This paper is the product of an international workshop aiming to make progress in our general understanding of adaptation. We met from 5-7 February 2025 in Hannover (Germany), funded by the foundation “Volkswagen Stiftung”. For our group of theoretical and empirical biologists, social scientists, and philosophers of science we set up a program to facilitate communication and collaboration between people with diverse backgrounds and viewpoints. The overall goal that the scientific community should strive for, we think, should be to obtain concrete conceptual, analytical, and experimental tools for researchers to understand and study all the processes of adaptation, and thereby the global phenomenon of adaptation. Our workshop aimed to contribute to this overall goal. For this, we discussed the relative strengths and weaknesses of different approaches, identified areas of consensus, identified areas of disagreement, and resolved (sub)areas of disagreement. Here we briefly report on the progress we have made during the workshop. We lay out the problem, discuss terminology, present a visual framework to think about adaptation, suggest useful approaches for its study, and provide recommendations for practitioners and policymakers.

Variation in social traits can be attributed to direct individual effects (DIEs) of the focal individual and indirect individual effects (IIEs) due to its social partners eliciting behavioural change, analogous to indirect genetic effects. Indirect effects affect the expressed phenotypic variation upon which selection can act, especially when they covary with direct effects, providing a potential explanation for slower or faster evolution than predicted by classic theory. However, little is known about the among-trait covariance of DIEs and IIEs, or whether IIEs are consistent across time and context and how this can affect evolutionary dynamics. Here we tested game theoretical predictions of producer-scrounger tactic use during social foraging games within a DIE-IIE framework in wild house sparrows (Passer domesticus). We used automated high-throughput phenotyping, where we assayed individuals repeatedly against different social partners. We provide evidence for small IIEs in producer-scrounger behaviour, and show high cross-year consistency. We found tight among-trait covariance, which is expected to impose strong constraints on the evolution of the DIEs and IIEs. Indirect effects decreased the potential heritable variation in producing and scrounging behaviour, which appear temporally stable. Overall, these effects may provide a potential mechanism for the long-term maintenance of stable social foraging strategies.

Social interactions mediate the phenotypic expression of fitness-relevant traits. The expression of such labile social traits includes three distinct components: an individual's mean trait value (direct effect), its social responsiveness, and its social impact (indirect effects). Traditional methods, such as variance-partitioning or trait-based models, usually only partition individual variation into direct and indirect effects. However, individual variation in social responsiveness and its covariation with direct effects and social impact will affect responses to selection. To date, no studies have explored the performance of models that allow the decomposition of responsiveness from impact. Here, we describe a model for studying variation in phenotypic expression caused by social interactions, and we use simulations to explore its performance under various experimental designs. Our analyses show that with adequate total sample sizes ($\ge \! 3,200$), variance components are estimated accurately across all study designs. In contrast, covariance estimation would benefit most from including more unique individuals, followed by more unique social partners per individual, whereas repeated interactions with the same partners added the least improvement to the covariance estimation. We also found that failing to model individual variation in responsiveness, and neglecting measurement error, increases bias and imprecision in trait-based approaches. Hence, disregarding individual variation in responsiveness would ignore a key component of social behaviour, and hamper our ability to acquire unbiased estimates of indirect genetic or social effects.

Variation in traits expressed during social interactions can be attributed to direct individual effects (DIEs) of the focal individual's identity and indirect individual effects (IIEs) of social partner identity. When of genetic origin and covarying with direct effects, indirect effects affect the expressed variation upon which selection can act; this can explain why evolution is slower or faster than predicted by classic theory. Little is known about how DIEs and IIEs covary across traits, even though such relationships should affect micro-evolutionary trajectories. We also do not know whether IIEs change over time or contexts. Here, we tested game-theoretical predictions of producer-scrounger tactic use during social foraging games in wild house sparrows (Passer domesticus). We used automated high-throughput phenotyping, where we assayed individuals repeatedly against different social partners. We provide evidence for IIEs and DIEs in producer-scrounger behaviour, and high cross-year repeatability. Both IEEs and DIEs were correlated among traits: producers depressed producing-but elicited increased scrounging-in others, and vice versa. This structure likely strongly constrains behavioural evolution. Indirect effects decreased the phenotypic variation in both behaviours. IEE-DIE correlations among and within traits may thus explain the long-term maintenance of stable social foraging strategies.

Repeatability, more generally known as intraclass correlation, represents an important quantity of interest in many scientific fields. It represents a metric for summarizing variance decomposition to identify sources of variation in an outcome of interest (e.g. organismal traits). The estimation of variance components is often achieved through linear mixed-effects models or their extension, generalized linear mixed-effects models. Here, we review variants of calculating repeatabilities from mixed-effects models for a variety of conditions and applications. We also recommend which variant might be appropriate under what conditions, focusing on behavioural biology/ecology examples. However, the decision is ultimately with the researcher, since it depends upon their research question, and there is no one-size-fits-all solution. We also highlight the importance of the scope of inference, which affects how repeatabilities are used and interpreted. We recommend transparent reporting of statistical results, including all variance components, which are the building blocks of repeatability. This review aims to assist empiricists in choosing an appropriate repeatability variant and interpretation concerning their questions and the scope of inference.

Density-dependent effects on individual vital rates play a critical role in determining a population's equilibrium size and rate of return following stochastic disturbances. In fragmented landscapes, spatial variation in density dependence across life-history stages causes heterogeneity in limitations of population growth, which influences extinction-persistence dynamics. As habitat loss leads to increased landscape fragmentation, understanding the factors driving variation in the strength of density dependence across life-history stages is necessary to assess population viability and guide conservation strategies. We quantified spatial variation in the strength of density dependence across life-history stages in a house sparrow metapopulation encompassing 11 islands in northern Norway, collected from 1994 to 2022. Our findings reveal that increased density negatively affects the demographic contributions of individuals to population growth. We found a pronounced negative effect of density, especially on survival, which was stronger in juveniles than in adults. In contrast, there was no clear impact on nestling production. We recorded stronger density dependence on islands further from the mainland, likely due to harsher environmental conditions during winter. Furthermore, the density dependence of survival was stronger in juveniles relative to adults on islands where adult survival contributed more to population growth than on islands where juvenile survival was more important. This indicates that the strength of density dependence at different life-history stages was related to each island's average pace-of-life. Thus, this study demonstrates potential links between spatial variation in density dependence across life-history stages, population dynamics, and life-history evolution.

The phenotypes of individuals within a population create a dynamic social environment that influences phenotypic selection and population growth. Fluctuations in the frequencies of these phenotypes can influence population mean fitness and the relative fitness of phenotypes, and thus impact both population dynamics and phenotypic evolution. Various theoretical frameworks have been used to study the impact of frequency dependence on ecological and evolutionary dynamics. However, their diversity and mathematical complexity have obscured the relationship between theoretical models and empirical work. To bridge this gap, we discuss the effects of frequency dependence on phenotypic selection and population growth from a statistical perspective. We classify frequency-dependent effects on fitness according to their additive, relative or multiplicative effects, and explore how their interaction with population density can affect the eco-evolutionary dynamics of continuous traits. We show how these different effects can be mapped onto the parameters of simple linear regression models and derive how their magnitude is expected to affect the population carrying capacity and equilibrium mean phenotype. We then use individual-based simulations to complement our analytical results and demonstrate that quantifying frequency-dependent effects on fitness is key for understanding how populations will respond to environmental change.

ABSTRACT Nocturnal animals inhabiting northern latitudes face prolonged periods of reduced foraging times in summer due to short light nights. The energetic challenges of reduced foraging times are further heightened in reproductive mammals that allocate substantial resources to offspring care with peak energy demands in mid‐summer. However, little is known about responses to variation in natural light conditions at high latitudes in light‐averse species, such as slow‐flying gleaning bats, especially during reproduction. Here, we investigate the impacts of natural light levels and other environmental conditions (i.e., temperature, rain and wind) on individual‐level activity patterns (emergence time, return time, proportion of night utilised) in reproductive and non‐reproductive female brown long‐eared bats, Plecotus auritus (Nind = 27) in Norway (60.1° N) collected across three summers (2019–2021). We found that bats delayed the start of evening foraging trips on lighter nights, typically emerging from the roost only when light levels decreased below 5 lux, likely because higher light levels are associated with increased predation risk. However, no such effect was found in morning return times to the roost, for which bats showed greater light tolerance. Lactating females took apparently higher risks and left the roost approximately 20 min earlier than non‐reproductive females, presumably because of their greater energetic requirements. They also spent a larger proportion of the night away from the roost compared to pregnant and non‐reproductive individuals, although this proportion was influenced by variation in environmental conditions, such as temperature, rainfall and windspeed. Our results highlight the dynamic nature of responses in light‐averse bats balancing risks of predation against foraging gains during reproduction at northern latitudes. Reduced foraging times during short northern nights may thus represent a hard constraint to range expansion in slow‐flying gleaning bats, even if other environmental conditions improve with climate change.

The house sparrow (Passer domesticus) is a small passerine known to be highly sedentary. Throughout a 30-year capture-mark-recapture study, we have obtained occasional reports of recoveries far outside our main metapopulation study system, documenting unusually long dispersal distances. Our records constitute the highest occurrence of long-distance dispersal events recorded for this species in Scandinavia. Such long-distance dispersals radically change the predicted distribution of dispersal distances and connectedness for our study metapopulation. Moreover, it reveals a much greater potential for colonization than formerly recorded for the house sparrow, which is an invasive species across four continents. These rare and occasional long-distance dispersal events are challenging to document but may have important implications for the genetic composition of small and isolated populations and for our understanding of dispersal ecology and evolution.

Changes in avian breeding phenology are among the most apparent responses to climate change in free-ranging populations. A key question is whether populations will be able to keep up with the expected rates of environmental change. There is a large body of research on the mechanisms by which avian lay-dates track temperature change and the consequences of (mal)adaptation on population persistence. Often overlooked is the role of males, which can influence the lay-date of their mate through their effect on the prelaying environment. We explore how social plasticity causing male indirect genetic effects can help or hinder population persistence when female genes underpinning lay-date and male genes influencing female’s timing of reproduction both respond to climate-mediated selection. We extend quantitative genetic moving optimum models to predict the consequences of social plasticity on the maximum sustainable rate of temperature change, and evaluate our model using a combination of simulated data and empirical estimates from the literature. Our results suggest that predictions for population persistence may be biased if indirect genetic effects and cross-sex genetic correlations are not considered and that the extent of this bias depends on sex differences in how environmental change affects the optimal timing of reproduction. Our model highlights that more empirical work is needed to understand sex-specific effects of environmental change on phenology and the fitness consequences for population dynamics. While we discuss our results exclusively in the context of avian breeding phenology, the approach we take here can be generalized to many different contexts and types of social interaction.

Maximising reproductive success is crucial to animal production systems, particularly in meeting global demands for animal products and improving commercially important traits. However, while social interactions and mating strategies are known to influence reproductive success in wild populations, their consideration in agricultural systems remains limited. Using an interdisciplinary framework that combines concepts from behavioural ecology and quantitative genetics in an animal breeding context, we investigated the role of sperm limitation and polygynous mating strategies (female polyandry, male monopolisation of females and male polygamy) in limiting female reproductive success in farmed Pekin ducks ( Anas platyrthynchos domestica ). We assessed the impact of these behaviours on chick production and quantified their genetic and environmental (co)variance. Our results revealed that the number of dam mates positively influenced chick production in female ducks. However, contrary to our expectation, skew in chick paternity (our measure of male monopolisation) was associated with increased female chick production, challenging the hypothesis that male monopolisation limits the sperm available to females and reduces their reproductive success. We found no evidence that male polygamy led to decreased female chick production. Genetic analysis revealed that female mate number and reproductive skew exhibit genetic variance, providing opportunities for targeted selection to enhance chick production. However, there was a negative genetic association between female polyandry and skew in chick paternity, suggesting a trade-off between these traits that would need to be considered in future selection programmes. Our findings highlight how concepts from behavioural ecology can be incorporated into breeding programmes, providing new opportunities to develop effective and sustainable breeding strategies. ### Competing Interest Statement The authors have declared no competing interest.

The causes of variation in multiple paternity (MP) in socially monogamous birds have received considerable attention. Traits like age and age-dependent morphology have been shown to be important for the distribution of MP. However, most studies fail to separate between the different processes underlying age-effects on MP, such as selective disappearance (that is, higher survival probability for certain phenotypes) versus individually plastic changes in morphology and mating behaviour with age. Using Bayesian multi-level path analysis on a long-term dataset from a house sparrow ( Passer domesticus ) metapopulation, we disentangle the effects of age on male and female MP, both independently from and through morphological traits. Age was a key determinant of MP for males when accounting for morphological traits, with males having MP more often as they get older (plastic component of age) and not due to disappearance of males unsuccessful to get MP (selective disappearance), challenging ‘good genes’ explanations for MP. Conversely, selective disappearance explained the higher levels of MP in older females, suggesting that higher quality females have MP more often than lower quality females. We show how an appropriate statistical decomposition of age-components and age-dependent processes provides insights to the biological drivers of MP.### Competing Interest StatementThe authors have declared no competing interest.

Assessing the biological relevance of variance components estimated using MCMC-based mixed-effects models is not straightforward. Variance estimates are constrained to be greater than zero and their posterior distributions are often asymmetric. Different measures of central tendency for these distributions can therefore be very different, and credible intervals cannot overlap zero, making it difficult to assess the the size and statistical support for among-group variance. This is often done through visual inspection of the whole posterior distribution, and so relies on subjective decisions for interpretation. We use simulations to demonstrate the difficulties of summarising the posterior distributions of variance estimates from MCMC-based models. We compare commonly used summary statistics of posterior distributions of variance components showing that the posterior median is predominantly the least biased. We also describe different methods for generating null distributions (i.e. a distribution of effect sizes that would be obtained if there was no among-group variance) that can be used to aid in the interpretation of variance estimates. We further show how null distributions could be used to derive a p-value that provides complimentary information to the commonly presented measures of central tendency and uncertainty and also facilitates the implementation of power analyses within an MCMC framework.

Strong seasonality at high latitudes represents a major challenge for many endotherms as they must balance survival and reproduction in an environment that varies widely in food availability and temperature. To avoid energetic mismatches caused by limited foraging time and stochastic weather conditions, bats employ the energy-saving state of torpor during summer to save accumulated energy reserves. However, at high-latitude small-bats-in-summer face a particular challenge: as nocturnal foragers, they rely on the darkness at night to avoid predators and/or interspecific competition, but live in an environment with short, light summer nights, and even a lack of true night at the northernmost distributions of some bat species. To predict optimal behaviour in relation to latitudinal variation in diurnal cycles, we constructed a stochastic dynamic programming model of bats living at high latitudes. Using a stochastic dynamic programming framework with values that are representative for our study system, we show that individual energetic reserves are a strong driver of daytime use of torpor and night-time foraging behaviour alike, with these linked effects being both temperature- and photoperiod-dependent. We further used the model to predict survival probabilities at five locations across a latitudinal gradient (60.1° N to 70.9° N), finding that combinations of photoperiod and temperature conditions limited population distributions in the model. To verify our model results, we compared predictions for optimal decisions with our own empirical data collected on northern bats (Eptesicus nilssonii) from two latitudes in Norway. The similarities between our predictions and observations provide strong evidence that this model framework incorporates the most important drivers of diurnal decision-making in bat physiology and behaviour. Comparing empirical data and model predictions also revealed that bats facing lighter night conditions further north restrict their mass gain, which strengthens the hypothesis that predation threat is a main driver of bat nocturnality. Our model findings regarding state-dependent decisions in bats should contribute to the understanding of how bats cope with the summer challenges at high latitudes.

Telomeres, the nucleotide sequences that protect the ends of eukaryotic chromosomes, shorten with each cell division and telomere loss may be influenced by environmental factors. Telomere length (TL) decreases with age in several species, but little is known about the sources of genetic and environmental variation in the change in TL (∆TL) in wild animals. In this study, we tracked changes in TL throughout the natural lifespan (from a few months to almost 9 years) of free-living house sparrows (Passer domesticus) in two different island populations. TL was measured in nestlings and subsequently up to four times during their lifetime. TL generally decreased with age (senescence), but we also observed instances of telomere lengthening within individuals. We found some evidence for selective disappearance of individuals with shorter telomeres through life. Early-life TL positively predicted later-life TL, but the within-individual repeatability in TL was low (9.2%). Using genetic pedigrees, we found a moderate heritability of ∆TL (h2 = 0.21), which was higher than the heritabilities of early-life TL (h2 = 0.14) and later-life TL measurements (h2 = 0.15). Cohort effects explained considerable proportions of variation in early-life TL (60%), later-life TL (53%), and ∆TL (37%), which suggests persistent impacts of the early-life environment on lifelong telomere dynamics. Individual changes in TL were independent of early-life TL. Finally, there was weak evidence for population differences in ∆TL that may be linked to ecological differences in habitat types. Combined, our results show that individual telomere biology is highly dynamic and influenced by both genetic and environmental variation in natural conditions.

The density and frequencies of interacting phenotypes create a type of environment which affects both phenotypic selection and population growth. Fluctuations in population density create temporal variation in population mean fitness, driving population dynamics, while fluctuations in phenotypic frequencies create variation in the relative fitness of phenotypes through frequency-dependent selection. Different modelling frameworks have been used to study these (social) environment effects and the eco-evolutionary dynamics produced by their interaction. However, the diversity and mathematical complexity of these models can represent an obstacle for empiricists aiming to study the social factors shaping the eco-evolutionary dynamics of natural populations. Here, we reformulate components of these models using generalized linear regression equations to provide a statistical decomposition of how different frequency- and density-dependent processes influence phenotypic selection, population growth, and the expected equilibrium density and mean phenotype of a population. We complement these results with individual-based simulations to illustrate how quantifying the different ways the social environment affects an individual's fitness can improve our understanding of the feedback that links the evolutionary dynamics of phenotypes with the carrying capacity of natural populations.

Closer integration between behavioral ecology and quantitative genetics has resulted in a recent increase in studies partitioning sources of variation in labile traits. Repeatable between-individual differences are commonly documented, and their existence is generally explained using adaptive arguments, implying that selection has shaped variation at the among- and within-individual level. However, predicting the expected pattern of non-adaptive phenotypic variation around an optimal phenotypic value is difficult, hampering our ability to provide quantitative assessments of the adaptive nature of observed patterns of phenotypic variation within a population. We argue that estimating the strength of selection on trait variation among and within individuals provides a way to test adaptive theory concerned with phenotypic variation. To achieve this aim, we describe a nonlinear selection analysis that enables the study of the selective pressures on trait means and their among- and within-individual variation. By describing an integrative approach for studying the strength of selection on phenotypic variation at different levels, we hope to stimulate empirical studies investigating the ecological factors that can shape the repeatability, heritability, and coefficients of variation of labile and other repeatedly expressed traits.

A recent study on the diving behaviour of European shags (Gulosus aristotelis (L.)) foraging in kelp forests off rocky coasts of Norway suggests surface durations are related only to the duration of the preceding dive, and hence are being used for respiratory recovery. These results contrast with earlier reports concerning shags foraging in highly tidal estuarine waters off the coast of Lundy Island, SW England, where there was a stronger relationship between dive durations and preceding pre-dive surface durations, suggesting the use of preparatory variation in oxygen loading. These two datasets were collected using different methods, and statistically analysed in quite different ways, so the contrasting results here could be due to different methodologies rather than the ecological differences between the two foraging environments. Here, we re-analyse the two datasets using similar statistical methods, and we confirm the contrasting results produced by the two datasets. We, therefore, conclude that shag breathing strategies do differ between these two marine environments, presumably reflecting adaptive facultatively plastic responses to differences in predictability of foraging dive durations. Off the Norwegian coast, unpredictable variation in the depth and availability of pelagic prey in complex environments may require more responsive post-dive respiratory recovery on the surface after each dive. In the more uniform English near-shore environment, however, pre-dive preparatory oxygen loading customised to match predictable dive-to-dive variation in benthic prey depths and foraging durations, may be more time and energy efficient.

Extra-pair paternity (EPP) influences the relatedness between social parents and offspring. Therefore, one might expect the level of EPP to influence levels of paternal investment. Here, we investigated the effect of variation in EPP rates on male contributions to parental care within a phylogenetic framework of up to 271 primarily socially monogamous bird species representing 85 families. We used proportion of male provisioning and occurrence of male incubation and nestbuilding as measures of paternal care. We tested the relationship between EPP rates and different components of paternal care while controlling for various life-history traits, namely lifespan, clutch size, and body mass in a phylogenetic path analysis framework. EPP was significantly negatively associated with the occurrence (i.e., whether males participate or not) of male nestbuilding and incubation, but not with the relative amount (proportion) of nestbuilding or incubation performed by the male. Importantly, the proportion of provisioning and biomass delivery by males was clearly negatively associated with EPP. These analyses thus confirm that the effect of EPP on proportion of provisioning visits by males is similar to proportion of biomass delivery, an often assumed but rarely tested assumption. Analysing only Passerine species provided similar results, although only proportion of provisioning was significantly negatively associated with EPP. This study, therefore, provides the most comprehensive support to date of a negative relationship between EPP and paternal care across species. However, a causal relationship between EPP and paternal care cannot necessarily be concluded. We also identify key methodological improvements for future research within the topic.

Social evolution and the dynamics of social interactions have previously been studied under the frameworks of quantitative genetics and behavioural ecology. In quantitative genetics, indirect genetic effects of social partners on the socially plastic phenotypes of focal individuals typically lack crucial detail already included in treatments of social plasticity in behavioural ecology. Specifically, whilst focal individuals (e.g. receivers) may show variation in their 'responsiveness' to the social environment, individual social partners (e.g. signallers) may have a differential 'impact' on focal phenotypes. Here we propose an integrative framework, that highlights the distinction between responsiveness versus impact in indirect genetic effects for a range of behavioural traits. We describe impact and responsiveness using a reaction norm approach and provide statistical models for the assessment of these effects of focal and social partner identity in different types of social interactions. By providing such a framework, we hope to stimulate future quantitative research investigating the causes and consequences of social interactions on phenotypic evolution.