Evolution, Medicine, and Public Health

Early environmental conditions can have long-lasting effects on survival and reproductive fitness. Using a 23 year-long monitoring data set of captive mouse lemur's life history traits, we tested a relationship between maternal allocation to offspring (N = 1365) and their adulthood survival. Maternal characteristics such as age or body condition did not affect allocation to offspring whatever the litter size (1 to 3). Although birth mass depended on size and composition of the litters, mouse lemur survival was highly correlated with body mass acquired after weaning in both sexes, through potential sibling competition. Moreover, female's reproductive success correlated with this body mass and was consistently associated with increased longevity. These findings suggest the presence of a silver spoon effect under constant captive conditions. However, cumulative effects of genetic and adulthood social conditions strongly interact to affect adult survival. Deaths related to intra- or inter-sexes aggressive social interactions may outweigh effect of early environment and therefore minimize the silver-spoon effect on captive mouse lemur's survival. However, having a high body mass after weaning appeared to be a determinant factor in individual survival for mouse lemurs.

Cancer progression often requires the accumulation of multiple driver mutations, yet the same set of drivers may be acquired in different orders. How these alternative mutation-order pathways jointly shape tumor clonal structure remains unclear. We develop a multitype branching-process model in which malignant transformation requires two driver mutations and distinguish malignant cells both by their mutation order and by the independent transformation event that founded their clone. Under a successive exponential approximation, we establish point-process limits for the pathway-specific clone-size processes and derive a closed-form expression for the limiting expected Simpson's index of the combined malignant population. When the two mutation orders produce malignant cells with the same net growth rate, the limiting index separates into effective pathway weights, determined by mutation rates and birth-death dynamics at preceding stages, and within-pathway concentration terms, determined by intermediate-to-malignant growth-rate ratios. This decomposition shows that a driver's effect on heterogeneity depends critically on when it is acquired. A strong driver acquired early expands the intermediate lineage and increases the supply of independent malignant founders, whereas the same driver acquired at the final transition strengthens the growth and age advantage of early-founded malignant clones. Under additive fitness effects, these counteracting mechanisms can produce a non-monotone relationship between selective advantage and clonal concentration. We further show that threshold-like non-additive fitness effects can generate highly concentrated malignant populations, while order-dependent terminal fitness leads the faster-growing pathway to dominate asymptotically. Together, these results reveal how mutation order, mutational accessibility, selection, and epistasis jointly determine lineage-level intratumor heterogeneity.

Infected individuals drive infectious disease transmission, yet empirical data on social interactions during acute infection remain limited. We conducted a prospective longitudinal study of 1,000 medically attended acute gastroenteritis and acute respiratory infection cases and 709 household members in the northwestern United States from 2024 to 2025 to quantify illness-associated changes in social contacts and their implications for transmission modeling. Cases substantially reduced social contacts during peak illness; weighted mean number of contacts increased 2.0-fold (95% confidence interval, 1.8-2.2) over the two-week follow-up. In transmission modeling, the simulated outbreak trajectory and estimated effectiveness of interventions differed substantially between models that did and did not account for temporal reductions in social contacts during acute infection. These findings show that behavioral responses during acute infection could substantially influence transmission dynamics and intervention evaluation, and provide an empirical framework for incorporating illness-associated behavioral change into infectious disease models.

Torpor is a physiological strategy used by which some species conserve energy by lowering metabolic rates and body temperatures. While mammalian hibernation is well-studied, the molecular mechanisms governing the rapid entry into avian torpor remain poorly understood. This study represents the first multi-tissue whole-transcriptomic analysis of torpor entry in an avian system. By combining real-time thermal imaging and respirometry with transcriptomic profiling across seven tissues, we identified the key molecular pathways orchestrating this transition in Anna's hummingbirds (Calypte anna). Our results reveal that avian torpor entry is characterised by a subtle transcriptomic change (<5% of the genome differentially expressed across tissues) compared to mammalian hibernation (10%-60%). Most of these genes are differentially expressed before torpor entry is complete, rather than tracking body temperature linearly. This indicates that active transcriptional reprogramming precedes and potentially drives the transition. Thus, hummingbird torpor entry is not a passive shutdown, but a highly regulated, tissue-specific process. Vital organs, such as the heart and lungs, remain transcriptionally stable to maintain essential functions, while the liver and gut exhibit extensive metabolic rewiring. We found evidence of global transcriptional and translational suppression, and cell-cycle arrest: the three major contributors to cellular energy budgets. We hypothesise that avian torpor entry is orchestrated through an arrest of apoptosis, a metabolic switch from carbohydrate to lipid utilisation, coupled with alterations in alternative splicing and circadian rhythm regulation. Finally, the counterintuitive upregulation of genes involved in mitochondrial metabolism suggests a preparation for rapid arousal despite the depressed metabolic state.

Primates maintain cooperative relationships largely through grooming reciprocity, yet studies disagree on the timescales over which grooming is returned. Part of this disagreement stems from statistical issues: inconsistent definitions of a reciprocation window, combined with the exclusion of unreciprocated grooming, leave the debate unresolved. We still do not know if primates exchange grooming within the short windows predicted by biological market theory or over much longer timescales. Whilst survival analysis can identify reciprocation windows objectively by retaining unreciprocated grooming as censored data, previous studies have applied it inconsistently. This inconsistency leaves the true time frames over which primates reciprocate grooming somewhat blurred. Here, we use a statistical approach that addresses these problems to examine grooming reciprocity in crested macaques (Macaca nigra), a socially tolerant macaque species. We applied survival analysis to identify each group's short-term reciprocation window without imposing arbitrary cut offs, and Bayesian survival models to test whether social bond strength and eigenvector centrality predicted reciprocation latency. Short-term reciprocation occurred rarely (29% of events in one group and 5% in the other); most reciprocation occurred over longer timescales than previous research commonly assumes. Stronger bonds predicted faster reciprocation; this finding suggests that crested macaques express social investment in valued partners through how promptly they return grooming, meaning that the timing of exchange carries information about relationship value. Eigenvector centrality showed no credible effect, though our small sample size and dense groups prevent us from interpreting this outcome more robustly. Substantial variation between dyads remained unexplained, pointing to partnerspecific properties we have yet to identify. These findings caution against assuming that reciprocity operates over short windows. They show that social predictors, such as bond strength, remain essential to understanding when primates return grooming. This work provides a baseline for testing whether grooming is exchanged for other social commodities, and for comparing reciprocation timescales across macaque species that differ in social tolerance.

Humans and other animals live in interconnected social communities that are essential for survival and well-being. In principle, social communities can persist despite changes in constituent members. However, gradually accumulating changes to membership could push systems past critical thresholds into alternate states. We investigated social network dynamics for 443 wild female Asian elephants over 12 years by tracking community membership and structure. Following accumulated demographic turnover, networks fractured into smaller and more isolated social communities. Connections were changed not only due to turnover of members but also rewired among remaining individuals. This provides the first empirical evidence of a social network state change in a long-lived mammal.

Male-biased cooperative behaviours that produce public goods (male services) are widespread in primates. Recent behavioural studies suggest that male services may function beyond paternal investment, either as displays of competitive ability or as honest signals eliciting female choice. Because these hypotheses make contrasting predictions about the physiological state of service providers, endocrine measures offer a complementary approach for distinguishing between them. We investigated whether long-term testosterone and cortisol concentrations, measured from hair samples, were associated with four male services in wild vervet monkeys: alarm calling, participation in between-group conflicts (BGCs), leading river crossings, and sentinelling. Testosterone and cortisol concentrations increased with male competitive status, measured as a combination of dominance rank and recent rank trajectory, particularly during the mating period, establishing the endocrine profile associated with competition. In contrast, most male services showed few relationships with hormone concentrations: participation in BGCs was associated with lower cortisol concentrations and, to a lesser extent, lower testosterone concentrations. Because BGC participation is the strongest predictor of male mating frequency in this population, these findings indicate that males contributing to BGCs present an endocrine profile distinct from that associated with competitive state and instead support the prediction that high-quality males, irrespective of their dominance rank, can perform costly cooperative behaviours while maintaining relatively low physiological costs. More broadly, our results provide physiological support for reputation-based partner choice as a mechanism contributing to the maintenance of male services in primates.

Social relationships are strongly linked to health and biological fitness in humans and other animals. For example, social support may buffer individuals against harsh ecological conditions. Reciprocally, better health may allow for greater investment in social relationships. We examined links between health and social behaviour in an endangered population of northern bottlenose whales (Hyperoodon ampullatus). Using photo-identifications spanning 37 years, we visually classified whether individuals had poor body condition on each day they were observed (N = 3,086 whale-days). We then used generalized linear mixed models to explore how body condition varied with sex, age, and within-individual deviations in group size or other social metrics derived from Bayesian social networks. Approximately 11% of individuals were classified as being in poor condition at any time, though the prevalence of poor condition varied across raters and years. Visually assessed condition was successfully validated with morphometric measures, though there was only weak evidence that poor condition predicted apparent survival. Contrary to expectations, whales in poor condition were found in larger groups and had more social partners than their average across the study period. These findings run counter to the expectation that health and social connectedness are positively correlated in social mammals.

High-sugar diets in mammals are associated with a number of metabolic diseases including diabetes and obesity. However, frugivorous and nectivorous bats specialize in consuming diets high in simple sugars while remaining healthy. To investigate the metabolic adaptations in bats with high-sugar diets, we conducted targeted metabolomics on blood samples collected during the first ten minutes after glucose or fructose feeding in five bat species whose natural diets vary in sugar content. By comparing changes in blood metabolite levels over time both between feeding treatments and across dietary guild, we have been able to identify shifts in sugar processing that likely support the healthy metabolism of simple sugars mammals with high-sugar diets. Notably, despite both sugars being readily absorbed, only fructose feeding induced significant changes in other metabolites within ten minutes after feeding. Species who naturally consume more sugar showed both faster production of metabolic intermediates and lowered production of common deleterious byproducts of fructose metabolism than species with comparatively lower-sugar diets. The prioritization and refinement of fructose metabolism is therefore likely a more key aspect of how bats with high-sugar diets are able to avoid the diseases typically associated with excessive sugar consumption than modifications to glucose metabolism.

A major transition in the evolution of eukaryotic cells was phagocytosis: the ability of one cell to engulf another. In some cases, the engulfed cell persists, forming an endosymbiosis which is often mutually beneficial. However, it is unclear whether the interaction between the ancestors was also cooperative. There are clear benefits to developing the ability to eat a competitor, but can it ever be beneficial to eat a cooperator? We use a dynamical systems framework to identify the ecological conditions under which phagocytosis can evolve from a system of cooperating microbes.Beginning with a system of two cooperating species, we use invasion analysis to identify the parameter regimes under which a phagocytosing mutant can invade and the subsequent population dynamics. We identify a critical threshold that determines whether an invading predator will coexist with the cooperative community or replace members. Repeatedly invading with different predators, we find that the system saturates when there are three coexisting species, though this community does not have a single optimal composition. However, if we incorporate additional costs, we find there is a single evolutionarily stable community of two cooperators and a pure predator. Ultimately, we show that it is possible for phagocytosis to evolve from a cooperative community and, moreover, coexist with the original cooperating species.

Endosymbiosis is a major source of biological innovation, enabling hosts to explore new ecological niches. While genome reduction in endosymbionts is well documented, its impact on the evolutionary transition from parasitism to mutualism remains unclear. Regulatory genes are lost disproportionately as bacterial genomes shrink, as we confirm in endosymbiont-containing lineages. We therefore model metabolic competition between hosts and endosymbionts, representing endosymbiont genome size by the number of independently regulated sets of reactions. In our simulations, hosts that grow poorly alone adapt best when harboring moderately genome-reduced endosymbionts rather than large-genome ones. This advantage goes beyond the lower resource use of smaller endosymbionts. By decomposing endosymbiont metabolic responses to changes in host-provided metabolites into "adaptability" and "sensitivity," we demonstrate that moderate genome reduction increases the sensitivity of endosymbionts to host metabolic conditions, rendering them more exploitable through host evolution. This suggests that genome reduction plays a more constructive role in mutualism evolution than previously appreciated, facilitating host-level selection for exploitable endosymbionts rather than being merely a passive consequence of endosymbiosis. Our systems-level approach, focusing on regulatory capacity rather than specific gene functions, provides a general framework for understanding how genome reduction affects the evolutionary trajectory from parasitic to mutualistic endosymbionts, and may help explain the frequent replacement of highly reduced endosymbionts in nature.

Ancestral recombination graphs (ARGs) represent the coalescent history at every locus in the genome, subject to mutation and recombination. Modelling viruses using the coalescent with recombination remains less explored, due to the potential for complex, multi-scale dynamics. Using perturbative methods, we establish mathematical bounds that describe the balance between inter-host and within-host coalescence, and demonstrate that the highly-recombinant Epstein-Barr Virus (EBV) coalesces predominantly at the inter-host level. We then modify SINGER, a tool from diploid genetics, for haploid ARG inference with fine-grained variable recombination rates. We provide insight into the paradox of high recombination rates and slow linkage disequilibrium decay, as ARG-based methods potentially allow us to disentangle epistatic effects from shared ancestry, and illustrate this using the coalescent topologies of EBV's host cell-entry glycoproteins. Our findings demonstrate that ARG-based methods recover gene-level selective structure, and provide a general method for analysing virus phylodynamics in the limit of small quantities.

Offspring can inherit a physiological or molecular state from their parents, and the carry-over in phenotypes across generations that results is often called epigenetic or phenotypic "memory''. Most theory on the evolution of such memory asks how the rate of switching between pre-existing phenotypic variants evolves in a fluctuating environment. Here we ask instead how the variants themselves originate. We model an asexual population in a periodically alternating environment in which a genotype specifies two inherited states, the phenotypic value each produces and the probability with which each is retained from parent to offspring. Starting from an ancestor that produces a single phenotype, mutations creating small differences between inherited states have no first-order selective advantage. Their spread depends on second-order selection and requires strong selection against phenotype-environment mismatch. A maternal effect of the same small size, conditioned on the maternal environment, is under first-order selection in the same model. Once phenotypes have diverged, genotypes whose retention probabilities match the duration of the environmental states are strongly favored, and the evolved strategy persists under conditions in which it could not originate. Memory is a consequence of individual selection and its origin probably follows earlier selection for cue-dependent maternal effects.

Stressful events are associated with long-term adverse health impacts, but the causal mechanisms linking these exposures to subsequent disease remain poorly understood. One plausible mechanism is persistent changes to hormonal signaling that regulate metabolic homeostasis. Here, we examine the long-term impacts of stress associated with exposure to the 2004 Indian Ocean tsunami on levels of free triiodothyronine (FT3), a circulating measure of biologically active thyroid hormone, using data from the Study of Tsunami Aftermath and Recovery. We measure biomarkers 20 years after the tsunami in a population-representative sample age 35y+ in 2024 who, at the time of the tsunami, were living along the coast of Aceh, Indonesia, the most affected part of the Indian Ocean Basin. Because respondents were first interviewed before the tsunami and subsequently tracked regardless of migration, the sample avoids a major source of selection bias common in long-term disaster studies. We identify the causal effect of tsunami exposure using variation in community-level tsunami mortality, comparing communities within the same sub-district. We find that greater tsunami exposure reduced levels of FT3 20 years later. Exposure also altered relationships between FT3, body composition, and cortisol and increased cardiometabolic risk. These findings identify persistent alteration of thyroid hormone physiology as a potential pathway linking severe stressful exposures to long-term cardiometabolic disease risk.

Background Long-acting HIV pre-exposure prophylaxis (LAP) could transform HIV prevention in sub-Saharan Africa, but its public-health impact and value-for-money will depend on whether programmes can reach people at greatest risk. We quantified the potential impact and efficiency of targeted LAP deployment across sub-Saharan Africa. Methods We developed a deterministic, age-structured compartmental model of HIV among adults aged 15 years or older in African countries accounting for 98% of adults living with HIV in sub-Saharan Africa. Antiretroviral therapy (ART) was assumed to reach 95% coverage by 2030 in all countries. LAP was assumed to reduce HIV acquisition by 96%. In model analyses, LAP was allocated according to rules that specify the degree to which programmes can effectively allocate it to those at greatest risk of acquiring HIV, including using a person's district, sex and age-group, and other signals of risk. Findings Across 34 countries, allocating a fixed quantity of LAP by district, sex and age averted 2 - 21 times as many infections as uniform allocation; perfect identification of within-group risk, an upper bound, raised this to 5 - 32 times. Multipliers were largest in low-prevalence countries, where absolute impact was smallest; in higher burden settings, they were 2 - 6 and 5 - 11, respectively. If 3 million people used LAP from 2028 in the 11 countries currently receiving Global Fund-supported lenacapavir, uniform allocation would avert about 41,000 infections by 2050, targeting by district, sex and age about 601,000 (range 360,000 - 1,660,000), and perfect risk targeting about 972,000 (490,000 - 3,570,000). Discounted averted ART costs fully offset LAP costs by 2050 at a total cost of up to ~US$50 per person-year with perfect risk targeting, $30 with targeting by district, sex and age, and $2 with uniform allocation. Expansion to 15 million users by 2035 brought modelled adult incidence across sub-Saharan Africa below 0.1 per 1000 person-years in 2049, about a decade earlier than projected without LAP, under the assumption that ART coverage reaches 95% in every country by 2030. Interpretation Targeting determines both the impact and the net cost of LAP. Allocation by district, sex and age is feasible now and multiplies the infections averted by each person-year of LAP. LAP offsets its cost through averted treatment only when it is targeted and its fully loaded cost is near the low end of current estimates. Better identification of people at highest risk within demographic groups, and sustained use among them, could add substantially to impact. The same approach could guide the allocation of LAP in other settings where HIV risk is concentrated.

Male and female animals often differ remarkably in their reproductive behaviours, typified by competing males and caring females. One influential explanation traces sex-role divergence to anisogamy: sex differences in gamete size. This explanation remains controversial, because existing models have not fully resolved how anisogamy leads to secondary sex-role divergence, and exceptions to typical patterns are empirically well-documented. Here, we develop mathematical models linking anisogamy to secondary sex-role evolution, showing that anisogamy drives sex-role divergence along contrasting evolutionary trajectories depending on ecological context. When the sexes differ only in gamete size, male-biased mating competition and offspring care evolve under scramble competition, whereas uncertain paternity shifts the population towards typical patterns. By contrast, anisogamy generally drives typical sex roles under contest competition, although sex-role reversal can emerge as an alternative stable state when care must be apportioned among offspring. Anisogamy can thus explain the striking diversity of sex differences across taxa.

Despite sharing nearly identical enzymes, strains of the same microbial species display wide variability of growth rates when growing on the same nutrients. Here, we show that such growth rate variability can emerge as a consequence of evolutionary adaptation to different patterns of environmental fluctuations. We develop a mathematical model combining cellular proteome allocation with eco-evolutionary dynamics in fluctuating environments. We find that different patterns of environmental fluctuations select for distinct growth profiles. Slow-growing strains repeatedly outcompete fast-growers when evolved in rapidly changing environments. In a range of environments with asymmetric nutrient availability, evolution reproducibly leads to the robust diversification of two coexisting strains. We develop a theoretical framework based on adaptive dynamics which quantitatively predicts these evolutionary outcomes and explains them in terms of the growth-lag tradeoff that all cells face. Finally, we show that the model can reproduce several patterns in growth-rate data from closely-related strains. Our results highlight that bacterial growth rates can rapidly adapt to their recent environmental history by proteome allocation alone, in ways that often oppose expectations based on nutrient quality.

Background Ecological surveillance indicates (regional) establishment of West Nile virus (WNV) and Usutu virus (USUV) in the Netherlands. However, the incidence of human infections remains unknown. Methods We identified WNV and USUV infections in the Netherlands in 2020-2023, using a longitudinal, nationwide population-based cohort, applying a two-tiered serology approach based on protein micro-array and virus neutralization. IgG data were used to estimate overall seroprevalence and infection incidence, and to study antibody longevity. Study set-up enabled tracking of the period of seroconversion and linking of self-reported travel history. The analyses took account of the survey design and weights were incorporated to match the Dutch population distribution following census data. Findings Among 4290 participants, we identified 113 probable WNV and/or USUV cases, including 55 seroconversions. Thirty-six cases were confirmed by antibody neutralization, of which 12 seroconverted. Six confirmed seroconverters were identified as autochthonous infections. The overall weighted seroprevalence was estimated at 0.54% (0.21-0.87%) for WNV and 0.25% (0.09-0.40%) for USUV. The weighted incidence for autochthonous WNV and USUV infection was estimated as respectively 0.06% (95% CI 0-0.12%) and 0.04% (95% CI 0-0.09%). Interpretation The estimated infection incidence and seroprevalence of WNV and USUV indicate a substantial underdiagnosis of clinical cases in the Netherlands in 2020-2023. These observations highlight the need for increased awareness among physicians to identify cases and the need for continued paired syndromic-laboratory surveillance for these viruses.

Understanding the transmission and persistence of zoonoses in wild reservoirs is crucial for outbreak prevention, demographic assessment, and conservation planning. Host demography, ecology, behaviour and within-host viral kinetics together underpin transmission dynamics but are rarely united within epidemiological models. Here we characterised the transmission dynamics of European bat lyssavirus 2 (EBLV-2) infection dynamics by uniting a long-term study (2010-2018) of the ecology of five greater mouse-eared bat (Myotis myotis) colonies in France (~2700 marked individuals) with diagnostic evidence of virus circulation. Although no lyssavirus was detected in any of the 442 saliva samples tested, seropositivity showed synchronised peaks across colonies (0-100% annually), with the highest seroprevalence in 2015-2017, but remaining low otherwise. Fitting Bayesian compartmental epidemiological models, we found evidence that shifts in EBLV-2 prevalence were primarily driven by the gregarious behaviour of populations during the mating season and age-dependent variation in transmission. The duration of maternal immunity emerged as a key, and still empirically unknown factor, which shaped whether EBLV-2 maintenance was enzootic or required an external force of infection. Lyssavirus outbreaks in M. myotis were associated with few deaths overall and had a negligible impact on population dynamics. These findings underscore the value of integrating ecological and individual-level data with mechanistic modelling to enhance our understanding and management of zoonotic pathogens in wildlife.

Disentangling whether gut microbiome diversification reflects host evolutionary history or spatial processes remains challenging because these factors are often confounded. The Macaronesian chaffinch (Fringilla) radiation provides a replicated natural experiment to separate these effects. Using long-read 16S sequencing, diet metabarcoding, and whole-genome data, we evaluated competing models of community assembly, testing host-mediated dispersal versus neutral assembly, and environmental filtering versus host control. We show that microbiome alpha diversity across host species was largely shaped by island size and isolation, aligning with Island Biogeography Theory, rather than host colonisation history. This indicates that microbial dispersal occurs independently of their hosts. Moreover, spatial and host factors influenced different aspects of the microbiome: geography and diet primarily predicted which taxa were present, whereas host genetic relatedness and heterozygosity were associated with the abundance of established residents. Notably, we show that inbreeding compromises a host's ability to regulate microbial abundance, leading to highly variable microbiomes. While we detected phylosymbiosis (closely related hosts harboring similar microbiomes), we found no evidence of co-diversification between birds and microbes. Instead, associations likely arise from convergent ecological filtering, where shared host traits filter similar bacteria from the local environment. Together, our results show that avian microbiomes are shaped by distinct spatial and host-mediated processes, and that separating microbial presence from abundance is important for understanding the mechanisms driving microbiome assembly.