Biological Reviews

Defaunation, defined as the loss or severe decline of animal populations, is increasingly recognized as a major dimension of global environmental change, yet its role in zoonotic disease ecology remains poorly understood. Although defaunation significantly influences ecological dynamics, current metrics often focus on species loss without fully addressing its cascading impacts, which can affect ecological processes such as zoonotic disease transmission. In this perspective, we propose a defaunation-cascade framework linking anthropogenic pressure to zoonotic pathogen dynamics through four ecological steps: host trait filtering, abundance restructuring, interaction rewiring, and altered pathogen circulation or proliferation, and potential spillover hazard. We illustrate this framework by using a global review of empirical studies and combining georeferenced data on disease prevalence with environmental and human-related factors. Our exploratory models suggest pathogen-specific associations: parasitic and bacterial prevalence were more strongly associated with anthropogenic pressure and mammal diversity metrics, whereas viral prevalence showed weaker support. We contend that incorporating defaunation cascades into One Health surveillance will improve ecological understanding of pathogen circulation and enhance the ecological accuracy and predictive power of zoonotic disease models, thereby identifying landscapes where faunal simplification may elevate spillover hazard.

Why birds evolved complex songs has been a long-standing puzzle. Here, by quantifying song complexity in 4,940 species (83% of all passerines), we show a latitudinal gradient in song complexity, with the most complex songs occurring in temperate, open-habitat bird assemblages characterized by relatively inconspicuous plumage. Using comparative analyses controlled for common ancestry, we find little support for traditional explanations based on sexual selection intensity, intelligence signaling, or species richness. Instead, complex songs evolve primarily in song-learning passerines, especially migrants, habitat generalists, and species breeding in open habitats where structured acoustic signals transmit more efficiently. Together, our findings indicate that global variation in song complexity emerges from the interaction between vocal learning, ecological context, and evolutionary history rather than from a single selective driver.

Sexually selected traits often impose fitness costs on their bearers. Yet, the relative costs and benefits of conspicuous traits can vary through space and time, driving variation in selection acting on those traits. For insects, an important but overlooked source of such variation is seasonal shifts in the local abundance of migratory insectivorous birds. Smoky rubyspot damselflies (Hetaerina titia) exhibit a marked seasonal polyphenism in wing pigmentation throughout much of North America, with individuals emerging in the summer exhibiting conspicuous dark wings. Here, we test the hypothesis that this variation is an adaptive response to seasonal and geographical variation in predation risk. First, we find evidence for strong constraints acting on wing phenotypes outside of the summer season, consistent with a seasonal shift in the relative costs and benefits of pigmentation. Second, using a continentally distributed predation experiment, we find that predation risk covaries with spatiotemporal variation in wing pigmentation and is linked to shifts in the local abundance of migratory birds. Overall, our analyses establish an eco-evolutionary link between tropical and temperate regions, underscoring the importance of considering both the evolutionary and ecological consequences of spatiotemporal variation in biotic interactions as species assemblages shift in response to global change.

Mixed-species groups (MSGs) pose a particular challenge for our understanding of sociality in animals. Though MSGs are widespread social assemblages that form to enhance foraging success and reduce predation risk of participants, the role of traits in mediating grouping has received less attention. In particular, the role of body colour has not been tested quantitatively, despite the fact that visual similarity can reduce individual predation risk. Here, we examine whether plumage colour structures mixed-species bird flocks (MSFs) at a global scale. Using data spanning four continents, we developed a new metric that quantifies colour similarity among flock participants and compared observed flocks to null assemblages constructed from all flocking species at each site. We further examined whether MSF participants represented a colour subset of the available colours in the regional species pool. We found striking evidence that birds in MSFs were more similar in colour than expected by chance across all sites, indicating that plumage colour is a non-random structuring trait that shapes assembly of flocks globally. The strength and prevalence of colour structuring varied across geographies, but not flock size. Within communities, MSF participants differed systematically in colour composition from the regional species pool, occupying a restricted region of colour space dominated by yellow and brown plumage. Thus, plumage colour affects MSFs influencing both overall flock participation as well as species co-occurrence within flocks. Our findings illustrate the importance of visual traits in structuring interspecific social systems, by highlighting that birds of a feather do indeed flock together.

Post-reproductive lifespan is an evolutionary puzzle. In most mammals female fertility tracks survival, yet humans and a few toothed whales show survival after reproduction ends. Explaining when and why post-reproductive lifespan evolves is central to understanding the evolution of ageing, social structure, and intergenerational helping across species. Kinship-dynamics theory predicts that when males are philopatric, a females local relatedness--especially to male descendants--increases with age, potentially favoring late-life helping over continued reproduction. We develop an age-sex-structured kin-selection model to test whether a rare menopause-inducing modifier allele can invade an initially non-menopausal population through its direct effects on survival and fecundity and its indirect effects on relatives. We consider two evolutionary pathways: stop early, where reproduction ceases earlier with little change in lifespan, and live long, where lifespan extends beyond reproduction under disposable-soma trade-offs. Parameterized with demographic, dispersal, and helping-effect estimates from eight mammalian taxa, the model predicts empirically plausible ages of reproductive cessation and post-reproductive representation in humans and killer whales, but no invasion across plausible cessation ages in non-menopausal taxa. Global sensitivity analyses identify male dispersal and the effect of post-reproductive help on male survival as determinants of whether menopause evolves, motivating the "mamas boy hypothesis": menopause is most strongly favoured by selection when late-life care increases the survival and lifetime fitness of philopatric sons and grandsons.

Resilience is a central concept in ecology and environmental policy, yet its meaning and quantification remain inconsistent across subfields. Clarifying how resilience is defined and measured across the subfields of ecology is therefore a critical step towards delivering coordinated efforts to strengthen resilience research. Here, we analyse 594 studies published between 1977 and 2025 to determine how resilience is quantified across ecological contexts. Using large language models to extract structured data and conditional inference forests to assess predictors of metric choice, we show that resilience is most commonly ([~]25%) quantified using recovery rate and recovery degree, but no single metric dominates. Crucially, study attributes like organisational level, methodological approach, and disturbance regime explain only a small fraction of variation in metric selection. Despite this apparent inconsistency, more than 90% of studies draw from a shared set of six quantitative dimensions of resilience. This combination of weak constraint and latent convergence suggests that resilience metrics function as a flexible but implicitly standardised toolkit rather than as context-specific constructs. We argue that this hidden structure provides a foundation for a unified, multidimensional resilience framework that can support synthesis across ecological systems and improve the translation of resilience science into conservation and policy.

AimBiodiversity-rich regions often lack the scientific infrastructure needed to document and curate their own biodiversity, creating inequalities in access to taxonomic reference material. We investigated how biological, institutional, and geopolitical factors shape the retention, extraction, and appropriation of reptile holotypes, the name-bearing specimens upon which species descriptions are based. LocationGlobal. TaxonReptiles. MethodsWe compiled a historical dataset of reptile holotype origins and destinations spanning 1758-2024 to reconstruct long-term patterns of retention and international specimen flows. We then quantified species-level holotype retention, holotype flows between country pairs, and country-level patterns of retention, appropriation, and network centrality for the period 1990-2024, and used generalised linear mixed models to assess the biological, institutional, and geopolitical determinants of these contemporary circulation processes. ResultsAlthough nearly 90% of reptile species described originated in the Global South, less than a quarter of their holotypes remain housed there. Historically, exported holotypes consistently outnumbered retained holotypes on a decadal basis until the early twenty-first century. Retention was promoted by local scientific capacity, institutional infrastructure, collector involvement in species descriptions, and environmental governance. In contrast, extraction was concentrated in highly endemic regions with limited scientific infrastructure and was associated with taxonomic revisions, socioeconomic interest, and disparities in political stability and colonial history. Appropriation of foreign holotypes was greatest in countries with high research investment, strong environmental governance, and historical geopolitical influence. Main conclusionsGlobal patterns of holotype circulation reflect a persistent geography of scientific inequality. The distribution of taxonomic reference material emerges from the interaction of retention, extraction, and appropriation processes, linking local biodiversity discovery to uneven global scientific capacity. Reducing these inequalities will require investments in taxonomic expertise, institutional infrastructure, and governance frameworks that promote more equitable stewardship of biodiversity knowledge and its material foundations.

The Salas y Gomez and Nazca Ridges (SGNRs) in the Southeast Pacific, recognized as an Ecologically or Biologically Significant Area (EBSA), host unique marine ecosystems with one of the highest rates of endemism on the planet. This study provides the first comprehensive trait-based assessment of seabird functional diversity in this globally significant region, focusing on their ecological contributions as top predators. Using at-sea abundance data from 11 oceanographic surveys (2014-2017) across 3,500 km of transects, we recorded 36 seabird species (8,179 individuals). We analysed functional diversity through ten foraging-related traits, including diet, foraging strata, and morphology. Multidimensional trait analyses revealed a seabird assemblage characterised by low functional richness (FRic = 0.0587), moderate-to-low evenness (FEve = 0.3649), and high divergence (FDiv = 0.6609), with non-random patterns confirmed by null models. Nesting (17 species) and non-nesting (19 species) groups showed distinct functional structures, with nesting seabirds exhibiting higher functional divergence and non-nesting seabirds greater functional evenness, though with 61% trait-space overlap. Low functional redundancy suggests that the loss of seabird species would likely translate into the loss of unique functional roles, potentially compromising ecosystem processes such as cross-system nutrient subsidies. With 73% of the SGNRs beyond national jurisdiction, seabirds face threats from unregulated fishing, plastic pollution, and seabed mining. These findings underscore the urgent need for conservation strategies under the High Seas Treaty (BBNJ treaty) to protect not only species richness but also functional roles, ensuring ecosystem resilience in this biodiversity hotspot of over 110 seamounts.

Cooperatively breeding species are disproportionately found in extreme and unpredictable climates globally, suggesting that cooperation is beneficial to persistence in climatically challenging conditions. Notably, other dimensions of sociality, like group living and tendency to engage in affiliative social behaviors, offer fitness-related benefits that could make them similarly advantageous in such climates. Here, I present a phylogenetic analysis of Primates aimed at testing whether these two dimensions of sociality--average group size and average percent time spent social grooming--are predicted by climatic challenges in species environments. Results show that time spent grooming is highest in extreme and unpredictable climates, with how dry conditions are explaining the greatest amount of variation. Thus, climate may influence the evolution and/or persistence of social grooming. While multiple mechanisms could mediate this association, subsequent analyses point to the benefits of social affiliation in environments where groupmates have highly competitive dynamics as one potential explanation.

Anseriformes (waterfowl) and Galliformes (landfowl) are among the worlds most recognisable groups of birds, together comprising the clade Galloanserae. Despite their close evolutionary relationship, the skulls of adult anseriforms and galliforms exhibit strikingly distinct morphologies, the developmental basis and evolutionary history of which is poorly understood. To illuminate the developmental and evolutionary underpinnings of cranial disparity between and within these major extant bird clades, we quantitatively investigated ontogenetic changes in cranial morphology across galloanseran phylogenetic diversity, focusing on the previously unexplored post-hatching interval during which adult morphology takes shape. Our results reveal the combined effects of multiple heterochronic shifts early in galloanseran evolutionary history including anseriform hypermorphosis, along with influential non-heterochronic changes leading to substantially more disparate ontogenetic trajectories--and greater cranial variability--in anseriforms than galliforms. Key galloanseran fossils help clarify the polarity of evolutionary shifts in cranial development through galloanseran phylogenetic history and demonstrate that extant galliform cranial morphology is more constrained and retains a more plesiomorphic morphology than that of anseriforms. Our work helps illuminate the developmental basis of the iconic differences in cranial form between waterfowl and landfowl and illustrates the importance of broad phylogenetic and ontogenetic sampling for clarifying patterns of post-hatching developmental divergence among major vertebrate clades.

Early life adversity (ELA) causes lasting psychological and behavioral consequences in humans, with parallel stress and fear responses observed in captive animals. How naturally occurring ELA shapes behavior in non-captive animals remains poorly understood. Investigating these effects across socioecological environments is important for identifying the evolutionary pressures shaping early life sensitivities, and determining whether these behavioral syndromes have deep evolutionary roots. Here, we tested the effects of eight forms of ELA on adult behavior using a large dataset (N=313 males, 346 females) from a free-ranging population of rhesus macaques (Macaca mulatta). We measured adult behaviors indicative of stress and the tenor of social interactions, including agonism, vigilance, self-directed behaviors, and affiliation. Individuals exposed to more ELA received more aggression and less grooming, were more submissive, and were less vigilant. While high status buffered the effects of ELA on grooming, it amplified effects on aggression. Our findings demonstrate that in a large sample of a free-ranging primate, early life environments impact behavior into adulthood. While some behaviors are consistent with patterns in humans and captive animals, suggesting an evolutionarily conserved ELA syndrome, other findings add new insights into the various ways individuals interact with their environment as a function of ELA.

Maximising ecosystem service (ES) benefits while minimising ecosystem disservices (EDS) is essential for ecological intensification in annual crop systems. Yet ecosystem disservices are often underreported in the scientific literature, potentially biasing how agroecosystem functioning is understood. To test this, we built a predicted network of plausible links between ecosystem service providers (taxa or functional groups that can deliver ecosystem services or disservices), and the ES or EDS they may provide. We then compared this with a realised network based on systematic Scopus searches of annual crop literature. The predicted network contained 47 nodes and 103 links, whereas the realised network contained 33 nodes and 58 links, representing declines of 29.8% in nodes and 43.7% in links. Overall connectivity declined, especially for highly connected nodes, and four of the nine predicted disservice nodes were absent from the literature. ES links were more likely to be documented than EDS links, and EDS links were three times more likely to be absent. Across all links, ES were reported in 6.6 times more papers than EDS. Projected networks, which map indirect connections by linking ES directly to EDS if they share common providers, showed that these bundled interactions were strongly reduced, obscuring multifunctionality and trade-offs. This systematic underrepresentation of EDS, reflecting a cognitive bias, can inflate perceived benefits, distort the evaluation of key taxa and interactions, and create unrealistic expectations about intervention outcomes in biological crop protection. Addressing EDS alongside ES is therefore essential for more balanced assessments of crop-system management and better-informed decisions.

Directional selection is often viewed as a transient force in macroevolution, with its signal eroded over time by stabilizing and fluctuating selection. Yet, transitions into new adaptive zones are predicted to impose strong and sustained selective pressures that may leave a detectable signature even across deep timescales. We test this prediction by comparing the rates of multivariate skull morphological evolution required to traverse the boundaries between adaptive zones against genetic drift expectations. Our dataset includes 11,793 specimens spanning 231 species from 12 mammalian clades, each containing unique ecological transitions into new adaptive zones. Using a quantitative genetics framework, we estimated the phenotypic distances between ancestral and derived adaptive zones and contrasted them with null expectations under genetic drift. While a few adaptive zone invasions (e.g., marsupials and rodents) are consistent with drift, most exhibit substantially elevated rates of evolution. These results suggest that directional selection has recurrently shaped mammalian cranial evolution during major ecological shifts. We propose that adaptive zone transitions represent evolutionary contexts in which adaptation leaves a persistent macroevolutionary signal, challenging the prevailing view that long-term patterns are dominated by static forces.

Data on the shape of a group of organisms can be conceptualized as forming a point cloud in the multivariate space of measurement. This is literally true for traditional linear measures, while in a geometric morphometric context the cloud resides in Kendalls shape space, tangent to the true shape space. Regardless of the method of construction, the topology of this point cloud, or phenotypic (hyper)ellipse, is a beguiling target for evolutionary analysis. Reordination of the axes will not change the geometry of this phenotypic ellipse, and the notion that its geometry carries a meaningful biological signal is an old idea; but the character of this signal is often elusive. This paper explores the application of the most commonly used parameter designed to summarize differences in phenotypic ellipse geometry (relative eigenvalue variance, or Vrel), and demonstrates that it is incapable of differentiating between several plausible ways in which phenotypic ellipse geometry might differ among species, because it confounds three separate parameters necessary to describe the ellipse. Two example data sets are analyzed to illustrate variability in phenotypic ellipse geometry and draw conclusions about observed differences. The first case compares wolves to domestic dogs, and replicates previous findings of much greater variance yet tighter integration in dogs. This calls into question the simple model of a single peak in the fitness landscape of dogs. The second example comprises geometric morphometric landmarks from the jaws of a clade of sigmodontine rodents, and allows comparison of ellipse geometry in a phylogenetically controlled setting with qualitative ecological categories. Three parameters are found to vary in concert along a grade of most to least ecologically specialized: the phenotypic variance, the effective rank (dimensionality), and the degree of covariance (Vrel and related metrics). Use of all three of these quantities to characterize the geometry of the phenotypic ellipse is advocated, as all are necessary to characterize how variance is distributed in the ellipse in different taxa. The phenotypic ellipse geometries illustrated here appear to reflect something of the geometry of the adaptive peak upon which each taxon sits; or that they represent (aspects of) the mapping function of adaptive peak to phenotypic ellipse, in ways first predicted by Simpson in the twentieth century.

The recent large-scale circulation of High Pathogenicity Avian Influenza (HP AI) viruses H5Nx of clade 2.3.4.4b has been responsible for massive die-offs in wild species, notably in long-lived seabirds, with unknown implications for the immunity of surviving individuals. In the North Atlantic, northern gannet colonies were heavily affected in 2022, with more than 40% mortality observed among breeding adults and some surviving individuals developing dark irises. Using samples collected in 2023 and 2024 on Rouzic colony (France), we report persistent individual anti-AI antibody levels and seroneutralisation titres, with most of the immune individuals showing dark irises. A modelling approach further stressed the importance of long-lasting immunity in such species by showing that the proportion of individuals which kept their immunity between years strongly limited decreases in population size in case of repeated outbreaks. Overall, our results highlight the existence and importance of long-lasting immunity in long-lived species for population persistence.

Gliding enables mammals to forage and escape from predators by moving between discontinuous forests. Its benefits depend not only on glide distance but also on the ability to decelerate and land safely. This study developed a theoretical framework linking glide distance, gliding velocity, aerodynamic braking, body mass, and braking distance. Twenty-two representative distance-velocity observations from eight studies and five species were compiled. Among distance-velocity models, log-distance and saturated with V0 models received nearly equivalent support. Both models predicted increasing velocity with glide distance, with the rate of increase declining at longer distances. For a 1 kg animal undergoing a 60% reduction in velocity, predicted kinetic energy remaining immediately before contact increased from 4.80-5.14J at 20 m to 8.13-8.90 J at 80 m. This velocity reduction corresponded to a dissipation of 84% of approach kinetic energy before contact. Over a braking distance of 1 m, the required mean deceleration increased from 2.57-2.75 g at 20 m to 4.35-4.77 g at 80 m. At 80 m, shortening the braking distance from 4 to 0.5 m increased the required deceleration from 1.09-1.19 to 8.70-9.53 g. These results indicate that the absolute energetic and deceleration requirements of landing increase with glide distance, even when velocity is close to an asymptote. These results provide a quantitative basis for considering aerodynamic braking and landing requirements alongside conventional measures of glide performance. Summary statementModels quantify how glide distance, aerodynamic braking, and braking distance affect pre-contact kinetic energy and deceleration requirements in gliding mammals.

High Pathogenicity Avian Influenza (HPAI) H5N1 clade 2.3.4.4b has spread beyond birds to affect seals across the Southern Ocean and sub-Antarctic region, with southern elephant seals (Mirounga leonina) particularly devastated. The virus, likely introduced via spillover from infected migratory birds, has killed tens of thousands of adult seals and pups throughout most of their range, though Macquarie Island remains unaffected so far. We used twenty years of elephant seal movement data from the southern Indian and Pacific oceans to assess whether seal-to-seal transmission could spread HPAI H5N1 between breeding colonies, despite the vast distances separating them (Marion Island, Iles Crozet, Iles Kerguelen, and Macquarie Island). There was substantial overlap in seals' at-sea distributions during their winter post-moult trips, when seals travel for weeks at average speeds of 3.5 km/h. Two transmission pathways were examined: (1) terrestrial "stepping stone" routes, where infected seals could pass the virus between colonies during short intervals to remain infectious were feasible from Marion Island to Kerguelen but not from Kerguelen to Macquarie Island; and (2) at-sea encounters between seals, which occurred frequently enough to enable transmission. The findings suggest that once established at Macquarie Island, the virus could potentially spread further to New Zealand's sub-Antarctic islands and mainland New Zealand. While seal-to-seal transmission appears possible, we conclude this is unlikely. Nonetheless, understanding at-sea contact rates enhances knowledge of H5N1 epidemiology and demonstrates the value of combining long-term population monitoring with movement data to understand wildlife disease dynamics.

Inbreeding depression is a critical driver of extinction risk in fragmented populations, yet its magnitude in the wild has historically been estimated using sparse pedigrees or low-resolution molecular markers. The transition to genomic metrics of inbreeding offers unprecedented precision in quantifying realized homozygosity and its fitness costs. We conducted a systematic review and multi-level meta-analysis of 91 effect sizes from 20 wild vertebrate populations to provide a global benchmark for genomic inbreeding depression. Our results confirm a pervasive and significant negative relationship between fitness and genomic inbreeding. Males exhibited significantly stronger inbreeding depression than females. Fitness costs were consistent across life stages (developmental, adult and lifetime) and across types of fitness traits (survival vs. reproduction). We found no significant association between the magnitude of inbreeding depression and IUCN conservation status or historical isolation (discretely measured as "isolated" or "non-isolated"). As we enter a genomics era that will provide realised estimates of inbreeding, future studies can be added to this meta-analysis to provide a more comprehensive view of inbreeding depression and potentially identify patterns pertinent to evolutionary biology and conservation science.

Land-bridge islands are former mainland areas isolated by post-glacial sea-level rise (<15,000 years) and the most common island type. Because of their recurrent connectivity with continents, it is unclear whether species on land-bridge islands can undergo evolutionary changes associated with the more isolated oceanic islands ( island syndrome). Here, we test the hypothesis that the selective environment on land-bridge islands exerts predictable and consistent evolutionary shifts in morphological traits of songbirds. We apply Bayesian hierarchical models to a morphological dataset of 6,917 individuals comprising 185 species of songbirds from four land-bridge islands (Bioko, Sri Lanka, Taiwan and Trinidad) and adjacent continents. Across all 185 species, we find that occurrence on a land-bridge island has clear directional effects on five morphological traits related to beak, wing, and tarsus, as well as a general increase in body size. At the species level, 57 out of 90 tested species exhibit significant morphological divergence between land-bridge island and mainland, yet for only 20 of these are the land-bridge island populations recognised as distinct endemic subspecies. Our results show that occurrence on land-bridge islands has a detectable effect on passerine morphology consistent with the island syndrome, and suggest these islands harbour previously unrecognized unique biodiversity.

Understanding the impacts of climate on migratory dynamics is a priority for global change research, especially as phenological mismatches have been identified as a primary concern for long-distance migration. The eastern monarch (Danaus plexippus) is a flagship species for insect migration, but there has been surprisingly little research on their spring and summer migratory timing and mismatches with their key breeding resources. We show that spring, but not summer, phenological mismatch with their milkweed host plants has the potential to be a limiting factor in the eastern monarchs yearly population growth. To better understand arrival at their breeding grounds, we developed models specifically to tease apart the factors that push (from departure grounds), pull (towards arrival grounds), or facilitate flow (in the flight corridor) during migration. We found wind (flow) dynamics to be the most important factor associated with arrival timing, especially in spring. Further, we show how spring and summer migratory flyways coincide with the large-scale Hadley and Ferrel atmospheric cells respectively, and how their dynamics appear to be essential for understanding monarch migratory timing.