Territorial Intrusions Research Articles

Regardless of personality, males show similar levels of plasticity in territory defense in a Neotropical poison frog

Animal personality traits are sometimes linked to an individual’s degree of plasticity, with certain personality types being more plastic than others. In territorial species, consistently high levels of aggression might increase the risk of harmful fights, while consistently low aggression might lead to the loss of a territory. Consequently, reacting plastically with an appropriate territorial response should be beneficial to avoid these risks. An integrative investigation of both personality traits and plasticity can help us better understand the dynamics of aggressive interactions during male-male competition. Here, we used a free-ranging Neotropical poison frog population to investigate the role of plasticity in male territorial aggression towards intruders. We conducted repeated standardized territorial intrusion experiments mimicking frogs of different body sizes via playback calls with different peak frequencies. We found individual repeatability for the latency to reach and approach a simulated intruder and observed that both aggressive and less aggressive males decreased their level of aggression towards big intruders. However, our results do not support a correlation between personality and plasticity in the context of male territory defense during the breeding season. We discuss how such a correlation between personality and plasticity might be conditional on the costs and benefits across contexts.

Females Are More Aggressive Than Males towards Same- and Opposite-Sex Intruders in the Blue Tit (Cyanistes caeruleus).

During the breeding season, aggression is expressed to gain access to resources such as territories and mates and protect offspring. Female aggressiveness has received much less attention than male aggressiveness, and few studies have examined female and male aggressiveness towards intruders of both sexes in the same species. We compared female and male aggressiveness towards same- and opposite-sex intruders during the egg-laying period in blue tits (Cyanistes caeruleus) using simulated territorial intrusions. For each sex, we examined the occurrence of different behavioural responses during agonistic encounters, and compared the intensity and individual consistency of intra- and inter-sexual aggression using same- and opposite-sex taxidermy mounts. Our results show that females are the more aggressive sex. Both sexes showed similar behaviours during simulated intrusions, although females were never observed singing and males never entered the nest box. In females, aggression was predominantly independent of the sex of the intruder, while males sang more from a distance during male-male encounters. The relative levels of aggression (pecking and perching on the mounts) during intra- and intersexual conflicts were consistent for females, but not for males. Females might be under stronger selection for aggressive phenotypes due to nest-hole competition and larger reproductive investments.

Soft calls do not modulate aggressiveness in male common cuckoos toward conspecific territorial intruders: A sequential playback study

In many bird species loud broadcast calls serve as aggressive signals with a large effective radius, whereas soft calls may indicate aggression at a closer distance, and are often directed at a nearby conspecific individual. Male common cuckoos (Cuculus canorus) are famous for their simple cu-coo calls, which are long-range broadcast calls, uttered in long sequences during the breeding season. Cuckoos also produce soft calls, including a single gowk (wah), a series of gowks (gowk series), or a harsher single variant, the guo call. To our knowledge, no previous study has attempted to understand the function of cuckoos’ soft calls. We conducted a sequential playback experiment by (i) first attracting male cuckoos with “cu-coo” calls, and then (ii) playing heterospecific control calls or one of several types of cuckoo soft calls. While the “cu-coo” call attracted focal males, neither the soft conspecific calls nor the heterospecific control calls elicited an additional response. Finally, the gowk series calls did not alter cuckoos’ approach to the speaker when played together with “cu-coo” calls. As cuckoos’ soft calls did not increase or decrease aggression, they instead may advertise the cuckoo’s presence and position for a nearby conspecific.

Male‐specific nocturnal song functions similarly to day song in a diurnal bird species

AbstractHistorically, birdsong research has been biased towards song of male birds at dawn and during the day, even though some diurnal birds sing at night. To address this gap, we studied how song in the willie wagtail, Rhipidura leucophrys—a diurnal species with prolific male‐specific nocturnal song during the breeding season—varies with time of day, breeding status and simulated intrusions. We recorded male nocturnal and dawn song over three breeding seasons and examined how this related to fertile and non‐fertile breeding stages of females. To test whether song functions for territory defence, we simulated territorial intrusion experimentally in both sexes with daytime and nighttime playback of male and female song. To test whether nocturnal song could function for mate guarding or post‐pairing mate attraction, we describe the mating system of willie wagtails using molecular genetic methods. We showed that both nocturnal and day song of male willie wagtails has roles in mate attraction and territory defence, while day song by females functioned primarily for territorial defence. Males increased song behaviour during fertile periods of resident females, suggesting possible roles in mate stimulation and mate guarding. Males and females responded similarly to simulated daytime intrusions and no differences were seen in male responses dependent on the time of day. We found 10%–14% of offspring were fathered by extra‐pair males, suggesting song may also function for mate guarding and post‐pairing mate attraction. In a species with small repertoires and simple songs like the willie wagtail, differences between males in overall song output achieved through nocturnal singing may be important in mate attraction and territory defence.

Individual territoriality in lekking species: the case of the Helmeted Manakin

ABSTRACT Neotropical manakins exhibit polygynous lekking systems in which males do not provide paternal care and exhibit elaborate courtship displays. Despite familial patterns, some historical field observations suggested that the Helmeted Manakin (Antilophia galeata) has evolved territoriality rather than lekking and a socially monogamous mating system. Furthermore, the evidence so far does not allow a clear understanding of the role played by resources, such as nesting sites and male behaviour in shaping the mating system in this species. Our objectives were to verify whether territorial behaviour occurs in the Helmeted Manakin and to provide a preliminary assessment of this species’ hypothesised social monogamy. Using an experimental approach with simulated territorial intrusions, we demonstrated that males in this species hold individual territories with nesting sites but do not provide parental care, similar to its congener, the Araripe Manakin (Antilophia bokermanni). Males’ responses occurred more aggressively in central areas of territories. Larger home ranges encompassed the smaller defended territories and the male display consists of repeated circular flights within the territory. Our results indicate that the Helmeted Manakin has a polygamous mating system. However, we suggest the species is better classified within the continuum of a resource-defence exploded-lek mating system. The distinct environmental conditions in which the species evolved may have played a significant role in modifying its behaviour, resulting in patterns that deviate from the typical lekking system found among manakins. This study highlights the importance of investigating atypical manakins to further our understanding of alternative reproductive tactics among birds.

Territoriality varies across elevation in a Hawaiian songbird

Abstract Reproductive territoriality can be influenced by external and internal variables. Trade-offs between reproductive behaviors and other costly biological processes, such as immunity, exist across taxa, but the effects of novel diseases on these trade-offs remain poorly understood. Since the introduction of avian malaria to Hawaii in the early 1900s, low elevation Hawaii Amakihi (Chlorodrepanis virens) populations, which have undergone strong selection by the disease, have evolved increased malaria resilience. However, the effects of malaria selection on trade-offs between immunity and reproduction in Amakihi remain largely unknown. To begin exploring this relationship, we conducted simulated territorial intrusions on Amakihi at low elevation (where malaria selection has been stronger) and high elevation (where selection has been weaker) on Hawaii Island during the breeding season. We hypothesized that selection by avian malaria has favored greater investment in avian malaria resilience at the cost of reproductive behaviors. We predicted that low elevation Amakihi would be less territorial compared with high elevation Amakihi, while recognizing that variables other than disease pressures that may differ across elevation (e.g., competition, predation) could explain behavioral variation. Territoriality was reduced in low compared with high elevation Amakihi as measured by proximity and chases in response to the intrusion. Low elevation Amakihi generally flew less than high elevation individuals, although this relationship varied across the breeding season. Our correlational results demonstrate that territoriality is greater in high compared with low elevation Amakihi. Further investigations would help determine which factors differing across elevation underlie this variation in territoriality.

Colorful facial markings are associated with foraging rates and affiliative relationships in a wild group-living cichlid fish.

Many animals use color to signal their quality and/or behavioral motivations. Colorful signals have been well studied in the contexts of competition and mate choice; however, the role of these signals in nonsexual, affiliative relationships is not as well understood. Here, we used wild social groups of the cichlid fish Neolamprologus pulcher to investigate whether the size of a brightly colored facial patch was related to 1) individual quality, 2) social dominance, and/or 3) affiliative relationships. Individuals with larger patches spent more time foraging and tended to perform more aggressive acts against conspecific territory intruders. We did not find any evidence that the size of these yellow patches was related to social rank or body size, but dominant males tended to have larger patches than dominant females. Additionally, patch size had a rank-specific relationship with the number of affiliative interactions that individuals engaged in. Dominant males with large patches received fewer affiliative acts from their groupmates compared to dominant males with small patches. However, subordinates with large patches tended to receive more affiliative acts from their groupmates while performing fewer affiliative acts themselves. Taken together, our results suggest that patch size reflects interindividual variation in foraging effort in this cichlid fish and offer some of the first evidence that colorful signals may shape affiliative relationships within wild social groups.

Fitness Costs of Female Competition Linked to Resource Defense and Relatedness of Competitors.

AbstractFemale reproductive success is often limited by access to resources, and this can lead to social competition both within and between kin groups. Theory predicts that both resource availability and relatedness should influence the fitness consequences of social competition. However, testing key predictions requires differentiating the effects of these two factors. Here, we achieve this experimentally by manipulating the social environment of house mice, a facultative communal breeding species with known kin discrimination ability. This allows us to investigate (1) the reproductive costs of defending a limited resource in response to cues of social competition and (2) whether such costs, or their potential mitigation via cooperative behavior, are influenced by the relatedness of competitors. Our results support the hypothesis that resource defense can be costly for females, potentially trading off against maternal investment. When the availability of protected nest sites was limited, subjects (1) were more active, (2) responded more strongly to simulated territory intrusions via competitive signaling, and (3) produced smaller weaned offspring. However, we found no evidence that the propensity for kin to cooperate was influenced by the relatedness of rivals. Communal breeding between sisters occurred independently of the relatedness of competitors and communally breeding sisters weaned fewer offspring when competing with unrelated females, despite our study being designed to prevent infanticide between kin groups. Our findings thus demonstrate that female competition has fitness costs and that associating with kin is beneficial to avoid negative fitness consequences of competing with nonkin, in addition to more widely recognized kin-selected benefits.

Density-dependent plasticity in territoriality revealed using social network analysis.

Territories are typically defined as spatially exclusive areas that are defended against conspecifics. Given the spatial nature of territoriality, it is inherently density dependent, but the economics of territoriality also depend on the distribution and abundance of defended resources. Our objectives were to assess the effects of changing population density and food availability on individually based territorial phenotypes. We developed a novel analytical framework that bridges spatially explicit territories with social network analysis to model density-dependent territorial phenotypes. Using the outputs from our data pipeline, we modelled plasticity in territory size and territory intrusion rates in a long-term study population of North American red squirrels Tamiasciurus hudsonicus. Red squirrels defend year-round territories around a central hoard (midden) of white spruce Picea glauca cones. Importantly, white spruce is a masting species that produces large cone crops every 4-7 years (i.e. mast years) in our study area interspersed with non-mast years when few cones are produced. In the spring following mast years, populations are approximately double in size, but are lower in the spring of non-mast years. We predicted that territory size and intrusion rates would decrease as resource abundance, and consequently population density, increased. By contrast, as resource abundance decreased via depletion, and therefore density decreased, territories should increase in size and intrusions should also increase. As we expected, individual territory size and territorial intrusions were negatively density dependent, such that increased density after mast years resulted in smaller territories and fewer intrusions. When considering between-individual variation in plasticity across a density gradient, individuals responded differently to changes in population density within their lifetime. Our results show that territory size and intrusion rates display negative density dependence. When food becomes available in the autumn of a mast year and density in spring of the following year increases, territories shrink in size to effectively a small area around the midden. While our findings for red squirrels are unique compared to other systems, they serve as a reminder that the direction and strength of fundamental ecological relationships can depend on the nature of the system.

Male aggressiveness during the female fertile phase in relation to extra-pair paternity, plumage ornaments and female traits

In many bird species, physical aggression between males become more frequent during the female’s fertile period, as female encounters with extra-pair males are more frequent and can entail paternity losses. Male aggressiveness during this stage has been proposed as crucial for ensuring male reproductive success. Thus, plumage ornaments could represent honest signals of individual quality that could reflect the aggressiveness of paired territorial males. Furthermore, male aggressiveness could be related to mate quality or defensive capacity. We quantified extra-pair paternity in the broods and investigated the association of male and female traits with the aggressive behaviour of territorial paired males in a Spanish population of Pied Flycatchers (Ficedula hypoleuca), where territorial intrusions were simulated during the female fertile period by placing a taxidermic male mount close to the nest. We predicted that (1) more aggressive males should better protect their mates from intruding males and thereby reduce their paternity losses, (2) males with larger white patches and higher UV reflectance of wing patches should respond more strongly to intrusions, and (3) that males should be more aggressive when mated with higher quality females. We found evidence that males that responded less intensely to a territorial intrusion suffered a higher paternity loss, which offers strong support to the basic tenet of the theory of territoriality as paternity defence. Moreover, both the level of male aggressiveness and control of the territory increased with male UV reflectance of wing patches. Finally, we found, contrary to our prediction, that males were less aggressive when mated with more ornamented 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.

Soft song provokes stronger aggressive responses than broadcast song in the Plain Laughingthrush (Pterorhinus davidi)

Many bird species produce low‐amplitude acoustic signals, known as soft songs, which are widely believed to function primarily in aggressive interactions. However, there are surprisingly few experimental studies that have examined whether soft songs are reliable signalling strategies for escalating aggressive interactions. Here, we investigate the functions of soft song in a colour‐marked population of the Plain Laughingthrush Pterorhinus davidi, using playback experiments combined with specimen‐based simulated territory intrusions. We found that playback of male soft song provoked quicker response and stronger aggressive intent by territorial pairs compared with playback of male broadcast song. Responses by territorial pairs to male soft song playback were significantly stronger compared with responses to broadcast song playback, with a greater number of flights over the speaker, more time spent within 1 m of the speaker and closer approaches to the speaker. Furthermore, closer approach with the production of soft song by focal males was the best predictor of aggressive intent in combined playback and specimen intrusion experiments. Our results suggest that male soft song is used as an aggressive signal in the Plain Laughingthrush, and represents the first evidence of such for a monogamous temperate duetting species.

Experimental evidence that chronic outgroup conflict reduces reproductive success in a cooperatively breeding fish.

Conflicts with conspecific outsiders are common in group-living species, from ants to primates, and are argued to be an important selective force in social evolution. However, whilst an extensive empirical literature exists on the behaviour exhibited during and immediately after interactions with rivals, only very few observational studies have considered the cumulative fitness consequences of outgroup conflict. Using a cooperatively breeding fish, the daffodil cichlid (Neolamprologus pulcher), we conducted the first experimental test of the effects of chronic outgroup conflict on reproductive investment and output. 'Intruded' groups received long-term simulated territorial intrusions by neighbours that generated consistent group-defence behaviour; matched 'Control' groups (each the same size and with the same neighbours as an Intruded group) received no intrusions in the same period. Intruded groups had longer inter-clutch intervals and produced eggs with increasingly less protein than Control groups. Despite the lower egg investment, Intruded groups provided more parental care and achieved similar hatching success to Control groups. Ultimately, however, Intruded groups had fewer and smaller surviving offspring than Control groups at 1-month post-hatching. We therefore provide experimental evidence that outgroup conflict can decrease fitness via cumulative effects on reproductive success, confirming the selective potential of this empirically neglected aspect of sociality.

Song functions for joint territory defence and within-pair communication in female and male lovely fairy-wrens

Song has been extensively studied in birds and is considered a classic example of a sexually selected trait. However, this interpretation is based predominantly on studies of males. There is growing evidence that female song is common and phylogenetically widespread, but there are still relatively few species for which song similarities and differences between the sexes have been described. We investigated the function of female and male song in the lovely fairy-wren, Malurus amabilis , a tropical species that maintains territories year-round. We tested whether song structure and sex-specific seasonal variation in song rates across breeding and nonbreeding stages were consistent with expectations of key hypotheses for song function. We did this with temporarily caged and free-living birds. We also experimentally examined female and male responses to simulated territorial intrusion, using playbacks simulating solo intruders of either sex singing songs that varied in complexity (i.e. the variety of elements in song). Females and males had similar song structure, complexity and natural song rates and both sang more during nonbreeding than breeding periods. Within each breeding stage, males sang more than females during incubation, a period of female-only investment in parental care. Both sexes sang more when they were more than 5 m apart than when close together in their dense tropical habitat. Females and males responded similarly to simulated intrusion and coordinated their responses. They responded more strongly to male than female song playbacks and to simple playback songs after they were exposed to complex songs, during both breeding and nonbreeding periods. Our observational and experimental data show that female and male songs are similar and suggest that convergent selective pressures may explain similar functions of song, primarily in joint territorial defence of resources and within-pair communication for coordination. • More systematic studies comparing female and male birdsong are needed. • We investigated birds' song form and function in a female and male tropical bird. • Females and males have similar songs which suggests convergent selective pressures. • Both sexes sang more during nonbreeding periods than during breeding periods. • Both sexes have similar song functions for territorial defence and pair communication.

Female aggression towards same-sex rivals depends on context in a tropical songbird

Agonistic conflict is ubiquitous throughout taxa, although the intensity of aggression observed is often highly variable across contexts. For socially monogamous species, a coordinated effort by both pair members can improve the chances of successfully warding off challengers and reinforce pair bonds. However, the intensity of aggression exerted by any one pair member may vary with respect to contextual factors, including the intensity of their mate’s aggression. Thus, experimentally exploring how individuals respond to potential rivals via multiple assays with varying social contexts can advance our basic understanding of how aggression varies in socially monogamous systems. We used simulated territorial intrusion and mirror image simulation assays to explore this issue in white-shouldered fairywrens (Malurus alboscapulatus moretoni) of Papua New Guinea. While males tended to be more responsive than females during simulated territorial intrusions, females were more aggressive towards their mirrored reflection than males. Further, individual females that were most aggressive in mirror image simulations were the least aggressive during simulated territorial intrusions, whereas males were inconsistent. These results suggest that female behavioral phenotypes appear to be flexible, relative to context. We discuss how multiple commonly used measurements of aggression might in fact measure different types of responses.

Social status mediates behavioral, endocrine, and neural responses to an intruder challenge in a social cichlid, Astatotilapia burtoni

Most animals encounter social challenges throughout their lives as they compete for resources. Individual responses to such challenges can depend on social status, sex, and community-level attributes, yet most of our knowledge of the behavioral and physiological mechanisms by which individuals respond to challenges has come from dyadic interactions between a resource holder and a challenger (usually both males). To incorporate differences in individual behavior that are influenced by surrounding group members, we use naturalistic communities of the cichlid fish, Astatotilapia burtoni, and examine resident dominant male responses to a territorial intrusion within the social group. We measured behavior and steroid hormones (testosterone and cortisol), and neural activity in key brain regions implicated in regulating territorial and social dominance behavior. In response to a male intruder, resident dominant males shifted from border defense to overt attack behavior, accompanied by decreased basolateral amygdala activity. These differences were context dependent – resident dominant males only exhibited increased border defense when the intruder secured dominance. Neither subordinate males nor females changed their behavior in response to a territorial intrusion in their community. However, neural activity in both hippocampus and lateral septum of subordinates increased when the intruder failed to establish dominance. Our results demonstrate how a social challenge results in multi-faceted behavioral, hormonal, and neural changes, depending on social status, sex, and the outcome of an intruder challenge. Taken together, our work provides novel insights into the mechanisms through which individual group members display context- and status-appropriate challenge responses in dynamic social groups.

Pleiotropic opposing dominance within a color gene block contributes to a nascent species boundary via its influence on hybrid male territorial behavior.

The divergence of plumage color genes contributes to songbird radiation. However, the mechanisms by which color gene divergence counteracts gene flow to maintain reproductive isolation during the formation of new species boundaries remain elusive. The hybrid zone between Setophaga occidentalis (SOCC) and S. townsendi (STOW) in the Cascade Range provides a natural observatory to investigate potential behavioral mechanisms underlying divergent selection on color genes. Recently, we found that selection within a single gene block associated with plumage color variation has maintained a stable and narrow hybrid zone. Here, we investigated the potential role of plumage signals in moderating a behavioral mechanism of selection. Specifically, we assessed whether two plumage traits are associated with body size among breeding males and if trait mismatch predicted aggressive behavior within hybrid and parental individuals in response to simulated territorial intrusion. The two plumage signals, cheek and flank coloration, though associated with the same gene block, reflect opposing dominance of SOCC and STOW alleles. We found that both plumage traits significantly predict the body size in the territorial sex (i.e. males). The opposing dominance of the single color gene block resulted in plumage signal discordance in heterozygotes, which in turn was associated with reduced hybrid territorial performance, an important proxy of fitness in this system. Taken together, these observations point to a single-locus-two-alleles mechanism of incompatibility in shaping a natural species boundary in the early stage of speciation.

Male song stability shows cross-year repeatability but does not affect reproductive success in a wild passerine bird.

Predictable behaviour (or 'behavioural stability') might be favoured in certain ecological contexts, for example when representing a quality signal. Costs associated with producing stable phenotypes imply selection should favour plasticity in stability when beneficial. Repeatable among-individual differences in degree of stability are simultaneously expected if individuals differ in ability to pay these costs, or in how they resolve cost-benefit trade-offs. Bird song represents a prime example, where stability may be costly yet beneficial when stable singing is a quality signal favoured by sexual selection. Assuming energetic costs, ecological variation (e.g. in food availability) should result in both within- and among-individual variation in stability. If song stability represents a quality signal, we expect directional selection favouring stable singers. For a 3-year period, we monitored 12 nest box plots of great tits Parus major during breeding. We recorded male songs during simulated territory intrusions, twice during their mate's laying stage and twice during incubation. Each preceding winter, we manipulated food availability. Assuming that stability is costly, we expected food-supplemented males to sing more stable songs. We also expected males to sing more stable songs early in the breeding season (when paternity is not decided) and stable singers to have increased reproductive success. We found strong support for plasticity in stability for two key song characteristics: minimum frequency and phrase length. Males were plastic because they became more stable over the season, contrary to expectations. Food supplementation did not affect body condition but increased stability in minimum frequency. This treatment effect occurred only in 1 year, implying that food supplementation affected stability only in interaction with (unknown) year-specific ecological factors. We found no support for directional, correlational or fluctuating selection on the stability in minimum frequency (i.e. the song trait whose stability exhibited cross-year repeatability): stable singers did not have higher reproductive success. Our findings imply that stability in minimum frequency is not a fitness quality indicator unless males enjoy fitness benefits via pathways not studied here. Future studies should thus address the mechanisms shaping and maintaining individual repeatability of song stability in the wild.

Androgen responsiveness to simulated territorial intrusions in Allobates femoralis males: Evidence supporting the challenge hypothesis in a territorial frog

Territoriality has been widely described across many animal taxa, where the acquisition and defence of a territory are critical for the fitness of an individual. Extensive evidence suggests that androgens are involved in the modulation of territorial behaviours in male vertebrates. Short-term increase of androgen following a territorial encounter appears to favour the outcome of a challenge. The “Challenge Hypothesis” proposed by Wingfield and colleagues outlines the existence of a positive feedback relationship between androgen and social challenges (e.g., territorial intrusions) in male vertebrates. Here we tested the challenge hypothesis in the highly territorial poison frog, Allobates femoralis, in its natural habitat by exposing males to simulated territorial intrusions in the form of acoustic playbacks. We quantified repeatedly androgen concentrations of individual males via a non-invasive water-borne sampling approach. Our results show that A. femoralis males exhibited a positive behavioural and androgenic response after being confronted to simulated territorial intrusions, providing support for the Challenge Hypothesis in a territorial frog.

Male White-shouldered Fairywrens (Malurus alboscapulatus) elevate androgens greater when courting females than during territorial challenges

Androgens like testosterone mediate suites of physical and behavioral traits across vertebrates, and circulation varies considerably across and within taxa. However, an understanding of the causal factors of variation in circulating testosterone has proven difficult despite decades of research. According to the challenge hypothesis, agonistic interactions between males immediately prior to the breeding season produce the highest levels of testosterone measured during this period. While many studies have provided support for this hypothesis, most species do not respond to male-male competition by elevating testosterone. As a result, a recent revision of the hypothesis (‘challenge hypothesis 2.0’) places male-female interactions as the primary cause of rapid elevations in testosterone circulation in male vertebrates. Here, we offer a test of both iterations of the challenge hypothesis in a tropical bird species. We first illustrate that male White-shouldered Fairywrens (Malurus alboscapulatus) differ by subspecies in plasma androgen concentrations. Then we use a social network approach to find that males of the subspecies with higher androgens are characterized by greater social interaction scores, including more time aggregating to perform sexual displays. Next, we use a controlled experiment to test whether males respond to simulated territorial intrusion and/or courtship competition contexts by elevating androgens. We found that males elevated androgens during territorial intrusions relative to flushed controls, however, males sampled during courtship competitions had greater plasma androgens both relative to controls and males sampled while defending territories. Ultimately, our results are consistent with challenge hypothesis 2.0, as sexual interactions with extra-pair females were associated with greater elevation of androgens than territorial disputes.