Conspecific Intruders Research Articles

Testing the dear enemy relationship in fiddler crabs: Is there a difference between fighting conspecific and heterospecific opponents?

Reduction of aggressiveness toward familiar neighbors, when compared to aggressiveness toward unfamiliar strangers, can decrease the costs of territory defense. This phenomenon is known as the “dear enemy effect”. Individuals may shift their aggressiveness toward neighbors or strangers from the same or different species, depending on the relative threat associated with different opponents. Therefore, a reduced level of aggressiveness between heterospecific neighbors is expected in relation to conspecific intruders, since the latter compete not only for territory, but also for mates. Herein we investigated the occurrence of the dear enemy effect in territorial fights between conspecific pairs of Leptuca leptodactyla and heterospecific pairs of L. leptodactyla versus Leptuca uruguayensis. Across both conspecific and heterospecific fights, medium- and high-intensity fight components were more used in resident–stranger than in resident–neighbor fights. Thus, residents showed a dear enemy response, regardless of opponent species. Moreover, conspecific fights induced a greater number of low- and medium-intensity fight components than did fights between heterospecifics, both neighbors and strangers. Finally, conspecific resident–stranger fights took longer than heterospecific resident–stranger fights. Our results indicate that fiddler crabs adjust their territorial response according to the species and resident status of intruders, consistent with the risks posed by different intruder types.

Sing softly to evoke a response only from a recent intruder

Low-amplitude soft songs have been described in many birds, but in most species, research has addressed only broadcast songs. Soft songs may have a similar or distinctive structure in comparison to broadcast songs produced in order to defend a territory and attract females. In some species, such soft songs were found to be produced in an aggressive context and were the best predictors of conflict escalation and later physical attack. However, such observations are not consistent across all species studied. Several hypotheses have been put forward to explain the function(s) of soft songs and why they are so quiet. Studies on the ortolan bunting (Emberiza hortulana) show that males produce soft songs similar in structure to broadcast songs during close interactions with conspecific intruders. However, experiments with the playback of loud and soft songs as well as taxidermic mount presentation revealed that soft song in this species does not fulfil aggressive signal criteria. Here we performed an experiment with two loudspeakers simulating movements of the intruder in order to test whether the soft songs are used to evoking a response from a nearby but not localised rival. We found that males responded with similar strength to songs played back from single and double speaker systems. Simultaneously, males produced more soft songs during and just after the phase of the experiment in which we simulated short flights of the intruder. Our results indicate that soft songs might be used during close interactions with rivals without being an aggressive signal and are used as short-range signals to check if the rival is still around. Our results also provide an alternative explanation of soft song behaviour in comparison to the hypotheses of eavesdropping avoidance and readiness to fight.

Male and female signaling behavior varies seasonally during territorial interactions in a tropical songbird

In many tropical birds, both sexes use conspicuous vocal signals during territorial interactions. Although a growing number of studies examine male and female signals in the context of coordinated vocal duets, the use of vocal signals by both sexes in non-duetting species is poorly documented, even though these species are more numerous than duetting species. Furthermore, few studies of tropical non-duetting species test for seasonal variation in signaling behavior. We studied season-specific and sex-specific variation in signaling behavior of a tropical resident songbird, the Rufous-capped Warbler (Basileuterus rufifrons), by conducting a playback experiment where we simulated conspecific territorial intruders producing three types of vocalizations (male songs, female calls, or a “pair” with simultaneous male songs and female calls) and a heterospecific control. We repeated playback during the pre-breeding and breeding seasons. Response intensity to playback varied with season and sex of the focal birds. During the pre-breeding season, both sexes showed strong physical approach responses and vocal responses to all conspecific intrusions, especially paired intrusions. During the breeding season, males responded strongly to all conspecific treatments, whereas females showed little response. Although females primarily used calls in response to conspecific playback, many females also sang, especially during the non-breeding season. Our results therefore suggest that both male and female signals are used for shared territory defense, but that the contributions of each sex to territory defense vary seasonally. Our results also contribute to our understanding of the evolution of combined male and female signaling during territory defense. In resident tropical animals, both males and females often use conspicuous signals during territorial interactions with conspecifics. Seasonal and sex-specific variation in vocal behavior of tropical resident birds during these territorial interactions has received little research attention. We conducted a playback experiment to investigate season- and sex-specific variation in signaling behavior of tropical Rufous-capped Warblers (B. rufifrons). In the pre-breeding season, both sexes responded strongly to all conspecific intrusions, especially paired intrusions, by approaching the playback speakers and vocalizing. In the breeding season, males actively defended the territory alone whereas females rarely responded. Our results suggest that both sexes of Rufous-capped Warbler use vocal signals for shared territory defense, although the contributions of each sex to territory defense differ seasonally.

An experimental test of the relationship between yolk testosterone and the social environment in a colonial passerine

Maternal hormones can be transferred to offspring during prenatal development in response to the maternal social environment, and may adaptively alter offspring phenotype. For example, numerous avian studies show that aggressive competition with conspecifics tends to result in females allocating more testosterone to their egg yolks, and this may cause offspring to have more competitive phenotypes. However, deviations from this pattern of maternal testosterone allocation are found, largely in studies of colonial species, and have yet to be explained. Colonial species may have different life‐history constraints causing different yolk testosterone allocation strategies in response to conspecific competition, but few studies have experimentally tested whether colonial species do indeed differ from that of solitary species. To test this, we collected eggs from zebra finchesTaeniopygia guttata, a colonial species, in the presence and absence of conspecific intrusions. Females did not alter the concentration of testosterone deposited in eggs laid during intrusions despite becoming more aggressive. These results suggest that maternal effects are not characterized by a uniform response to the social environment, but rather need to be contextualized with life‐history traits.

Dear enemies or nasty neighbors? Causes and consequences of variation in the responses of group-living species to territorial intrusions

Territorial behavior is widespread throughout the animal kingdom, with responses to conspecific intruders differing depending on various ecological, life history, and social factors. One factor which has received considerable research attention is rival identity. Early work provided many examples of species exhibiting relatively stronger responses to strangers versus neighbors (the “dear-enemy” effect) or the opposite (the “nasty-neighbor” effect). However, those studies focused predominantly on single or pair-bonded territory-holders. There is increasing evidence of neighbor–stranger response differences in group-living species (where 3 or more individuals share a territory), and of within-species variation in the relative responses shown to these 2 intruder types. Considering social species is important both because group territoriality is widespread and because group responses include the actions of multiple individuals whose interests and motivations differ. We begin our review with a summary of territoriality in group-living species. We then discuss causes of variation in territorial responses depending on intruder neighbor–stranger identity, considering both between-species differences and those within species arising from context-dependent variation and from individual group members responding differently to the same intrusion. We next detail the consequences of different territorial responses, in terms of both postinteraction behavior and individual benefits and costs. Finally, we suggest 3 key areas—theoretical modeling, hormonal mechanisms, and anthropogenic disturbances—that could be developed when considering the relative responses of territory-holders to neighbors and strangers. Since conflict is a powerful selective force, determining the causes and consequences of variation in group-territorial behavior is important for a full understanding of sociality.

The role of extragroup encounters in a Neotropical, cooperative breeding primate, the common marmoset: a field playback experiment

In cooperatively breeding species, encounters with intruders may serve multiple functions ranging from reaffirming group territory ranges to facilitating assessments for additional breeding opportunities. While these distinctive events offer the opportunity to investigate the delicate balance of these social dimensions within animal societies, their unpredictable occurrence makes witnessing and controlling these events in the wild particularly challenging. Here we used a field playback approach to simulate conspecific territorial incursions in cooperatively breeding common marmosets (Callithrix jacchus) to distinguish between the three following non-mutually exclusive functions of intergroup encounters in this species of New World primate: territorial defense, mate defense, and assessment of breeding opportunities. For these experiments, we systematically broadcast species-typical long-distance contact calls - phees - commonly used in intergroup interactions from the core and periphery of the groups' territories using either male or female vocalizations. Consistent with a territorial defense hypothesis, a group's reaction was independent of the simulated intruder's sex and the response strength was greater when the playback stimulus was broadcast from the core areas of groups' territories relative to stimulus broadcast from periphery areas. However, sex differences in some facets of their responses suggest that this is not the only potential function for these encounters. Mated males and females started to move first in response to simulated intruders of the opposite sex, suggesting that these events offered opportunities to assess extra-pair breeding opportunities, while the occurrence of females' piloerection towards simulated female intruders is suggestive of mate-guarding. These data provide unique experimental evidence for the theory that excursions by conspecific intruders may serve multiple functions in a cooperatively breeding vertebrate and are reflective of the known complexities of common marmoset sociobiology.

Sex differences in parental defence against conspecific intruders in the burying beetle Nicrophorus vespilloides

In species with biparental care, females often provide more care than males. Previous work has focused on sex differences in parental food provisioning and defence against predators. However, parents often also defend their offspring against conspecific intruders, which could be male or female. Thus, there is a need for studies examining sex differences in the behaviour of both caring parents and intruders, and whether sex differences in the behaviour of caring parents depend upon the intruder's sex. We conducted an experiment on the burying beetle Nicrophorus vespilloides where a single female or male resident caring for a brood was confronted with a male or female intruder. Female residents were more successful in defending their brood and engaged in more fights against an intruder than males. Residents engaged in more fights against male intruders and, among those that successfully defended their brood, residents spent more time provisioning food to larvae when confronted with female intruders. There was no evidence that sex differences in the behaviour of caring parents depended upon the intruder's sex. There were no sex differences in any measures of reproductive success among those residents that successfully defended their brood and no sex differences in the life span or mass gain of either residents or intruders. Our study extends the study of sex differences in parental care to the context of defence against conspecific intruders by demonstrating sex differences in the behaviour of both residents and intruders and sex differences in reproductive success in the presence of conspecific intruders.

Baseline and stress-induced blood properties of male and female Darwin’s small ground finch (Geospiza fuliginosa) of the Galapagos Islands

Birds are renowned for exhibiting marked sex-specific differences in activity levels and reproductive investment during the breeding season, potentially impacting circulating blood parameters associated with stress and energetics. Males of many passerines often do not incubate, but they experience direct exposure to intruder threat and exhibit aggressive behaviour during the nesting phase in order to defend territories against competing males and predators. Nesting females often have long bouts of inactivity during incubation, but they must remain vigilant of the risks posed by predators and conspecific intruders approaching the nest. Here, we use 33 free-living male (n = 16) and female (n = 17) Darwin's small ground finches (Geospiza fuliginosa) on Floreana Island (Galapagos Archipelago) to better understand how sex-specific roles during the reproductive period impact baseline and stress-induced levels of plasma corticosterone (CORT), blood glucose and haematocrit. Specifically, we hypothesise that males are characterised by higher baseline values given their direct and relatively frequent exposure to intruder threat, but that a standardised stress event (capture and holding) overrides any sex-specific differences. In contrast with expectations, baseline levels of all blood parameters were similar between sexes (13.4 ± 1.9 ng ml−1 for CORT, 13.7 ± 0.4 mmol l−1 for glucose, 58.3 ± 0.8% for haematocrit). Interestingly, females with higher body condition had lower baseline haematocrit. All blood parameters changed with time since capture (range 1.2–41.3 min) in both sexes, whereby CORT increased linearly, haematocrit decreased linearly, and glucose increased to a peak at ∼20 min post-capture and declined to baseline levels thereafter. Our results do not support the hypothesis that sex-specific roles during the reproductive period translate to differences in blood parameters associated with stress and energetics, but we found some evidence that blood oxygen transport capacity may decline as finches increase in body condition.

Investment in territorial defence relates to recent reproductive success in common loons Gavia immer

As the value of a limited resource such as a territory increases, animals should invest more in the defence of that resource. Because reproductive success often depends on the quality of a breeding territory, reproductive success or failure may alter the perceived value of territory and affect an animal's investment in territorial defence. We used common loons Gavia immer to test the hypothesis that animals with recent breeding success would show stronger territorial defence than those with no recent breeding success. Surprisingly, successful loons responded less, not more, to a simulated intrusion. However, birds with success in the previous season also increased their territorial response as the breeding season progressed. In conjunction with past data showing that recently successful loons experience an increase in conspecific intrusions on their territories, we interpret our data to suggest that loons with recent success offset the cost of increased intrusions by adopting a more efficient strategy for territorial defence (e.g. limiting investment in resource defence until the time of the season when it is most critical).

Shared territorial defence in the suboscine Aphrastura spinicauda

ABSTRACTTerritorial behaviour is an active and typically aggressive behaviour used to defend resources. Here, we investigated the presence of shared territorial defence behaviour during conspecific intrusions in the Thorn-tailed Rayadito (Aphrastura spinicauda), a suboscine species that generally does not maintain long-term pair bonds. We found that, compared to females, males displayed more alarm calling during their response, approached closer and were also more physically aggressive towards conspecific intruders. Despite these differences, the defence behaviour of males and females was highly correlated during territorial responses to simulated intruders. To our knowledge, this is the first report of coordinated or joint territorial defence in a South American suboscine that generally only maintains short-term pair bonds.

Territory exclusivity and intergroup encounters in the indris (Mammalia: Primates: Indridae: Indri indri) upon methodological tuning

Territorial, socially monogamous species actively defend their home range against conspecifics to maintain exclusive access to resources such as food or mates. Primates use scent marks and loud calls to signal territory occupancy and limit the risk of intergroup encounters, maximizing their energetic balance. Indri indri is a little-studied territorial, socially monogamous singing primate living in family groups. The groups announce territory occupancy with long-distance calls, and actively defend their territories from conspecific intruders. This work includes data collected in three forests in Madagascar on 16 indri groups over up to 5 years. We aimed (1) to estimate the extent of territories using minimum convex polygon (MCP), implementing minimum sampling effort requirements; (2) to quantify territorial exclusivity, measuring the overlap between territories; and (3) to evaluate the intergroup encounter rate and to quantify the dynamics of group encounters. Our results showed that indris range evenly within exclusive small territories with no or little overlap. Intergroup encounters are rare (0.05 encounters per day), and are located on the periphery of the territories. Disputes were mostly solved with vocal confrontation and only in 13% of the cases ended in physical fights. This frame underlines a cost–benefit explanation of territoriality, favouring a strategy that efficiently limits overlap and avoids costly intergroup encounters. We hypothesize that territorial behaviour in indri is related to mate-guarding strategy and that vocal behaviour plays a fundamental role in regulating intergroup spacing dynamics.

In front of a mirror: visual displays may not be aggressive signals in nocturnal tree frogs

ABSTRACTSome evidence indicates that in anuran amphibians, visual signalling can be important during social interactions such as territorial disputes among males, especially in diurnal species. The correct identification of a signal is not a trivial matter. A visual signal provides a visual cue during a social interaction, and to be effective it must elicit an immediate response in the receiver. We tested the hypothesis that visual displays in an agonistic context constitute aggressive signals, in three nocturnal species of Hylidae. We predicted that the production of visual displays would increase in the presence of a conspecific intruder male. Males of Hypsiboas raniceps, Dendropsophus nanus and Lysapsus limellum were submitted to two treatments: (1) Self Image, a reflection in a mirror, simulating the presence of an intruder; and (2) Control, a black rectangle covering the mirror. We observed three visual displays: vocal-sac display (inflate the vocal sac and maintain it inflated for some time), limb lifting (rapid up-and-down movements of one or more limbs), and toe/finger trembling (rapid up-and-down movements of one or more toes and/or fingers). This last display was observed only in H. raniceps males. Contrary to our hypothesis, the emission rates of all visual displays of the focal animals did not differ between treatments; and the behavioural response did not differ among species. Therefore, we suggest that these behaviours could not be used directly for communication in agonistic contexts, and may represent displacement activities (involuntary responses). Alternatively, an aggressive bimodal stimulus may be necessary to trigger a behavioural response by using visual signals during territory defence in these three species.

Residency in white-eared hummingbirds (Hylocharis leucotis) and its effect in territorial contest resolution.

BackgroundTerritory owners usually defeat intruders. One explanation for this observation is the uncorrelated asymmetry hypothesis which argues that contests might be settled by an arbitrary convention such as “owners win.” We studied the effect of territorial residency on contest asymmetries in the white-eared hummingbird (Hylocharis leucotis) in a fir forest from central Mexico.MethodsTwenty white-eared male adult hummingbird territories were monitored during a winter season, recording the territorial behavior of the resident against intruding hummingbirds. The size and quality of the territory were related to the probability that the resident would allow the use of flowers by the intruder. Various generalized models (logistical models) were generated to describe the probabilities of victory for each individual resident depending on the different combinations of three predictor variables (territory size, territory quality, and intruder identity).ResultsIn general, small and low quality territory owners tend to prevent conspecific intruders from foraging at a higher rate, while they frequently fail to exclude heterospecific intruders such as the magnificent hummingbird (Eugenes fulgens) or the green violetear hummingbird (Colibri thalassinus) on any territory size. Our results showed that the identity of the intruder and the size and quality of the territory determined the result of the contests, but not the intensity of defense.DiscussionInitially, the rule that “the resident always wins” was supported, since no resident was expelled from its territory during the study. Nevertheless, the resident-intruder asymmetries during the course of a day depended on different factors, such as the size and quality of the territory and, mainly, the identity of the intruders. Our results showed that flexibility observed in contest tactics suggests that these tactics are not fixed but are socially plastic instead and they can be adjusted to specific circumstances.

Social behavior and cooperative breeding in a precocial species: The Kalij Pheasant (Lophura leucomelanos) in Hawaii

We examined social behavior and documented cooperative breeding in Kalij Pheasants (Lophura leucomelanos) in an introduced population in Hawaii, only the third reported instance of cooperative breeding in Phasianidae. Birds in 31 distinct social groups occupied overlapping home ranges, and group composition remained relatively constant over the 3-yr study period. Each social group contained 1 breeding female and 1 to 6 males. One male was socially dominant in each multiple-male group. Age was the only factor found to determine within-group dominance, suggesting that subordinate males may gain dominance and breeding status by staying in the group. All adults exhibited cooperative behavior, including caring for chicks, agonistic behaviors against conspecific intruders, and vigilance against predators. Average population density was high (3.21 residents per ha), which may lead to habitat saturation in this population. The adult sex ratio was male-biased, with an average M:F ratio of 2.1:1.0. Genetic sex identification of egg samples revealed unbiased primary and secondary sex ratios, which suggests that the bias in adult sex ratio may be caused by differential survival and/or dispersal. Paternity analysis of 13 broods revealed that ~68% of offspring were fathered by the dominant male of the social group, while ~17% were fathered by subordinate males in the group, suggesting that helpers gained some direct benefit by sharing reproduction. We suggest that cooperative breeding may be more common in precocial species than conventionally recognized.

Female splendid and variegated fairy-wrens display different strategies during territory defence

Contrary to the traditional view that territory defence is a male behaviour, there is now evidence that female birds actively engage in territory defence, either alone or with their mate. In males, song sharing between neighbours has been shown to facilitate territory defence, but little is known about the importance of song sharing for such behaviour in females. Here, I examined sex roles and song element sharing in two related species: the splendid fairy-wren, Malurus splendens , and the variegated fairy-wren, Malurus lamberti . I first described song structure and song element sharing between partners and neighbours in both male and female songs. I then simulated conspecific intrusions by broadcasting a female or a male song within a pair's territory. Both splendid and variegated fairy-wren females sang songs as complex as their male counterparts and responded to playback of simulated conspecific intrusions. Song element sharing with neighbours was common in both species, but only splendid fairy-wren females shared more elements with their mate than with other recorded males on average. Females in each species engaged in different strategies when responding to territory intruders: female splendid fairy-wrens competed with female intruders, while variegated fairy-wren females coordinated their defence with their mates and exhibited cooperative defence behaviours toward male intruders. I discuss the ideas that variation in levels of extrapair paternity and/or female competition may drive the evolution of different female strategies when responding to territory intruders.

Temporal and sex-specific patterns of breeding territory defense in a color-polymorphic cichlid fish

In biparental species, the costs and benefits of parental investment can vary between the sexes and shift over time. However, such sex-specific and temporal changes in territory defense are not well understood. Here, we experimentally investigated parental investment in breeding territory defense in a feral population of the color-polymorphic, biparental cichlid fish, the red devil (Amphilophus labiatus). We presented either gold- or dark-colored conspecific intruder models (i.e., dummy models) to A. labiatus pairs at three key stages during the breeding cycle (i.e., after pair formation, after eggs have been laid, and when fry were free-swimming). We found that males were more aggressive when the pair first formed, whereas females significantly increased their territory defense with time, and were most aggressive when fry were free-swimming. These results show that parental roles in territory defense can markedly shift over key stages of the breeding cycle. Our results demonstrate that parental behaviors may not only vary between the sexes, but can also shift dramatically over the course of the brood cycle.

Seeing orange: breeding convict cichlids exhibit heightened aggression against more colorful intruders

Female convict cichlids (Amatitlania siquia) exhibit bright orange ventral coloration that males lack. The behavioral implications of this color are poorly understood, particularly in naturally occurring populations where female coloration could play a role in the expression of territorial nest-guarding behaviors. In this field experiment, monogamous breeding pairs of convict cichlids were presented with 3D printed model conspecific intruders of three body sizes (small, medium, and large) exhibiting three orange patch sizes (large, small, or none) to observe how territorial aggression varied as a function of intruder size and female coloration. Individuals occupying breeding pairs that were defending hatched offspring were significantly more aggressive toward intruders with small and large amounts of orange than toward models lacking orange, indicating that color is an important context-dependent elicitor of aggression in this species. Males were significantly more aggressive toward the intruder than females, and male aggression was strongly influenced by their size relative to the intruder. When males were smaller than the intruder, they performed significantly more aggressive acts than when they were the same size or larger than the intruder; this trend persisted across three putative populations in Lake Xiloa, Nicaragua. A potential explanation for these findings is that the orange color functions as a signal of individual quality or breeding readiness and that breeding pairs increase aggression to repel intruders that pose the greatest threat to pair bond and nest maintenance. One or both sexes of many animal species possess brightly colored features that might communicate information about overall heath or reproductive status. In convict cichlid fish, males and females establish pair bonds and jointly defend their nest and offspring. Single females exhibit striking orange coloration that males and breeding females lack. Our field-based experiment provided evidence that more colorful females, which likely pose a threat to pair bond stability and nest maintenance, incite more aggression from breeding pairs than drab females. Our study suggests that color provides salient information about, perhaps, female quality or readiness to breed in natural populations, and adds to a growing body of research that seeks to understand the varied roles that colorful ornaments play in animal communication.

Anthropogenic noise weakens territorial response to intruder's songs

AbstractNoise pollution degrades natural acoustic conditions, potentially interfering with bird communication. However, exactly how noise impacts the ability of the signal receiver to detect and discriminate vocalizations from conspecifics remains understudied in field settings. We performed a natural experiment to determine the effect of noise pollution on the territory‐defense behaviors of two emberizid sparrows exposed to carefully constructed playbacks of conspecific intruder songs. Although all birds reacted to the playbacks, response latency increased with noise levels. This suggests that noise interferes with signal reception and may indicate impaired signal discrimination. We place these results in the context of a receiver's “listening area” and the significant impact of noise pollution on this receiver‐centric perceptual acoustic range. This work informs conservation efforts and provides a much needed field‐based examination of the disruptive impact of noise pollution on behaviors directly related to reproduction and fitness.

Not just for males: females use song against male and female rivals in a temperate zone songbird

Birdsong in temperate zone passerines is a trait under sexual selection in males. Female song is still thought to be rare in this group. Here we show that female song is common in a temperate zone population of house wrens, Troglodytes aedon, and we provide evidence for its functional role in defending against male and female conspecifics. We observed that females sang most frequently at the onset of egg laying, with song becoming less common as incubation approached. Thus, females sang most during the time when eggs were left unguarded and susceptible to conspecific attack. We also conducted playback experiments to test whether conspecific stimuli would induce female song in focal individuals. Playback from both male and female conspecifics elicited strong song responses from resident females, who often vocalized independently from their partners. However, females were more physically aggressive towards female songs than male songs. Finally, females that sang more during these simulated conspecific intrusions ultimately lost fewer eggs to house wren ovicide. These results suggest that female house wren song may have evolved, at least in part, for use in intra- and intersexual competition. These results also highlight how investigating these traditionally male behaviours in female animals can lead to key insights regarding the evolution of sexual dimorphism.

Males of Hypsiboas goianus (Anura; Hylidae) do not assess neighbor fighting ability through acoustic interactions

In frogs, acoustic signals are the most important communication mechanism, since they may be used in several social contexts. In many anurans, the dominant frequency of calls is negatively related to body size, and in such species, this spectral parameter may be considered a good predictor of fighting ability. We experimentally investigated the vocal behavior of 30 male Hypsiboas goianus in central Brazil to answer the following questions: (1) Do males change the acoustic parameters of their calls in response to conspecific intruders? and (2) Does the acoustic behavior of H. goianus depend on the simulated body size of their opponent? We used playback of synthesized calls with high (3573 Hz) and low (3123 Hz) dominant frequency to simulate small and large males, respectively. Males reduced the rate of advertisement calling in response to playback but did not change vocal behavior in response to low-frequency and high-frequency playback. So, while males adjust their calling activity in response to simulated conspecifics, there was no evidence that they assess the fighting ability of their opponents through acoustic interactions.