionary processes.

Research topics

• Genetics
• Evolutionary biology
• Biology
• Zoology
• Ecology
• Geography
• Engineering
• Gerontology
• Internal medicine
• Medicine
• Endocrinology
• Paleontology

Selected publications

• Integrating mechanistic and evolutionary views on phenotypic integration through hormonal pleiotropy

  Integrative and Comparative Biology · 2026-05-18

  article1st authorCorresponding

  Organismal biologists have studied phenotypic integration from diverse perspectives that span molecular, cellular, physiological, and developmental mechanisms up through evolutionary patterns. Nonetheless, integration across these scales is rare, such that studies of mechanism can be difficult to link to evolutionary dynamics, and vice versa. Here, we integrate mechanistic and evolutionary perspectives on phenotypic integration through hormonal pleiotropy, which occurs when one hormone influences the expression of multiple traits. We focus on trait covariances as unifying measures of phenotypic integration that can be quantified at multiple scales of biological organization and linked to evolutionary dynamics through associated measures of genetic covariance. Drawing from our work on steroid hormones in Anolis and Sceloporus lizards, we illustrate experimental approaches for quantifying the hormonal regulation of phenotypic integration at both organismal and transcriptomic scales. By combining these experimental approaches with quantitative genetic theory and breeding designs, we illustrate how hormonal pleiotropy structures not only phenotypic integration, but also the underlying genetic variances and covariances that shape evolutionary responses to selection. We also explore how estimates of genetic variance and covariance in phenotypic responsiveness to a hormonal signal can reveal constraints imposed by hormonal pleiotropy that may shape the evolution of integrated phenotypes. Finally, we review recent work showing that responsiveness to testosterone can evolve at the transcriptomic level to produce divergent patterns of hormonal pleiotropy and phenotypic integration in closely related species.

  PublisherDOI

• Correction to: The landscape of natural selection during early population establishment in an invasive lizard

  Evolution · 2025-11-20

  erratum

  International audience

  PublisherDOI

• DNA methylation–based age prediction and sex-specific epigenetic aging in a lizard with female-biased longevity

  Science Advances · 2025-01-31 · 6 citations

  articleOpen access

  Sex differences in life span are widespread across animal taxa, but their causes remain unresolved. Alterations to the epigenome are hypothesized to contribute to vertebrate aging, and DNA methylation–based aging clocks allow for quantitative estimation of biological aging trajectories. Here, we investigate the influence of age, sex, and their interaction on genome-wide DNA methylation patterns in the brown anole ( Anolis sagrei ), a lizard with pronounced female-biased survival and longevity. We develop a series of age predictor models and find that, contrary to our predictions, rates of epigenetic aging were not slower in female lizards. However, methylation states at loci acquiring age-associated changes appear to be more “youthful” in young females, suggesting that female DNA methylomes are preemptively fortified in early life in opposition to the direction of age-related drift. Collectively, our findings provide insights into epigenetic aging in reptiles and suggest that early-life epigenetic profiles may be more informative than rates of change for predicting sex biases in longevity.

  PublisherDOI

• The landscape of natural selection during early population establishment in an invasive lizard

  Evolution · 2025-09-13

  article

  Populations during early stages of establishment are sensitive to forms of demographic regulation coinciding with rapid growth, which may also coincide with specific patterns of natural selection due to demographic variation. Understanding how selection varies during the establishment of new populations, however, is complicated by the constraint of knowing the precise age of a population as it grows over time. To address this, we established six brown anole (Anolis sagrei) populations on spoil islands in Florida and manipulated initial sex ratios to understand how natural selection is influenced by the demographic composition of founding populations. We found that initial sex ratios of founding populations led to age-specific patterns of natural selection. Juveniles experienced stronger selection in populations that began with a female-biased sex ratio, and the strength of natural selection on juvenile size strengthened with increasing population density. We also found substantial variation in selection, suggesting that the relationship between phenotypes and fitness across early generations of a population is not consistent over time. As a result, variation in natural selection driven by demographic aspects within populations may provide opportunities for rapid population growth and novel evolutionary trajectories during the earliest stages of establishment.

  PublisherDOI

• Widespread male-female expression imbalance of X-linked genes across phrynosomatid lizards

  bioRxiv (Cold Spring Harbor Laboratory) · 2025-10-07

  preprintOpen accessSenior authorCorresponding

  Abstract Classic theory on sex chromosome evolution predicts that selection should restore ancestral diploid expression for hemizygous X-linked genes in males. However, this dosage compensation is often incomplete, leaving X enriched for genes with female-biased expression. In this context, iguanian lizards are noteworthy among vertebrates because several species from separate families appear to exhibit both near-complete dosage compensation and male-female expression balance across their ancient, homologous X chromosomes. We tested for this pattern in a phrynosomatid iguanian, Sceloporus undulatus (Eastern Fence Lizard), and instead found that both ancient and more recently sex-linked regions of the X chromosome are enriched for genes with female-biased expression, regardless of age (neonate, maturing, adult) or tissue (brain, liver, muscle). By expanding our analysis across 10 phrynosomatid species spanning 4 genera, we found that male-female expression imbalance on the ancestral region of X is phylogenetically conserved. We also found that an inferred chromosomal rearrangement in the S. jarrovii lineage has resulted in the novel acquisition of female-biased expression by a formerly autosomal region. Whereas sex-biased expression of the ancestral region of X is primarily due to females overexpressing X-linked genes relative to autosomal genes, sex-biased expression of these formerly autosomal genes in S. jarrovii is primarily due to males underexpressing this putative neo-X region. We conclude that male-female expression imbalance on X is widespread across phrynosomatids, potentially reflecting both overexpression in females for ancestral regions that have evolved dosage compensation and underexpression in males for neo-X regions in which dosage compensation has yet to evolve.

  PublisherOA PDFDOI

• Sex-specific consequences of juvenile dispersal for survival and reproduction in an island lizard

  Behavioral Ecology · 2025-09-01 · 2 citations

  articleSenior author

  Abstract Dispersal is a costly, though potentially rewarding, behavior with important fitness consequences for juveniles. When the costs or benefits of juvenile dispersal differ between sexes, sex-biased dispersal should be favored, though such sex-specific consequences are rarely measured for multiple components of lifetime fitness in the wild. Here, we use detailed mark–recapture data from 4 annual cohorts of juveniles with associated estimates of juvenile survival and adult reproductive success from genetic parentage to measure natural selection on 2 dispersal phenotypes (binary dispersal propensity and continuous dispersal distance) in an island population of lizards (Anolis sagrei). Juvenile dispersal was consistently male-biased, with males exhibiting a higher propensity to disperse and dispersing twice as far as females. Males that dispersed had higher survival to adulthood and total fitness than males that remained philopatric, whereas fitness components did not differ between dispersing and philopatric females. Although this result indicates that the fitness benefits of dispersal are sex-specific, we found no difference in fitness components between dispersing and philopatric males after accounting for body size, which was positively correlated with dispersal. Likewise, we did not consistently detect direct selection on dispersal distance in either sex when including body size in multivariate selection analyses, nor did we find consistent correlational selection on combinations of body size and dispersal distance in either sex. We conclude that selection on juvenile dispersal propensity is sex-specific, but likely indirect and mediated through its positive association with body size in males.

  PublisherDOI

• Parental age effects on offspring fitness in a wild population of a short-lived reptile

  Journal of Evolutionary Biology · 2025-10-17

  articleSenior author

  As organisms age, the fitness of the offspring they produce can decline, which is often attributed to parental senescence. However, few studies have tested for effects of parental age on offspring fitness in wild populations or in short-lived vertebrates, and only recently have studies begun to examine such effects in male and female offspring independently. Here, we use five generations of mark-recapture and genetic parentage data from an island population of a short-lived lizard, the brown anole (Anolis sagrei), to test for effects of maternal and paternal age on the survival to adulthood, first-year reproductive success, longevity, and lifetime fitness of their offspring. When comparing parents of different ages within the same offspring cohort, survival to adulthood increased with paternal age in sons, but we found no effects of maternal or paternal age on any component of fitness in daughters and no evidence that parental age effects differed based on the sex of the parent or the offspring. When considering repeated measures of individual parents sampled at multiple ages, we found that first-year reproductive success of sons decreased with paternal age, but longevity of sons increased with maternal age. However, when pooling sons and daughters, we found no overall effects of parental age on any component of offspring fitness, and little evidence that parental age effects differed between sons and daughters. Our study adds to the growing literature suggesting that negative effects of parental age on offspring fitness may not be as prevalent as once thought, particularly in wild populations.

  PublisherDOI

• Evolutionary Loss of Male-Specific Coloration Is Associated with the Loss of Androgen Receptor Expression in Skin of <i>Sceloporus</i> Lizards

  Ecological and Evolutionary Physiology · 2024-08-15 · 2 citations

  articleSenior author

  Hormones can induce trait development in one species yet have no effect on the same trait in a closely related species, but the mechanisms underlying these differences are unclear. Here, we compare two closely related lizard species to explore the cellular mechanisms associated with the evolutionary loss of hormonally mediated ventral coloration. The eastern fence lizard (Sceloporus undulatus) has sexually dimorphic blue and black ventral coloration that develops when maturational increases in androgens induce melanin synthesis in males. The closely related striped plateau lizard (Sceloporus virgatus) has sexually monomorphic white ventral skin that does not produce melanin in response to the same signal. We used immunohistochemistry to localize the androgen receptor (AR) in the skin of both species and to test whether the loss of ventral coloration in S. virgatus corresponds to the loss of AR in the skin. We found that the ventral skin of S. virgatus displays little or no AR staining in the pigment cell layer, potentially explaining the loss of androgen sensitivity in this tissue, relative to the robust AR staining in the same layer of S. undulatus. Based on the location of three markers for melanophores (microphthalmia-associated transcription factor, dopachrome tautomerase, and tyrosinase), AR appears to be present in melanophores in S. undulatus. However, we could not detect these melanophore markers in the skin of S. virgatus. Therefore, the evolutionary loss of ventral coloration may have occurred via the loss of the AR-producing melanophore in mature ventral skin, preventing the development of a male-typical trait and sexual dimorphism in this tissue.

  PublisherDOI

• Effects of Testosterone on Gene Expression Are Concordant between Sexes but Divergent across Species of <i>Sceloporus</i> Lizards

  The American Naturalist · 2024-07-15 · 3 citations

  articleSenior author

  Hormones mediate sexual dimorphism by regulating sex-specific patterns of gene expression, but it is unclear how much of this regulation involves sex-specific hormone levels versus sex-specific transcriptomic responses to the same hormonal signal. Moreover, transcriptomic responses to hormones can evolve, but the extent to which hormonal pleiotropy in gene regulation is conserved across closely related species is not well understood. We addressed these issues by elevating testosterone levels in juvenile females and males of three Sceloporus lizard species before sexual divergence in circulating testosterone and then characterizing transcriptomic responses in the liver. In each species, more genes were responsive to testosterone in males than in females, suggesting that early developmental processes prime sex-specific transcriptomic responses to testosterone later in life. However, overall transcriptomic responses to testosterone were concordant between sexes, with no genes exhibiting sex-by-treatment interactions. By contrast, hundreds of genes exhibited species-by-treatment interactions, particularly when comparing distantly related species with different patterns of sexual dimorphism, suggesting evolutionary lability in gene regulation by testosterone. Collectively, our results indicate that early organizational effects may lead to sex-specific differences in the magnitude, but not the direction, of transcriptomic responses to testosterone and that the hormone-genome interface accrues regulatory changes over evolutionary time.

  PublisherDOI

• Sexual Dimorphism

  Elsevier eBooks · 2024-01-01

  book-chapterSenior authorCorresponding
  PublisherDOI

Recent grants

• DISSERTATION RESEARCH: Male fitness in a single cell: The evolutionary significance of male reproductive cell morphology

  NSF · $20k · 2015–2017

• MEETING: Evolutionary Endocrinology: Hormones as Mediators of Evolutionary Phenomena (SICB Symposium, January 5, 2016 in Portland, OR)

  NSF · $15k · 2015–2016

• Collaborative Research: Evolutionary reversals in hormonal modulation of growth-regulatory gene networks

  NSF · $653k · 2018–2023

• CAREER: Two phenotypes, one genome: genomic conflict as a research framework for professional development of science educators

  NSF · $900k · 2015–2020

Frequent coauthors

• Henry B. John‐Alder

  Rutgers, The State University of New Jersey

  45 shared

• Ryan Calsbeek

  Dartmouth College

  31 shared

• Christian L. Cox

  Florida International University

  29 shared

• Aaron M. Reedy

  Purdue University System

  23 shared

• Gregory J. Haenel

  Elon University

  16 shared

• Linda C. Smith

  16 shared

• Ariel F. Kahrl

  Stockholm University

  16 shared

• Albert K. Chung

  University of Hong Kong

  15 shared