Cross-Fostering, i.e., the exchange of eggs or hatchlings, is a widely used technique, to disentangle genetic from environmental effects or to manipulate the clutch size. In most bird species, this manipulation is easily accepted by the social parents, leading to the conclusion that fostering has no detrimental effect. Using a dataset of four cohorts (N=298) of zebra finches (Taeniopygia castanotis), in which we fostered routinely a single egg into another nest of zebra finches, we explored potential short- and long-time effects of fostering. Noteworthy, these experiments were not designed to test this hypothesis. The objective of the egg fostering experiments was to test for parental recognition (Caspers et al. 2017) and mate choice decisions (Golueke 2018). Consequently, the aim of the present study is purely explorative. Our study confirmed previous findings that fostering has no short-term effects on the morphology and growth rates of the chicks, neither in males nor in females. However, we found that fostering has a sex-specific long-term effect. Females originating from fostered eggs had a significantly reduced lifespan compared to those from non-fostered eggs. Conversely, the lifespan of fostered males was similar to that of non-fostered males. All birds were housed in large groups, experiencing the same conditions after nutritional independence (day 35). Therefore, we can only speculate that fostering might result in early developmental stress, which may affect the individual fitness of females later in life, ultimately leading to shorter lifespans. ### Competing Interest Statement The authors have declared no competing interest. Volkswagen Foundation, https://ror.org/03bsmfz84, Freigeist Fellowship to Barbara Caspers

Defensive coloration such as bright colors used to advertise secondary defenses (i.e., aposematic coloration) is very common but also shows high intraspecific variation. Similarly, consistent among-individual differences in behavior (i.e., animal personality) are pervasive in the animal kingdom. Therefore, aposematism and personality could be linked to produce an optimal defensive phenotype, however, this has not formally been investigated. Here, we used the European fire salamander (Salamandra salamandra) to study if personality traits correlate with an individual’s proportion of yellow and relative toxin gland size, using open field tests and observations during husbandry. Four of the five tested behaviors showed low to moderate but significant repeatabilities. However, only the activity during husbandry showed a positive correlation with the relative toxin gland size indicating a potential trade-off between foraging and the costs of chemical defenses. Furthermore, three of the four personality traits showed strong correlations between them, and all personality traits were expressed higher in fire salamanders collected in fall compared to spring, indicating the importance of seasonality effects on fire salamander personality. While we found little evidence for a potential role of trait integration maintaining individual variation in behavior and coloration of fire salamanders, future studies on personality traits in aposematic species should consider the potential of covariation of personality with coloration and/or toxicity. Variation in warning coloration is prevalent in many chemically defended species but represents a paradox nonetheless given the strong selection on this trait. Similarly, animal personality has been identified in different aposematic species but a dedicated test of the covariance of personality traits with warning coloration and/or toxicity has not been conducted. This study has tested the potential of trait integration of personality, warning coloration and toxicity in fire salamanders. We believe this research will motivate future studies on other aposematic species and can open a fascinating new field intersecting the research on warning coloration and animal personality.

Abstract Interdisciplinary research is widely acknowledged as a valuable means of knowledge production and as crucial for addressing complex scientific and societal problems. Interdisciplinary research takes many forms, and this article offers a novel case study of a distinctive form of interdisciplinarity, reflective interdisciplinarity. The article analyzes interdisciplinary research undertaken by philosophers of biology and empirically working biologists studying individualization. This kind of interdisciplinarity is reflective because biology is the target domain that philosophers of biology (and philosophically-minded biologists) reflect upon—as opposed to interdisciplinarity that occurs, for example, when insights from multiple distinct research domains are integrated. To analyze the case study, an analytic framework is developed that distinguishes four dimensions of interdisciplinarity: objects of interdisciplinary research, interdisciplinary research activities, communication of interdisciplinary results, and personal, institutional, and other conditions of interdisciplinary research. The case study shows, first, that reflective interdisciplinarity can involve interdependent, deeply immersive and cooperative interdisciplinary research processes (‘inquiry-embedded integration’) that include a form of knowledge-producing inquiry through which novel epistemic products emerge from joint commitments and goals, shared research questions, and intensive, collaborative discursive practices. The article adapts the notion of a plural subject in characterizing inquiry-embedded integration. Second, the analysis of the case study demonstrates how communicative choices, together with institutional and normative conditions, are partially constitutive of the interdisciplinary process. They shape the content of interdisciplinary products and play decisive roles in interdisciplinary activities such as knowledge integration. Finally, because the case study comprises the authors’ own research processes and publications, the analysis demonstrates that self-reflexive, qualitative analysis of interdisciplinary practice provides a valuable means of theorizing about interdisciplinarity with potential to offer transferable insights.

Animals gather information about their surroundings, including their social environment, using a wide range of sensory modalities. Variation in reception, processing and interpretation of information (cues or signals) can lead to differences in how individuals perceive their local environment. Yet, how individual differences in environmental perception influence behaviour and eco-evolutionary processes has been largely overlooked. Here, we highlight the importance of investigating such individual variation by focusing on perceived density, i.e. the individual's assessment of local conspecific density. This perceived density may diverge from the absolute number of conspecifics in a given area and may vary between individuals depending on their phenotype. Recognizing that not only absolute density but also individuals' perception of that density shapes behaviour offers novel insights into social and ecological dynamics. We summarize how individuals sense their environment and identify factors shaping variation in sensory uptake and processing. We argue that differential perception of the environment affects decision making and in turn impacts the way in which individuals realize their ecological niches, which occurs via three main processes: niche choice, niche conformance and niche construction. We propose that perceived density can have eco-evolutionary consequences by influencing resource use, competition and ultimately selective pressures. Finally, we provide methodological guidelines for studying perceived density, including experimental approaches and potential proxies for quantifying individual differences in environmental perception. By integrating the concept of perceived density into evolutionary and ecological perspectives, we aim at gaining a deeper understanding of how individual variation in environmental perception affects population-wide ecological patterns and evolutionary trajectories.

Urbanisation is a key driver of global environmental change and presents animals with novel stressors and challenges. It can fundamentally influence social behaviour and has the potential to reshape within- and between-species social interactions. Given the role of social behaviour in reproductive fitness and survival, understanding how social interactions change in response to urban conditions is crucial in addressing individual-, population-, and species-level responses to urbanisation, as well as the consequential ecological impacts. Here, we conducted the first systematic review addressing the impact of urbanisation on social systems and interspecific interactions. We synthesise the outcomes of the 227 studies from our literature search, organised across three key topics: ( i ) effects of urban stressors on social behaviour ( N  = 170), ( ii ) social system responses to urban environments ( N  = 75), and ( iii ) the impact of urbanisation on interspecific interactions ( N  = 12). Our review revealed that urbanisation is having a substantial impact on multiple facets of social behaviour, with 92% of studies finding a significant impact. We also identified several biases and gaps in the current literature. For example, 62% of all studies were conducted on birds, and 85% of studies testing urban stressors focused on anthropogenic noise. Given the diversity of animal social systems, there is obvious variation in social responses to urban conditions. However, we offer predictions for how social systems might change as urban environments continue to expand rapidly and suggest guidelines for future research to enhance generalisations across taxa. Our review brings together multiple areas of research, provides timely insights and outlines a framework for a unified and proactive approach to addressing social responses to urbanisation. This represents an essential foundation for anticipating species' responses to urban expansion and guiding effective conservation efforts.

Preen oil - the secretion from the uropygial gland of birds - may have diverse functions in avian reproduction: protection against eggshell bacteria, olfactory crypsis against nest predators and olfactory mate choice. To investigate such functions, we should first characterise variation in preen oil composition, but also confirm that previously described patterns are robust. Replication studies are crucial to test the reproducibility of previous findings, but are rarely undertaken in chemical ecology. Here, we conducted an almost exact replication of a previous study on the chemical composition of preen oil in a wild passerine bird, the pied flycatcher Ficedula hypoleuca. We aimed to estimate the reproducibility of the previous results using larger sample sizes and following a pre-registered analysis. In addition, we explored the ontogeny of preen oil composition by comparing nestling and adult preen oil. In line with previous findings, preen oil composition was similar between breeding partners and not repeatable within individual females across breeding stages. Female preen oil changed across breeding stages more clearly than in the original study (higher richness, diversity and volatility during incubation than nestling-rearing), further refuting a role of preen oil in olfactory crypsis in this species. Unlike the original study, we found no difference in chemical profiles between sexes (nestling-rearing), casting doubt on the proposed role of preen oil as a sex semiochemical in this species. Nestling preen oil differed from adults, was more similar to adult males than to adult females, but was not more similar to parents than to non-parents. We found family chemical signatures, which, along with the breeding pair signature, suggests an influence of the nest environment on preen oil composition. Our study highlights the importance of replication and provides novel insights into the function and development of preen oil.

Despite growing awareness of the importance of researcher diversity, barriers to inclusion and equity persist in science and at academic conferences. As hosts of the 37th International Ethological Congress, "Behaviour 2023", we studied gender disparities that unfold during question-and-answer (Q&A) sessions using observational and experimental behavioural data and surveys. We further used the surveys to investigate broader equity, diversity and inclusivity (EDI) issues at conferences in general. Attendees perceived as women asked fewer questions than those perceived as men because they raised their hands less often to ask questions, and not because they were chosen less often by the session host. Self-reports indicated that self-identified women felt more comfortable asking questions when their own gender was represented (in the audience, by the speaker, and/or by the host) and when the setting was smaller. However, this pattern was not reflected in the observational data as perceived women asked fewer questions regardless of the situation. We report potential reasons why women asked fewer questions using survey data, and experimentally tested whether we could reduce gender disparity in question-asking. Our results indicate that session hosts cannot mitigate the gender disparity in question-asking by actively selecting perceived women to start the Q&A session. We addressed further inclusivity barriers of underrepresented minorities beyond gender in a post-congress survey, which showed that underrepresented minorities did not have a more positive or negative congress experience but did perceive EDI issues as more severe. We conclude by providing recommendations for organising more inclusive scientific events.

Fire salamander larvae are top predators in limnic habitats and feed on a wide spectrum of prey. For our study, we hypothesized that the larvae have a top-down effect on meiofauna, but that this effect varies depending on the habitat the larvae originate from. Therefore, we collected larvae from ponds and streams and placed them individually into microcosms with sediment and benthos. After either one week or two weeks, we removed the larvae and counted the number of nematodes, oligochaetes, and rotifers. Already after week one, the abundance of meiofauna was significantly reduced, as was their biomass. Nematodes were more strongly reduced by pond larvae during this week, while oligochaetes were primarily affected by stream larvae but not by pond larvae, regardless of the sampling time. The rotifers were reduced by larvae from both habitats, but only during the first week and not during the second week. Our findings suggest that fire salamander larvae not only have a top-down effect on pelagic or macrobenthic organisms, as often described in other studies, but can also shape the community of small endobenthic organisms. Depending on habitat-specific adaptations in feeding behavior, morphology, or physiology, these effects can vary.

In a fire salamander population near Bonn, Germany, fire salamander females use two different habitats for larval deposition, i.e. ponds or streams. As it is known that females can discriminate between sexes and habitat types of the males based on chemical cues, we asked whether larvae show habitat specific differences in their surface chemical fingerprints. We took water samples and captured larvae at two ponds and two streams; we used polydimethylsiloxane and TD-GC-MS to collect and analyze samples of the environmental background (i.e. features that were found in the water) as well as from the surface of the larvae. We found a significantly different composition of chemical features in the environmental background samples. Additionally, we found the larvae to carry habitat type-specific chemical features. These finding allow to speculate that the features on the larval surface, if retained across metamorphosis, may be used for assortative mating in the adults, which should be further investigated.

BackgroundConspicuous color patterns are traditionally believed to advertise the toxicity of prey to potential predators. However, many aposematic species show drastic variation in coloration, indicating the possibility of other functions of coloration such as intraspecific communication. To study these other functions, we can investigate the influence of intrinsic (e.g., sex) and external factors (e.g., climate) on color variation. We used the aposematic European fire salamander (Salamandra salamandra) to study drivers of variation in the yellow-to-black ratio of the dorsal coloration based on citizen science data available in a new online database.ResultsOur results suggest a widespread sexual dichromatism in fire salamanders in Germany with males displaying a larger yellow-to-black ratio, i.e. are more yellow, than females. This dichromatism persisted even after correcting for a sex-difference in body shape that accounts for some of the variation in the yellow-to-black ratio. Among six investigated putative drivers of this color variation, the proportion of the aposematic yellow coloration increased with latitude but showed no association with other environmental variables such as temperature or the productivity of the habitat.ConclusionsIntegrating citizen science data in this study enabled a comparison of fire salamander populations across large parts of their distribution and highlights widespread sexual dichromatism. Future studies should further investigate potential mechanisms of mate choice in fire salamanders and other selective forces on coloration such as differential predation pressure between both sexes or the role in crypsis. Lastly, multi-national studies supported by citizen science data could fully unravel the extent of color variation in this species.

Assortative mating can play a major role in the divergence of a population with gene flow. In a fire salamander population near Bonn, Germany, the larvae differ genetically according to the habitat type they are found in, i.e. ponds or streams. Neither sound nor vision are a source for assortative mating in fire salamanders, but it is known that females can discriminate between sexes and habitat types of the males based on chemical cues. The adults share the same terrestrial habitat which leads to the question whether there is a habitat type-specific chemical fingerprint already existing in the larvae that may be used in the adult stage as the base for assortative mating. We took water samples and captured larvae at two ponds and two streams and used polydimethylsiloxane and TD-GC-MS to collect and analyze samples of the environmental background as well as from the surface of the larvae. We found a significantly different composition of chemical features in the environmental background samples. Additionally, we found the larvae to carry habitat type-specific chemical features. These finding allow to speculate that the features on the larval surface may be used for assortative mating in the adults, which should be further investigated. ### Competing Interest Statement The authors have declared no competing interest.

In mammals, olfaction plays a key role in social behaviour, for example, in identifying mating opportunities and potential rivals. However, we still have a limited understanding of how social information is encoded in animal odours, including the social determinants of chemical similarity and diversity. Here, we used gas chromatography to analyse the chemical composition of swabs taken from the facial and anogenital regions of Damaraland mole-rats, Fukomys damarensis, a highly social subterranean mammal that relies almost exclusively on olfactory and tactile social cues. We found no sign of individual identity across the two body areas sampled; samples from the facial region and samples of the anogenital region from the same individual were not similar to each other, suggesting that these regions carry different information. However, chemical profiles varied significantly by sex and breeding status; female breeders differed from nonbreeders in their anogenital profiles and had higher chemical diversity in their facial profiles compared with both males and nonbreeders. Interestingly, we found no signals of social group identity. Instead, individual identity may be conveyed through signature mixes that are learned through frequent contact, rather than through specific odours associated with genetic kinship or social group membership. Our results highlight the complexity of chemical communication systems in social species and suggest that signals of group level identity are not necessary for behavioural responses based on group membership. (c) 2024 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/ by/4.0/).

A variety of animals perform coprophagy to obtain nutrients but so far coprophagy remains unreported in bats. Here, we describe the existence of coprophagy and urine consumption among male greater short-nosed fruit bats Cynopterus sphinx . Males usually deposited their excreta outside their tents, but engaged in coprophagy and/or urine consumption, on occasions when the excreta fell inside the tent. Each coprophagous bout consists of licking the excreta, followed by chewing with occasional vigorous shaking of the head. We suspect that the odour of the excreta might interfere with the male-specific chemosignals smeared on the tent. Therefore, we hypothesise that the removal of excreta from the tent is essential and the bats engage in coprophagy opportunistically. However, more studies are needed to understand the functions of coprophagy in greater short-nosed fruit bats.

Amphibians exploit a variety of aquatic habitats for breeding, with local environmental factors shaping physiology, behavior, and morphology. Ponds and streams are representatives of contrasting environments. Due to the low ecological stability, reduced oxygen levels, and limited food availability, higher predation pressure and intra- and interspecific competition, ponds represent a less favorable habitat. Consequently, pond-dwelling larvae are expected to exhibit morphological traits that improve food intake and assimilation. Here, we demonstrate intraspecific morphological variation in early fire salamander larvae from a single population, where offspring are deposited in both pond and stream habitats. Contrary to expectations, larvae from streams exhibited significantly more vomerine and palatine teeth, shorter gill rakers and reduced inter-raker distances. These features may confer advantages in prey suction, handling, and retaining smaller prey items within the oral cavity. However, the observed variation in head shape, with pond-dwelling larvae having wider heads, aligns with the characteristic typically associated with cannibalism in food-poor environments. The gut length did not differ between the two types. Our results suggest habitat-driven intraspecific adaptations in feeding morphology in fire salamander larvae. While many previous studies did not focus on feeding morphology, our findings shed light on hidden or overlooked trait variation between pond- and stream-dwelling individuals. This could accelerate future studies by investigating the underlying causes and effects of these traits on feeding behavior. Such studies may improve our understanding of how environmental changes drive morphological adaptations and influence amphibian survival.

Olfactory kin discrimination occurs in many animal taxa, but its potential contribution to commonly observed kin-biased behaviours in birds has rarely been tested. In a previous odour discrimination experiment, 7-day-old blue tit, Cyanistes caeruleus, nestlings showed stronger begging responses to olfactory cues from conspecific nestlings from other nests than from their own. The authors hypothesized olfaction to mediate kin-biased sibling competition in nests with varying relatedness due to extrapair paternity. In the present study, we aimed to test this hypothesis. We therefore replicated the previous experiment with a crucial modification: we cross-fostered two nestlings of each brood the day after hatching. This allowed us to test for olfactory kin discrimination when nestmates differed in relatedness (due to being cross-fostered) but not in familiarity. We ascertained the relatedness of nestlings using genetic parentage assignment. We preregistered our research plan with the Open Science Framework (OSF) to increase research transparency and reduce researcher degrees of freedom. We found that nestlings did not differ in their begging responses to related versus unrelated (cross-fostered) nestmates' odours, indicating that nestlings do not discriminate kin from nonkin odours when these are both familiar. Moreover, in an exploratory analysis, cross-fostered nestlings did not differ in survival or size from their non-cross-fostered nestmates shortly before fledging, indicating that the presence of unrelated individuals did not affect the distribution of parental care in the nest. In conclusion, we found no evidence for olfactory kin discrimination in begging blue tit nestlings. (c) 2025 The Authors. Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/ by/4.0/).

Cities are expanding globally, exposing animals to novel ecological conditions that can alter the frequency and nature of their social interactions. Urban habitat features, such as built infrastructure and patchy resource distributions, can constrain movement and promote aggregation, potentially increasing encounters among conspecifics and introducing unique social challenges. Understanding how urbanization affects social behaviour is therefore crucial. However, these effects remain poorly understood, and studies on solitary or non-gregarious species are particularly scarce. Here, we investigate how urbanization influences social behaviour in the common wall lizard (Podarcis muralis), a territorial species and successful urban dweller. We constructed social networks using proximity-based association data from urban and non-urban lizard populations. Urban lizards had more social connections, stronger associations and were observed in more associations overall. These differences were not explained by variation in population density. We propose that spatial constraints and resource heterogeneity in urban habitats may enhance social tolerance. Our results reveal that urbanization can reshape social behaviour even in less gregarious species and suggest that shifts in social strategies may facilitate persistence in urban landscapes.

Nature and natural elements have been proven to promote mental health and well‐being but there is a research gap regarding psychiatric environments, despite the importance of psychiatric hospitals for mental health. It is essential to understand the particular characteristics relevant for patients in these settings. We conducted semi‐structured in‐situ interviews with patients on greenspaces around two psychiatric hospitals in Germany to identify positive and negative perceptions of natural elements and amenities. The perceptions are embedded into the perceived sensory dimensions (PSD) concept and combined with mapping data to achieve in‐depth descriptions of the psychiatric greenspaces. The results show the importance of greenspaces for psychiatric patients and their positive sentiments towards those spaces. The greenspaces have to provide for diverse needs and support different uses of patients at various stages of recovery. User‐sensitive and multifunctional greenspace design can reconcile conflicts of interest and contribute to a therapeutic landscape. We propose that the PSD can complement the concept of Therapeutic Landscapes, especially when the focus is on natural environments. ‘Policy implications’: Multifunctional greenspaces around psychiatric clinics can help patients to relax, enjoy the connection to nature and other people, and further their well‐being. The spaces have to be carefully designed and can potentially have a therapeutic effect on people with mental‐health problems and simultaneously enhance nature conservation and biodiversity. Read the free Plain Language Summary for this article on the Journal blog.

Tonic immobility is an antipredator defence in which the prey animal remains motionless after physical contact with the predator, pretending to be dead. This behaviour has been observed among a variety of taxa but has received only little attention in amphibian larvae. During our field studies with fire salamander larvae, we observed that larvae from different habitats display tonic immobility after handling. In our study site, we find larvae in pond and stream habitats, that differ in several aspects such as their stress response and their risk-taking behaviour, likely caused by the very different habitat conditions. We measured the time that the tonic immobility behaviour was displayed but found no difference between larvae from the two habitat types. Likewise, we also found no correlation between the size of the larvae and the duration of displaying the behaviour. In conclusion, we found that fire salamander larvae show tonic immobility, but found no evidence that the different habitat conditions influence the tonic immobility behaviour.

The larvae of the European fire salamander (Salamandra salamandra) can inhabit two different habitats: streams and ponds. Streams are characterized by lower predation risks and higher food availability. Thus, ponds are considered a less suitable habitat. To investigate the differential impacts of these two habitats on larval physiology, we measured the stress response of larvae. After successfully validating the measure of water-borne corticosterone release rates in fire salamander larvae, we measured the baseline and stress-induced corticosterone of 64 larvae from ponds and streams in the field. We found that larvae in ponds have a higher baseline and stress-induced corticosterone levels. Additionally, we performed a reciprocal transplant experiment (RTE) and tested whether larvae can adapt their stress responses to changing habitats. After two weeks, we did not find an increase in corticosterone levels when comparing stress-induced corticosterone values with baseline corticosterone values in larvae transferred into ponds, irrespective of their habitat of origin. However, larvae transferred into streams still exhibited an increase in the stress-induced corticosterone response in comparison with the baseline values. These results show that non-invasive hormone measurements can provide information on the habitat quality and potential adaptation and thus emphasize the potential for its use in conservation efforts.

Salamander larvae can impose a great impact on benthic organisms as they are keystone predators. Previous gut analysis revealed that their diet consists of invertebrates from the wa-ter column and benthic taxa. The impact on benthic communities, especially on meiofauna, is, however, not well understood yet, as these organisms cannot be detected reliably in the gut. To avoid this, we investigated the impact of fire salamander larvae on meiofauna by ana-lyzing consumed prey items from a microcosm. Fire salamander larvae can be found in dif-ferent habitats, such as streams and ponds. To test the impact of fire salamander larvae on the meiofauna and to test whether the effects differ in larvae from different habitats, we col-lected 20 larvae from both, ponds and streams from the wild and transferred them to the lab. We placed each larva individually into a microcosm with sediment and benthic organisms. We also added control samples without larvae. We kept the larvae either for one week or two weeks and counted the number of nematodes, oligochaetes and rotifers after we removed the larvae. Fire salamander larvae significantly reduced the meiofauna. Furthermore, fire salamander larvae show habitat specific differences, as nematodes were influenced more strongly by pond larvae during the first week. Oligochaetes were mainly influenced by stream larvae, but not by pond larvae, independent of the duration of the experiment. Rotifers were influenced by larvae from both habitats, but only during the first week and not during the second week.Our findings suggest that fire salamander larvae are able to reduce the meiofauna through predation. Furthermore, our finding indicates an adaptation towards the different environmen-tal conditions in the habitat of origin.