Conspecific Intruders Research Articles

Context-dependent multimodal behaviour in a coral reef fish.

Animals are expected to respond flexibly to changing circumstances, with multimodal signalling providing potential plasticity in social interactions. While numerous studies have documented context-dependent behavioural trade-offs in terrestrial species, far less work has considered such decision-making in fish, especially in natural conditions. Coral reef ecosystems host 25% of all known marine species, making them hotbeds of competition and predation. We conducted experiments with wild Ambon damselfish (Pomacentrus amboinensis) to investigate context-dependent responses to a conspecific intruder; specifically, how nest defence is influenced by an elevated predation risk. We found that nest-defending male Ambon damselfish responded aggressively to a conspecific intruder, spending less time sheltering and more time interacting, as well as signalling both visually and acoustically. In the presence of a model predator compared to a model herbivore, males spent less time interacting with the intruder, with a tendency towards reduced investment in visual displays compensated for by an increase in acoustic signalling instead. We therefore provide ecologically valid evidence that the context experienced by an individual can affect its behavioural responses and multimodal displays towards conspecific threats.

Elevational differences in territory defence response in native (endemic and non-endemic) forest birds on Viti Levu Island, Fiji

ABSTRACT Experimental approaches to measure territory defence can be used to interpret the relative magnitude of intraspecific aggression in different ecological contexts. The aim of this study is to examine the response of Fijian forest bird species to conspecific intruders using playback of previously recorded songs along an elevational gradient. We address two questions: Do different foraging guilds in Fiji’s forest birds show different patterns of territorial defence behaviour towards conspecific intruders? Secondly, do Fiji’s forest birds show stronger territorial defence at higher elevations? We explore these questions across two foraging guilds (insectivore and omnivore) on Viti Levu Island, Fiji. All four focal species had a stronger response at the highest elevation, and the endemic insectivore had the strongest response. Perhaps high elevation insects are less abundant and hence increased aggressiveness is favoured to defend the territory, and/or perhaps in general more aggressive birds colonise higher elevation areas, both ideas that require further testing. This study provides first insights into response to conspecifics in Fiji’s forest birds that can be explored in relation to avian behaviour also in the context of human activity, human disturbance and threats to the persistence of birds across elevational gradients.

Functional neurogenomic responses to acoustic threats, including a heterospecific referential alarm call and its referent, in the auditory forebrain of red-winged blackbirds

In animal communication, functionally referential alarm calls elicit the same behavioral responses as their referents, despite their typically distinct bioacoustic traits. Yet the auditory forebrain in at least one songbird species, the black-capped chickadee Poecile atricapillus, responds similarly to threat calls and their referent predatory owl calls, as assessed by immediate early gene responses in the secondary auditory forebrain nuclei. Whether and where in the brain such perceptual and cognitive equivalence is processed remains to be understood in most other avian systems. Here, we studied the functional neurogenomic (non-) equivalence of acoustic threat stimuli perception by the red-winged blackbird Agelaius phoeniceus in response to the actual calls of the obligate brood parasitic brown-headed cowbird Molothrus ater and the referential anti-parasitic alarm calls of the yellow warbler Setophaga petechia, upon which the blackbird is known to eavesdrop. Using RNA-sequencing from neural tissue in the auditory lobule (primary and secondary auditory nuclei combined), in contrast to previous findings, we found significant differences in the gene expression profiles of both an immediate early gene, ZENK (egr-1), and other song-system relevant gene-products in blackbirds responding to cowbird vs. warbler calls. In turn, direct cues of threats (including conspecific intruder calls and nest-predator calls) elicited higher ZENK and other differential gene expression patterns compared to harmless heterospecific calls. These patterns are consistent with a perceptual non-equivalence in the auditory forebrain of adult male red-winged blackbirds in response to referential calls and the calls of their referents.

Descending neurons of the hoverfly respond to pursuits of artificial targets

Many animals use motion vision information to control dynamic behaviors. Predatory animals, for example, show an exquisite ability to detect rapidly moving prey, followed by pursuit and capture. Such target detection is not only used by predators but is also important in conspecific interactions, such as for male hoverflies defending their territories against conspecific intruders. Visual target detection is believed to be subserved by specialized target-tuned neurons found in a range of species, including vertebrates and arthropods. However, how these target-tuned neurons respond to actual pursuit trajectories is currently not well understood. To redress this, we recorded extracellularly from target-selective descending neurons (TSDNs) in male Eristalis tenax hoverflies. We show that they have dorso-frontal receptive fields with a preferred direction up and away from the visual midline. We reconstructed visual flow fields as experienced during pursuits of artificial targets (black beads). We recorded TSDN responses to six reconstructed pursuits and found that each neuron responded consistently at remarkably specific time points but that these time points differed between neurons. We found that the observed spike probability was correlated with the spike probability predicted from each neuron's receptive field and size tuning. Interestingly, however, the overall response rate was low, with individual neurons responding to only a small part of each reconstructed pursuit. In contrast, the TSDN population responded to substantially larger proportions of the pursuits but with lower probability. This large variation between neurons could be useful if different neurons control different parts of the behavioral output.

Behavioral convergence in defense behaviors in pair bonded individuals correlates with neuroendocrine receptors in the medial amygdala

Monogamous, pair-bonded animals coordinate intra-pair behavior for spatially separated challenges including territorial defense and nest attendance. Paired California mice, a monogamous, territorial and biparental species, approach intruders together or separately, but often express behavioral convergence across intruder challenges. To gain a more systems-wide perspective of potential mechanisms contributing to behavioral convergence across two conspecific intruder challenges, we conducted an exploratory study correlating behavior and receptor mRNA (Days 10 and 17 post-pairing). We examined associations between convergence variability in pair time for intruder-oriented behaviors with a pair mRNA index for oxytocin (OXTR), androgen (AR), and estrogen alpha (ERα) receptors within the medial amygdala (MeA) and the anterior olfactory nucleus (AON), brain regions associated with social behavior. An intruder behavior index revealed a bimodal distribution of intruder-related behaviors in Challenge 1 and a unimodal distribution in Challenge 2, suggesting population behavioral convergence, but no significant correlations with neuroendocrine measures. However, OXTR, AR, and ERα mRNA in the MeA were positively associated with convergence in individual intruder-related behaviors, suggesting multiple mechanisms may influence convergence. Mice could also occupy the nest during intruder challenges and convergence in nest attendance was positively correlated with MeA OXTR. At an individual level, nest attendance was positively associated with MeA ERα. Vocalizations were positively associated with AR and ERα mRNA. No positive associations were found in the AON. Overall, neuroendocrine receptors were implicated in convergence of a monogamous pair’s defense behavior, highlighting the potential importance of the MeA as part of a circuit underlying convergence.

Aggression in a western Amazonian colonial spider, Philoponella republicana (Araneae: Uloboridae)

Group-living spiders are rare, and can be divided into multiple subcategories based on their tolerance of group mates. While social spiders are cooperative, colonial spiders are often antagonistic towards conspecifics. We examined colony dynamics in a colonial species, Philoponella republicana (Simon, 1891), focusing on aggressive behaviors to further understand this understudied species. We studied whether web region, sex ratio, web size, or spider size affected aggression. We also tested whether colony members discriminate against conspecific intruders, since this behavior, known as group closure, is prevalent in many other group-living animals but had not yet been tested in colonial spiders. Colony mates were often aggressive due to competition for limited resources, such as mates and orb webs, yet several characteristics of this species may reduce these competitive forces. First, female-biased secondary sex ratios appear to reduce male-male and female-male competition. Moreover, although some individuals defended orb webs, other areas in the communal web were not defended. Philoponella republicana also did not exhibit group closure. Our results further confirm that aggression between males decreases in colonies with more female-biased secondary sex ratios, and larger individuals correlate with a higher frequency of aggressive interactions. Moreover, we raise new questions concerning the evolutionary pressures that shape coloniality in spiders.

The presence of conspecific intruders alters the magnitude of sex differences in care in a burying beetle

Biparental care occurs when males and females cooperate to care for their joint offspring. It is associated with sexual conflict because the benefits stem from the combined effort of both parents, while any costs depend on each parent's own effort. Thus, biparental care involves a delicate balance between cooperation and conflict. Shifts in this balance are likely to be driven by environmental conditions that alter the costs and benefits to males and females. Here, we used the burying beetle Nicrophorus vespilloides to investigate whether the presence of conspecific intruders shifts the balance towards cooperation (detected as an increase in the duration of biparental care relative to uniparental care) or alters the magnitude of sex differences in care, and whether any such effects are dependent on resources available for breeding. We found that the presence or absence of conspecific intruders had no effect on the duration of biparental care relative to uniparental care. Thus, there was no evidence that the presence of intruders caused a shift towards more cooperation. Females, but not males, responded to the presence of conspecific intruders by increasing their time spent providing one form of care, but only when breeding on larger resources. Thus, there was some evidence that the presence of intruders altered the magnitude of sex differences in care and that this was conditional upon the amount of resources available for breeding. Overall, our results show that threats from conspecific intruders alters sex differences in parental care, while not changing the balance between cooperation and conflict. • We tested if parents cooperate more when facing threats from conspecific intruders. • There was no shift in cooperation, but increased sex differences in care. • Threats from intruders impacts differently the costs of care to males and females.

Superb fairy-wrens with extreme exploration phenotypes respond more strongly to simulated territory intrusions

Individuals can exhibit strong consistencies in behaviours across time and contexts, referred to as animal personalities. Yet studies linking personality differences measured in captivity to ecologically relevant social behaviours in the wild are sparse and contradictory. Here, we used wild superb fairy-wrens, Malurus cyaneus , to investigate whether two personality traits measured in short-term captivity (exploration and aggressiveness) predicted aggressiveness in the wild (measured as territory defence). We first identified behavioural phenotypes within our study population by temporarily capturing adult birds to measure their (1) emergence, activity and sector visits in a novel environment (exploration) and (2) aggression in response to mirror image stimulation (aggressiveness). We later simulated two conspecific intrusions within their territory using playback of vocalizations (female or male chatter song) within their territory, in a pairwise design. Both exploration and breeding status (breeder/dominant versus helper/subordinate) influenced the birds’ response to the simulated intrusions. Dominant individuals and individuals with extreme exploration phenotypes (i.e. fast or slow explorers) responded more strongly to playback compared to helpers or those with intermediate exploration phenotypes, respectively. However, aggressiveness measured in short-term captivity (mirror response) did not correlate with our measure of aggressiveness in the wild (playback response during territory defence). The personality matching of breeding pairs (whether their partner had a matching or mismatching exploration phenotype) also influenced playback response, as pairs with mismatching exploration phenotypes responded more strongly to conspecific intruders than pairs with similar phenotypes. Our results support a growing number of studies showing the importance of animal personalities for response strategies and social behaviours. • We explored associations between personality traits in wild superb fairy-wrens. • Birds with extreme exploration phenotypes were more aggressive against conspecifics. • Aggressiveness in captivity did not correlate with aggressiveness in the wild. • Pairs with mismatching exploration phenotypes responded more strongly to intruders. • Our results support the validity of personality in fitness-related contexts.

Differential investment in visual and olfactory brain regions is linked to the sensory needs of a wasp social parasite and its host.

Obligate insect social parasites evolve traits to effectively locate and then exploit their hosts, whereas hosts have complex social behavioral repertoires, which include sensory recognition to reject potential conspecific intruders and heterospecific parasites. While social parasites and host behaviors have been studied extensively, less is known about how their sensory systems function to meet their specific selective pressures. Here, we compare investment in visual and olfactory brain regions in the paper wasp Polistes dominula, and its obligate social parasite P. sulcifer, to explore the links among sensory systems,brain and behavior. Our results show significant relative volumetric differences between these two closely related species, consistent with their very different life histories. Social parasites show proportionally larger optic lobes and central complex to likely navigate long-distance migrations and unfamiliar landscapes to locate the specific species of hosts they usurp. Contrastingly, hosts have larger antennal lobes and calyces of the mushroom bodies compared with social parasites, as predicted by their sensory means to maintain social cohesion via olfactory signals, allocate colony tasks, forage, and recognize conspecific and heterospecific intruders. Our work suggests how this tradeoff between visual and olfactory brain regions may facilitate different sensory adaptations needed to perform social and foraging tasks by the host, including recognition of parasites, or to fly long distances and successful host localizing by the social parasite.

Parental male and female convict cichlids assess and respond to threats differently depending on intruder species

In this field experiment, we presented pairs of monogamous convict cichlids with different intruders (conspecific and either cichlid or non-cichlid heterospecific) and observed the aggressive behavior of the pair to determine if pairs adjust aggression based upon species of intruder. We predicted that pairs would be more aggressive to conspecifics as these intruders represent multiple threats. The overall pair aggression was significantly higher to the conspecific intruder than to either heterospecific intruder. We also predicted that within pairs, the sexes might differ in how they assessed the risk represented by the intruders and might respond differently. Indeed, females were significantly more aggressive to the conspecific than to either heterospecific whereas males were more aggressive to the conspecific only compared to the non-cichlid heterospecific. These results suggest parental convict cichlids differently assess the risk posed by an intruder and then adjust aggression to different species of intruder. It also supports that the aggressive roles within a pair are flexible and that males and females modulate their behavior differently in response to different species of intruders perhaps based on differences in risk assessment.

Highly divergent cuticular hydrocarbon profiles in the cleptobiotic ants of the Ectatomma ruidum species complex

In social insects, chemical communication is the main communication mode among colony members, which use the blends of cuticular hydrocarbons as recognition cues to discriminate between nestmates and non-nestmates and to prevent the exploitation of their nest resources by aliens. The aim of this study was to assess the variation of nestmate recognition cues in the ant Ectatomma ruidum, a species complex with a considerably conserved morphology and one of the few ant species where intraspecific thievery, a form of cleptoparasitism, has been reported. We analyzed the cuticular hydrocarbon profiles of ants collected from a number of geographically separated populations and examined DNA sequence data to assess their species identity. We focused on one species of the complex, E. ruidum sp. 3–4, whose species delineation remains controversial. We documented that several quantitative and qualitative traits of the cuticular hydrocarbon profiles varied significantly between populations, indicating that this species harbors more cuticular chemical phenotypic diversity than expected within a single species. In particular, there was a striking divergence among populations in the proportion of methylalkanes, alkenes, alkadienes and odd-chain components, which likely play a major role in nestmate/non-nestmate discrimination, a process which might have been crucial in these cleptobiotic ants. Further investigations are needed to test the hypothesis that biotic pressures, such as the need to discriminate conspecific intruders and limit thievery, could have played an important role in promoting the evolutionary divergence between populations in this ant species complex.

Age-based changes in kairomone response mediate task partitioning in stingless bee soldiers (Tetragonisca angustula)

Collective defense is one of the most ubiquitous behaviors performed by social groups. Because of its importance, complex societies may engage a set of defensive specialists, with physical and/or neurological attributes tuned for defense against specific invaders. These strategies must be balanced, however, with the need to flexibly respond to different threat levels and sources. Insect societies rely heavily on olfaction for detecting and communicating in the context of defense. We therefore asked whether threat detection via olfaction is specialized towards invader-specific cues and how this may be integrated into defense task specialization. Colonies of the stingless bee, Tetragonisca angustula, deploy a morphologically distinct sub-caste of larger-bodied workers (soldiers) for colony defense. These soldiers transition between two different guarding tasks as they age, progressing from guarding in a hovering position near the nest entrance to guarding in a standing position on the nest entrance tube. Hovering and standing guards intercept different types of invaders: primarily heterospecific versus conspecific, respectively. We asked whether hovering and standing guarding behaviors were modulated by differential sensitivity to invader-associated olfactory stimuli; then we compared their responses to these cues to those of smaller workers that perform predominantly non-defense tasks. We exposed bees under both field and lab conditions to citral, a kairomone produced by an obligate heterospecific nest robber, primarily intercepted by hovering guards. Consistent with their roles, hovering guards were more likely to move towards citral than were either standing guards or small-bodied bees within a Y-maze. We also presented guards at field nests with dummies of conspecific versus heterospecific invader types, varying whether they included citral odors. Standing guards were more responsive to conspecific intruder scenarios than hovering guards, but heterospecific response differed by presence of citral. Standing and hovering guards responded in similar proportions when citral was absent, but the addition of citral produced a marginally non-significant reduction in standing guard participation. Our results potentially demonstrate differentiated cue-specific responses that correspond to morphological task specialization and age polyethism in these eusocial societies. Group defense is a ubiquitous function of any society, but threats are often diverse. Though defense versus non-defense task specialization has been well studied, when and how individuals sub-specialize on particular threats is less understood. We asked how age-differentiated soldier sub-types in a stingless bee differ in their innate responsiveness to olfactory cues associated with nest invader type. Consistent with behavioral roles, younger soldiers were more innately reactive to odors of an obligate heterospecific parasite, and older soldiers were more reactive to odors of conspecific intruders. This suggests that either transitions in learning or discrete cue sensitivity mediate defensive sub-specialization. This is the first study to compare behavioral responsiveness to olfactory stimuli across age-specialized soldier sub-types. Results suggest benefits of flexible allocation and prompt further studies exploring the topic of defensive sub-specialization.

Shared transcriptional responses to con- and heterospecific behavioral antagonists in a wild songbird

The recognition of and differential responses to salient stimuli are among the main drivers of behavioral plasticity, yet, how animals evolve and modulate functional responses to novel classes of antagonistic stimuli remain poorly understood. We studied free-living male red-winged blackbirds (Agelaius phoeniceus) to test whether gene expression responses in blood are distinct or shared between patterns of aggressive behavioral responses directed at simulated conspecific versus heterospecific intruders. In this species, males defend territories against conspecific males and respond aggressively to female brown-headed cowbirds (Molothrus ater), a brood parasite that commonly lays eggs in blackbird nests. Both conspecific songs and parasitic calls elicited aggressive responses from focal subjects and caused a downregulation in genes associated with immune system response, relative to control calls of a second, harmless heterospecific species. In turn, only the conspecific song treatment elicited an increase in singing behavior and an upregulation of genes associated with metabolic processes relative to the two heterospecific calls. Our results suggest that aspects of antagonistic behaviors to both conspecifics and brood parasites can be mediated by similar physiological responses, suggestive of shared molecular and behavioral pathways involved in the recognition and reaction to both evolutionarily old and new enemies.

Distant neighbours: friends or foes? Eurasian beavers show context-dependent responses to simulated intruders

Neighbour-stranger discrimination is widespread in territorial animals, and depending on the relative threat posed by neighbours and strangers, residents commonly exhibit either the “dear enemy phenomenon” or the “nasty neighbour effect”. Different members of the same group may represent different threat levels, and the response of residents can be modified depending on, e.g. the sex and dominance status of the intruder. Neighbour-stranger discrimination is primarily investigated in neighbours with shared borders, and whether residents recognize their more distant neighbours remains unexplored. Here, using experimental scent marks, we investigated whether Eurasian beavers (Castor fiber) discriminate between distant neighbours (i.e. two territories away) and strangers (i.e. further away than the known dispersal distance). We tested the hypotheses that Eurasian beavers can discriminate between distant neighbours and strangers and that social status (i.e. sub-dominant or dominant) and sex of the intruder affect the responses of resident beavers. We predicted that resident beavers show the “dear enemy phenomenon” in response towards dominant distant neighbours due to their territory ownership and the “nasty neighbour effect” in response towards sub-dominants due to their likelihood to disperse. Sex of residents and social status of intruders were important in explaining territorial responses, with males exhibiting stronger responses to sub-dominant distant neighbours, especially males, than sub-dominant strangers. No such discrimination was found by females or between dominant distant neighbours and strangers. We suggest that the “nasty neighbour” response by male resident beavers towards sub-dominant distant neighbours relates to the relative threat levels due to repeated intrusions during dispersal attempts. Territorial animals discriminate between neighbours and strangers to allocate aggressive behaviour to conspecific intruders. This neighbour-stranger discrimination has primarily been investigated between adjacent neighbours, but extraterritorial movements of residents into distant territories occur. Whether residents can discriminate between distant neighbours and strangers has only been studied in skylarks (Alauda arvensis), based on acoustic communication. Yet, it is unknown whether distant neighbours are perceived as neighbours or strangers based on olfactory recognition. Here we investigated whether Eurasian beavers (C. fiber) can discriminate between distant neighbours (i.e. two territories away) and strangers, based on olfactory scent samples including information on social status (i.e. dominant or sub-dominant) and sex of the intruder. Our results show that male residents showed a “nasty neighbour” response towards sub-dominant distant neighbours. These findings highlight the sensitivity of territorial mammals to the familiarity and social status of intruders.

Nonparametric false discovery rate control for identifying simultaneous signals

It is frequently of interest to identify simultaneous signals, defined as features that exhibit statistical significance across each of several independent experiments. For example, genes that are consistently differentially expressed across experiments in different animal species can reveal evolutionarily conserved biological mechanisms. However, in some problems the test statistics corresponding to these features can have complicated or unknown null distributions. This paper proposes a novel nonparametric false discovery rate control procedure that can identify simultaneous signals even without knowing these null distributions. The method is shown, theoretically and in simulations, to asymptotically control the false discovery rate. It was also used to identify genes that were both differentially expressed and proximal to differentially accessible chromatin in the brains of mice exposed to a conspecific intruder. The proposed method is available in the R package github.com/sdzhao/ssa.

Game of webs: species and web structure influence contest outcome in black widow spiders

AbstractPopulation-level trait variation within species plays an often-overlooked role in interspecific interactions. In this study, we compared among-individual variation in web phenotype and foraging behavior between native black widows (Latrodectus hesperus) and invasive brown widows (Latrodectus geometricus). We staged repeated contests whereby native widows defended their webs against intruders of both species to 1) investigate how trait variation mediates web contest outcome among native widows and 2) see whether widow behavior differs in response to an invasive spider. In only one trait, the average number of foraging lines, did black widows differ from brown widows. Black widow residents that built more structural lines were more likely to successfully defend their webs from conspecific intruders (i.e., be the sole spider remaining on the web postinteraction). This association between web structure and contest outcome did not exist in trials between black widows and invasive brown widows; however, in interspecific interactions, these same residents were more likely to have intruders remain on the web rather than drive them away. Surprisingly, brown widows did not usurp black widows. Brown widows were never observed signaling, yet black widow residents signaled equally to intruders of both species. Our results suggest that among-individual variation among native species can influence the response toward invasive competitors and outcome of these interactions.

Consistency in boldness expression varies with ecological context in a jumping spider

AbstractAnimals can adjust their behaviours depending on ecological context (i.e., behavioural plasticity), and an individual's response to a given context may also vary from occasion to occasion (intra‐individual variability). Recognizing the roles of both behavioural plasticity and intra‐individual variability is important in understanding how behavioural diversity is maintained within populations. However, how the ecological context itself influences the individual behavioural response and intra‐individual variability (e.g., how variable an individual is in their behavioural expression) remains largely unexplored. Here, we examine boldness expression (the duration of startle response) in a specialised spider‐eating jumping spider, Portia labiata, across three contexts following a mild disturbance: presence of a conspecific intruder (most dangerous), environmental change but no conspecific intruder, and no conspecific intruder or environmental change (safest). We found that context does not significantly influence the average boldness expression at the population level. However, each individual responded to each context differently, and the repeatability of boldness expression—the proportion of behavioural variation attributable to the between ‐individual level—is context‐dependent. We also found that in the presence of a conspecific intruder, spiders behave less predictably than in the environmental change context, but not differently from the safest context. These findings may suggest that the presence of conspecifics influences behavioural consistency in individuals, but that this may occur without influencing the population average behaviour.

Perceived threat to paternity reduces likelihood of paternal provisioning in house wrens.

Biparental care is a critical and, occasionally, unequally shared obligation that ensures that young survive to maturity. Such care may be complicated in systems in which one parent, typically the male, is unsure of his genetic relatedness to the young. Males may reduce paternal provisioning when full paternity is not assured, as occurs in mating systems in which females engage in extrapair copulations. Moreover, other factors independent of extrapair matings, such as male personality traits, likely also affect the level of paternal care. In this study, we determined the effect of a paternity threat event (i.e., a conspecific or a heterospecific territory intrusion) and male personality (i.e., the level of aggressiveness) on provisioning effort by male house wrens (Troglodytes aedon). Males were more likely to attack a conspecific intruder than a heterospecific intruder. Males that were exposed to a conspecific intruder were less likely to provision young at all. Of those males that did feed the young in their nest, male aggressiveness did not relate to feeding effort. These findings suggest that the likelihood of paternal care is reduced by perceived threats to paternity but that the costs of not feeding potentially multisired young are high and feeding efforts are unrelated to male personality.

Vocal and visual communication in territorial rock pipit males (Anthus petrosus), focused on playback experiments. A study of the information content of displays

ABSTRACT Signals of aggressive intent are central in bird communication and my aim is to examine the use of vocal and visual displays during simulated and real territory intrusions in rock pipit males. Conspecific intruders are worth different energy investment because they constitute different threats. If owners are able to discriminate between various categories, they could act accordingly. I focused on calls as playback stimuli to judge their information content. I showed that various calls convey different signals to defending males. To identify vocal and visual displays elicited in real conflicts, I observed close encounters between neighbours in border zones and between owners and strangers within territories, respectively. A much discussed issue is whether aggressive displays predict approach, attack or even physical fight. My conclusion is that no single vocal component is a reliable signal leading to physical fights, but one particular, directed visual display is frequently seen before a clash and is then combined with intense vocalizations. The non-directed posture, with calls, is particularly common in the parallel walk display with neighbours. The vocalizations seem to express both aggressive intent, the directed calls, and low-frequency, defensive threat, the non-directed calls, while the males determine how to settle serious conflicts.

Male territorial aggression and fitness in collared flycatchers: a long-term study

Aggressive behaviour plays a fundamental role in the distribution of limiting resources. Thereby, it is expected to have consequences for fitness. Here, we explored the relationship between aggression and fitness in a long-term database collected in a wild population of the collared flycatcher (Ficedula albicollis). We quantified the aggression of males during nest-site defence by conducting simulated territorial intrusions in the courtship period. We estimated the fitness of males based on their pairing success, breeding output and survival to next year. Earlier arriving and older males had a higher probability to establish pair-bond, and males that started to breed earlier fledged more young. Aggression did not predict pairing and breeding performances. However, the probability of a male to return in the next year was significantly related to aggression in an age-dependent manner. Among subadult males, more aggressive individuals had higher chances to return, while among adult males, less aggressive ones did so. This finding is in harmony with our general observation that subadult collared flycatcher males behave more aggressively than adult males when confronted with a conspecific intruder. Subadult males may be socially inexperienced, so they should be more aggressive to be successful. In contrast, if adult males suffer from higher physiological costs, a lower level of aggression may be more advantageous for them. Our study shows that aggressive behaviour can be a fitness-related trait, and to understand its role in determining fitness, age should be taken into account.