Biological Reviews

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.

The morphology of cranial sutures (i.e., joints that connect the bones of the skull, are sites of growth, and absorb biomechanical stresses) is highly variable, especially in mammals. Exposure to different strain regimes as a result of development, function, and ecology can drive disparity in suture complexity at both the individual and taxonomic levels. Although the relationships between diet, muscle mass, static loading, and certain behaviors (e.g., headbutting) and suture morphology are well studied, the effects of fossoriality, particularly the use of the craniodental apparatus during substrate excavation, remain largely unknown. We synthesize data using an ordinal ranking scheme for digging mode importance in 107 rodent species; premaxillofrontal and nasofrontal suture complexity as measured by sinuosity index, power spectrum density, and spectral entropy, the latter of which is a novel suture complexity metric; and Bayesian multilevel models to examine the fossoriality-suture phenotype relationship at the suborder level and above and predict digging mode usage in 11 cryptic extant species. Most fossorial rodents utilize multiple digging modes, often at varying levels of importance. Chisel-tooth digging importance is associated with reduced nasofrontal suture complexity in multiple suborders, whereas scratch digging importance is correlated with elevated complexity in the premaxillofrontal suture. Suture complexity poorly reflects head-lift digging importance. We demonstrate that fossorial function is a complex, multivariate trait and reveal new insights into the functional morphology of cranial sutures across the most diverse mammalian order.

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.

Exotic species can reshape native communities, in the extreme, to local extirpation. Before that, natives may adjust where and when they use the landscape, in ways that allow coexistence and may be the earliest sign of trouble. Distinguishing coexistence built on such adjustment from one built on pre-existing niche differences requires comparing a native where the invader is absent with the same native where it is present, across sites at different invasion stages. Using eight years (2018-2026) of camera-trap data from 103 stations across Uruguay (154,590 camera-nights), we asked how the invasive axis deer (Axis axis) affects the space and time use of two native cervids: the forest-dwelling brown brocket (Subulo gouazoubira) and the grassland-restricted pampas deer (Ozotoceros bezoarticus). Activity-overlap, directional-avoidance, hierarchical activity, and two-species occupancy analyses showed sharply divergent dyads. A. axis and S. gouazoubira shared forest yet segregated in space and along the understory stratum the brocket requires and the invader avoids, and shifted their diel activity by context; understory cover fell with A. axis abundance, consistent with habitat modification. For A. axis and O. bezoarticus, segregation was intrinsic, opposite diel niches and habitats, with no support for a detectable shift in pampas deer behavior. The only behavioral signal was asymmetric: the invader, rather than the native, appeared to adjust, though few shared sites warrant caution. The same invader thus drives different mechanisms, but coexistence is provisional: A. axis is still expanding, far from equilibrium, so these shifts read best as early warning signals.

Multifunctional anatomical structures must accommodate potentially competing performance demands, leading to functional tradeoffs. The mammal rostrum (snout) is involved in multiple functions, including feeding, respiratory conditioning, olfaction, and, in most bats, echolocation. Therefore, it serves as an ideal system to investigate whether and how functional tradeoffs promote or constrain morphological diversification. While the external morphology of the rostrum has been linked with mechanical demands of feeding in bats, the factors and processes influencing its internal morphology, including the turbinals in the nasal cavity, remain comparatively unexplored. Here we investigate how internal rostral morphology reflects specialization and tradeoffs among respiratory air conditioning, echolocation, and olfaction by quantifying turbinal morphological diversity in a phylogenetically and ecologically diverse sample of bat species. We find that echolocation mode predominantly influences both the ethmoturbinal complex (ETc) and maxilloturbinal (MT) systems, with nasal-emitting bats having reduced and less complex turbinals compared to oral and non-laryngeally echolocating bat species. MT complexity and surface area are not associated with bioclimatic variables representing thermal variability and aridity. However, the ETc exhibits higher complexity in plantivorous bats than faunivorous bats, suggesting higher olfactory reliance. While MT morphological disparity was predominantly explained by echolocation mode, ETc morphological disparity was associated with both echolocation mode and diet, suggesting potentially opposing demands favoring turbinal reduction for sound transmission and elaboration for olfaction. Among nasal emitters, which must accommodate both demands within the nasal cavity, ETc complexity exhibited higher relative disparity compared to MT complexity. These patterns suggest that competing functional demands may promote or maintain morphological diversity, possibly through ecological specialization and/or many-to-one mapping between morphology and performance.

The size of an epidemic can influence the strength of selection acting on hosts and parasites, but whether epidemic size consistently shapes host-parasite (co)-evolution remains unclear. I conducted a meta-analysis of studies that simultaneously quantified epidemic size and evolutionary change in hosts, parasites or their interactions, synthesising 123 effect sizes from 11 studies. I tested whether epidemic size was associated with four components of evolutionary change: host evolution, parasite evolution, non-additive host-parasite coevolution and net coevolution. Larger epidemics were associated with greater host evolutionary change in invertebrates, whereas the relationship was positive in plants and negative but non-significant in bacteria. Parasite evolution, non-additive coevolution and net coevolution were not detectably associated with epidemic size, although parasite infectivity showed a relatively consistent increase across studies. These results provide limited evidence for a general relationship between epidemic size and host-parasite (co)-evolution and instead suggest that evolutionary responses depend on the host-parasite system and broader ecological and temporal context. Interpretation is constrained by the small and taxonomically biased evidence base, particularly the strong representation of Daphnia, and by substantial heterogeneity in how epidemic size and evolutionary change were measured and defined. More studies across diverse host-parasite systems are required that track both evolutionary change and disease prevalence through time. This work also provides an empirical basis for incorporating coevolutionary feedbacks into quantitative epidemiological models.

Urbanization is an accelerating evolutionary force, yet the patterns and drivers of urban gene flow remain largely obscured. This leaves a critical gap in testing the interplay between demographic and genetic models of urban connectivity. Specifically: the urban fragmentation model, where structural barriers restrict gene flow and drive genetic drift; the urban facilitation model, where permeable matrices enhance functional connectivity; and the urban sink hypothesis, a demographic model where high mortality creates unsustainable population sinks. We used high-resolution RAD-capture data to conduct landscape genomics and gene flow analyses to resolve these models and investigate contemporary evolution in urban coyotes (Canis latrans) across the greater Los Angeles, CA, USA metropolitan area. By incorporating fine-scale environmental, social-ecological, and historical variables of the built environment, we rigorously assessed how urban heterogeneity shapes intra-city evolutionary dynamics. We found weak population structure, heavily influenced by family groups. There was extensive landscape permeability and gene flow across the study area including across four major highways. We also identified heavily asymmetric gene flow from exurban source populations into the urban core, supporting the urban sink paradigm. However, rather than acting as a genetic dead-end, the urban matrix showed striking functional heterogeneity: one urban core functioned as a 'genetic sponge,' absorbing unique transient alleles from multiple surrounding sources, and both urban cores operated as permeable 'leaky sinks,' exporting critical gene flow to other populations. Consequently, urban demographic sinks can simultaneously act as vital genetic bridges. Our findings reject a pure urban fragmentation model while demonstrating that demographic sink dynamics and genetic facilitation are not mutually exclusive. This demonstrates that high-turnover urban sinks can buffer regional biodiversity against genetic erosion, highlighting the need to prioritize matrix permeability and inter-patch connectivity in urban conservation strategies.

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.

Social relationships are foundational to animal societies, but individual social connections differ hugely in importance and strength. Key relationships, such as those between breeding partners, may be prioritised and more highly invested in by individuals, but a key question is the extent to which social flexibility allows individuals to respond to the loss of these key social relationships. Here, we use a detailed longitudinal dataset of individual breeding and social behaviour in wild great tits (Parus major) to characterise flexibility in social behaviour after the loss of their breeding partner ('widowing'). We show transient increases in two measures of sociality following the widowing event. Individuals' relationships with their replacement partner were initially weaker than those between non-widowed birds, but converged by the breeding season. Hence, we show that birds are robust to the loss of key social relationships and that great tits show a flexible ability to return to social equilibrium. Strong social resilience has implications for the response of populations to change and loss.

Rapid evolution allows populations to persist in environments where they would otherwise go extinct. This phenomenon, known as evolutionary rescue, is typically studied in the framework of biological evolution, yet adaptive traits can also arise and spread through cultural evolution. The present study developed a stochastic eco-evolutionary model to compare rescue probabilities through biological and cultural evolution. Transmission bias governed the rescue probability under cultural evolution by setting how readily a rare adaptive trait was copied. Conformity bias suppressed population persistence because a rare trait was the least likely to be copied. Content bias toward the adaptive trait enabled evolutionary rescue when social learning was rapid, but it typically yielded a lower rescue probability than biological evolution. Only anticonformity bias, together with a high social learning rate, exceeded the rescue probability of biological evolution by enabling the adaptive trait to be established more rapidly. These results demonstrate that transmission bias alters the demographic consequences of cultural evolution and highlight the importance of transmission processes in evolutionary rescue theory. Understanding how adaptive behaviours are socially transmitted may also improve predictions of animal population persistence and inform conservation efforts in rapidly changing environments.

The palaeogeographic history of the Neotropics has inspired multiple diversification models, emphasizing either the Andes or Amazonia as primary drivers of biodiversity. However, the extensive Pebas wetland system that covered western Amazonia during the Miocene has gained attention as a potential barrier to dispersal and diversification. This system may have contributed to the modern diversity imbalance between the Andes and Amazonia, known as the Gentry pattern. Most evidence for this hypothesis comes from isolated case studies and fossils. Here, we conduct a meta-analysis including 1,361 species of Neotropical butterflies to evaluate the role of the Pebas wetland on the dynamics of dispersal and diversification. We found that Andean- and Amazonian-centred clades show complementary but temporarily distinct biogeographic patterns. High Andean diversity reflects an Andean origin, with early occupancy and diversification occurring within the Andes, followed by post-Pebas dispersal into Amazonia. By contrast, Amazonian-centred clades, reflect an Amazonian origin, with an early diversification in Amazonia and subsequent post-Pebas colonization of the Andes. While several clades show little evidence of Pebas influence, our study suggests that the Pebas wetland did not uniformly restrict diversification, but rather selectively constrained dispersal between the Andes and Amazonia, preventing simultaneous diversification in both regions and thereby creating a diversity imbalance that characterizes the modern Neotropic.

Global biodiversity recovery demands restoration at landscape scales, where ecological outcomes are shaped not only by habitat amount but also spatial arrangement. Alternative landscape restoration strategies can increase habitat amount, improve quality, and alter configuration in fundamentally different ways and may therefore vary in their efficiency. Here, we conceptualise five landscape restoration strategies and use a spatially explicit metapopulation framework to compare their ability to increase predicted species persistence across heterogeneous landscapes and trait space defined by dispersal ability and specialisation. We find that restoration outcomes are strongly trait-dependent, with restoration of existing degraded habitats yielding the highest benefits - particularly for specialists. Improvement of the landscape matrix to create low-quality habitat primarily benefits generalists, while strategies creating new habitat patches or enlarging patches provide more consistent but modest gains. These results reveal fundamental trade-offs among restoration strategies and highlight the utility of trait-explicit approaches to inform their implementation and evaluation.

Classic socio-ecological theory predicts that males and females experience different sources and mechanisms of social competition. Whether these differences translate into sex-specific structural properties of dominance hierarchies remains unclear. Here, we compiled 156 dominance interaction matrices from 80 published studies and extracted three commonly used metrics - hierarchy steepness, linearity and the directional consistency index - to investigate the structural characteristics of male and female dominance hierarchies across primates. All three metrics were strongly affected by methodological and demographic variables. Steepness increased with the number of recorded interactions and group size, linearity decreased as matrices became sparser, and directional consistency declined with increasing numbers of interactions. Steepness covaried positively with both linearity and directional consistency, indicating that groups with steeper hierarchies also exhibited more linear and more directionally consistent relationships. We found no sex differences in steepness, linearity or directional consistency. These results suggest that current metrics primarily reflect variation in the sampling effort and the rate of interaction of the recorded behaviour and appear therefore not to capture potential sex differences in the forms of competition. Our findings highlight the need for alternative measures of power asymmetries that are less confounded by sampling effort and demographic variation to better understand how competition and conflict are structured across primate societies.

Emerging infectious diseases constitute a major health threat and are often associated with zoonotic spillover processes. Pathogen host shifts may involve evolutionary adaptation to new host species, which is of central concern in relation to future pandemics. Evolutionary host shifts require certain conditions to be met, including physiological and ecological factors. A pathogen strain infecting a new host species to which it is not well adapted will generally experience reduced success, which depends on the effective similarity between the new host species and the original reservoir host species from the pathogen's perspective. The adaptation process involves intermediate strains, or phenotypes, that exist at the cost of reduced transmission and replication rates, and the evolutionary outcome depends on factors associated with host species physiology and population interactions. The interplay between host similarity and interaction, and pathogen adaptation trade-offs, is therefore important in steering pathogen evolution and possible evolutionary host-shift outcomes. Here we apply niche-evolution theory to host-pathogen systems to investigate the combined effect of host species similarity and intra- and interspecific interactions on evolutionary host shifts. We identify the conditions under which evolutionary host shifts occur and show, for a two-host-species system, that evolutionary outcomes fall into four distinct categories.

Changes in fire regimes negatively impact biodiversity worldwide1,2. The immediate behavioral responses of animals to fire, however, are largely unknown. Despite presenting obvious dangers, fire can also attract animals by providing unique foraging opportunities3. Such attraction to fire (pyrophilia) has already been observed in some bird species, yet its underlying motivation remains unidentified. Here, we examined the abundance and behavior of 16 bird species before, during, and after prescribed burns in the northern Australian savanna. Eight species were on average 3 to 23 times more abundant during fires. Raptors caught on average ten times more prey per hour during burns than in the absence of fire, and their foraging success increased with fire size. Black kites, the most pyrophilic birds observed, were more numerous during larger fires and foraged more efficiently on the ground than in the air. These results underscore the importance of investigating animal behavior in response to active fire and the effects of shifting fire regimes on the foraging ecology of pyrophilic animals.

Geographic isolation is a major driver of lineage divergence. However, strong ecological selection can promote reproductive isolation despite opportunities for gene flow in geographically continuous landscapes such as mountains. Here, we test whether ecological divergence across elevational gradients can promote speciation within a single mountain range. Integrating phylogenomic (ddRAD), morphological, and bioacoustic data, we test predictions of elevation-mediated parapatric speciation in Platymantis (Lahatnanguri) ground-dwelling frogs on Mindanao Island, Philippines. Phylogenomic analyses recovered an endemic radiation across neighboring mountain ranges and revealed a hierarchical pattern of diversification among three monophyletic lineages, with deeper divergence separating the mountain lineages Highland 1 and Highland 2 and more recent divergence occurring within a single mountain range. This topology identified a morphologically and acoustically distinct low-elevation lineage (<1,400 m a.s.l.) as closely related to the high-elevation Highland 1 lineage (>1,400 m a.s.l.), providing a natural test of divergence along an elevational gradient. The parapatric Lowland and Highland 1 lineages appear to have diverged relatively recently, as reflected in their shallow genome-wide differentiation, yet exhibit pronounced morphological and acoustic divergence. Population genomic analyses further recover discrete genomic clusters and no evidence of contemporary hybridization between the parapatric lineages, indicating independent evolutionary trajectories despite geographic continuity. Together, these findings are consistent with parapatric speciation along an elevational gradient and demonstrate how substantial phenotypic and reproductive-isolation divergence can accumulate despite limited genome-wide differentiation. More broadly, our results indicate that the exceptional biodiversity of oceanic archipelagos may arise not only from diversification among islands (and among mountains within islands) but also from lineage divergence along elevational gradients within mountains themselves. These empirical results support the biological plausibility of parapatric speciation within single mountain systems and highlight an underrecognized mechanism contributing to biodiversity in topographically complex oceanic archipelagos.

Host preference varies widely across mosquitoes, with many species feeding opportunistically on diverse vertebrate hosts, while others show strong fidelity to specific hosts. Anthropophilia, the behavioural preference for feeding on humans, is a defining characteristic of some mosquito species responsible for the transmission of major human diseases, including malaria, dengue, and yellow fever. The evolution of anthropophilia therefore has profound epidemiological implications because increased human biting elevates vectorial capacity and disease transmission potential. However, the ecological and evolutionary mechanisms driving this extreme specialisation have not yet been fully elucidated and remain difficult to unify across laboratory and field studies. Here we present an eco-evolutionary modelling framework that links genetically determined mosquito traits with spatially structured host environments. Our framework integrates innate olfactory sensitivity, blood meal-derived fitness benefits, and spatio-temporal host accessibility. Two complementary indices are introduced: a local specialisation index, capturing short-term ecological feeding strategies, and a co-evolutionary index, capturing long-term genetic coupling between host detection and resource utilisation. Our results demonstrate that host specialisation is not a default evolutionary outcome but an environmentally gated process, which is favoured in resource-poor or temporally varying habitats and strongly filtered by seasonality. The framework yields testable predictions regarding when specialisation emerges, persists, or collapses, with direct implications for predicting vector-borne disease risk in changing environments

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.