Aggressive Signals Research Articles

Responses to song playback vary with the vocal performance of both signal senders and receivers

Mating signals of many animal species are difficult to produce and thus should indicate signaler quality. Growing evidence suggests that receivers modulate their behaviour in response to signals with varying performance levels, although little is known about if and how responses are affected by receiver attributes. To explore this topic we conducted two experiments with swamp sparrows, Melospiza georgiana, in which we challenged territorial males with playback of songs with trill rates that were natural, digitally reduced, or digitally elevated (control-, low- and high-performance stimuli, respectively). In our first experiment, we found that males responded more aggressively to control songs than to low-performance stimuli, that low-performance stimuli with the most severe trill-rate reductions elicited the weakest aggressive responses, and that the subjects' own trill rates predicted aggressive responses. In our second experiment, we found that male responses to high-performance stimuli varied significantly, in ways predicted by two factors: the degree to which we had elevated stimulus performance levels of high-performance stimuli, and subjects' own vocal performance levels. Specifically, males were less aggressive towards stimuli for which we had elevated performance levels to higher degrees, and subject males with higher vocal performances themselves responded more aggressively. These findings together offer a novel illustration of how responses to aggressive signals may rely not just on signal attributes, but also on attributes of responding animals themselves.

Escalation of aggressive vocal signals: a sequential playback study

Rival conspecifics often produce stereotyped sequences of signals as agonistic interactions escalate. Successive signals in sequence are thought to convey increasingly pronounced levels of aggressive motivation. Here, we propose and test a model of aggressive escalation in black-throated blue warblers, presenting subjects with two sequential and increasingly elevated levels of threat. From a speaker outside the territorial boundary, we initiated an interaction (low-threat level), and from a second speaker inside the territory, accompanied by a taxidermic mount, we subsequently simulated a territorial intrusion (escalated threat level). Our two main predictions were that signalling behaviours in response to low-threat boundary playback would predict signalling responses to the escalated within-territory threat, and that these latter signalling behaviours would in turn reliably predict attack. We find clear support for both predictions: (i) specific song types (type II songs) produced early in the simulated interaction, in response to boundary playback, predicted later use of low-amplitude 'soft' song, in response to within-territory playback; and (ii) soft song, in turn, predicted attack of the mount. Unexpectedly, use of the early-stage signal (type II song) itself did not predict attack, despite its apparent role in aggressive escalation. This raises the intriguing question of whether type II song can actually be considered a reliable aggressive signal. Overall, our results provide new empirical insights into how songbirds may use progressive vocal signalling to convey increasing levels of threat.

Being angry, singing fast? Signalling of aggressive motivation by syllable rate in a songbird with slow song

Syllable rate has been shown to play a role in male–male aggressive interactions and has been proposed to serve as a male quality indicator in several bird species. In those with fast syllable rates, males often increase rates when singing in aggressive context, and respond differently to test stimuli of varying rates. We asked whether the syllable rate fulfils a similar signalling function in the chiffchaff (Phylloscopus collybita), a songbird species with a slow syllable rate. We confronted 36 chiffchaff males with one of three playback types differing in syllable rate: control (non-manipulated rate), fast, or slow (artificially increased and decreased syllable rate, respectively). We recorded tested males’ songs and behaviour before and during the experiment. Our results indicate that syllable rate might be an aggressive signal in chiffchaff. Males that physically attacked the loudspeaker during experiments sang faster songs spontaneously, and those that continued singing during the playback responded to fast and non-manipulated stimuli with substantial increase of syllable rate. Indirect evidence further suggests that syllable rate in chiffchaff is unlikely constrained by respiratory demands; thus, we propose that syllable rate in this species functions as a conventional signal of male aggressiveness rather than an index of quality.

Visual signals or displacement activities? The function of visual displays in agonistic interactions in nocturnal tree frogs

The efficiency of intraspecific communication directly affects male reproductive success. Acoustic signaling is the primary form of communication in nocturnal anurans. However, visual signaling can also be important in social interactions. We tested the hypothesis that open environments favor visual signals in a territorial defense context, in a nocturnal tree frog. We established three treatments each with eight males of Hypsiboas albomarginatus: (1) Clear Vision, with a mirror without visual obstacles; (2) Obstructed Vision, with half the mirror covered, and (3) Control, with mirror completely covered. We classified behavioral responses into orientation/locomotion, visual display, or acoustic signal. We calculated the mean emission rate per minute per behavior in each treatment and compared them among treatments using one-way ANOVA. Orientation and locomotion, visual display, and the advertisement call did not differ among treatments. However, the emission of aggressive calls in the Obstructed Vision treatment was significantly higher than in the Clear Vision treatment. The lowest rate of aggressive calls occurred in the Control. Thus, visual recognition of an intruder male was enough for resident males to adjust their rate of emission of acoustic aggressive signals, but not visual displays. Therefore, the recognition of the intruder male is not the only feature required for the evolution of visual signals in nocturnal tree frogs during agonistic interactions. This suggests that some visual displays may not be directly used for communication but rather constitute displacement activity.

Knowing your audience affects male–male interactions in Siamese fighting fish (Betta splendens)

Aggressive interactions between animals often occur in the presence of third parties. By observing aggressive signalling interactions, bystanders may eavesdrop and gain relevant information about conspecifics without the costs of interacting. On the other hand, interactants may also adjust their behaviour when an audience is present. This study aimed to test how knowledge about fighting ability of an audience affects aggressive interactions in male Siamese fighting fish. Subjects were positioned between two dyads of non-interacting males and allowed to observe both dyads shortly before the view to one of the dyads was blocked, and the dyads were allowed to interact. Subjects were subsequently exposed to an unknown opponent in the presence of either the winner or the loser of the seen or unseen interaction. The results suggest a complex role of the characteristic of an audience in the agonistic behaviours of a subject engaged in an interaction. The presence of a seen audience elicited more aggressive displays towards the opponent if the audience was a loser. This response was different in the presence of an unseen audience. Subjects then directed a higher aggressiveness against their opponent if the audience was a winner. These results also suggest a potentially more complex and interesting process allowing individuals to gain information about the quality and threat level of an unknown audience while it is interacting with a third party. The importance of information acquisition for an individual to adapt its behaviour and the role of communication networks in shaping social interactions are discussed.

A test of a hierarchical signalling model in song sparrows

Hierarchical signalling may be a common adaptation for aggressive signalling. In this strategy an animal progresses through a series of discretely different signals of escalating level of threat before eventually proceeding to physical aggression. A model of such hierarchical aggressive signalling has been proposed for song sparrows, Melospiza melodia , in which a core part of the sequence is: song type match → soft song → attack. The model predicts that song type matching is a strong predictor of soft song, but only a weak predictor of attack. We used a two-part playback design to test these predictions, with an initial edge playback from just off the subject's territory using a song type that the subject could match, followed by a centre playback from a speaker placed well within the territory. Each male was tested twice with this design. We found that matching the edge playback did not predict soft song production at the centre. A second strong threat, wing waving, was actually negatively associated with matching. Matching the edge playback also was not associated with physical measures of aggression such as approach and latency to approach. Thus, this particular model of hierarchical aggressive signalling was not supported for our study population. Song type matching in our study population may have some function not associated with aggressive signalling to the matched individual, or no function at all.

Is the Soft Song of the Brownish‐Flanked Bush Warbler an Aggressive Signal?

AbstractSoft songs have been detected in many songbirds, but in most species, research on soft songs has lagged behind studies of broadcast songs. In this study, we describe the acoustic features of a soft song in the brownish‐flanked bush warbler Cettia fortipes. Compared with the broadcast song, the warbler's soft song was characterized by a lower minimum frequency and longer duration, and it had a higher proportion of rapid frequency modulation notes. Using playback experiments, some in combination with mounted specimens, we found different responses to soft and broadcast songs, and we found that soft song can predict aggressive escalation (attack). We conclude that the soft song is an aggressive signal in this species.

Aggressive Waves in the Lemon-Clawed Fiddler Crab (Uca perplexa): A Regional “Dialect” in Fiji

A population of the lemon-clawed fiddler crab (U. perplexa) in Fiji (island of Vanua Levu) was studied for types of communication (i.e., signaling via waving the male’s larger claw). Two types of signals were observed. In addition to the expected territorial display of a large and complex vertical wave that conveys its message over a typical distance of 10–40 cm (with large males signaling to other large males over the greatest distance), a short, rapid, and horizontal wave was typically directed over a much shorter distance, rarely exceeding 10 cm. This latter wave type, seemingly of an aggressive nature, differs from the vertically directed aggressive signal observed in an Australian population of this species and thus appears to be a regional “dialect” for this mode of communication.

Gray tree frogs, Hyla versicolor, give lower-frequency aggressive calls in more escalated contests

As animal contests escalate, variation in the performance of aggressive signaling behaviors can give important insights into contest dynamics. In anuran amphibians, males of numerous species utilize distinctive aggressive vocalizations during disputes over calling spaces. Little is known, however, about the causes and consequences of variation in aggressive-call characteristics. We analyzed recordings of calls made during staged aggressive interactions between male gray tree frogs, Hyla versicolor, to determine how variation in a key aggressive-call characteristic, dominant frequency, was affected by increasing contest escalation. We found that dominant frequencies of aggressive calls were lower than those of advertisement calls used to attract females. Furthermore, we found that males lowered their aggressive-call frequencies with increases in escalation. Winners tended to have lower-frequency aggressive calls than losers. We conclude that aggressive calls in H. versicolor are similar to the graded aggressive calls that have been described in several other species. This gradation may allow males to balance the energetic costs of producing lower-frequency calls with the benefits of efficiently repelling rival males. Other processes related to motivation and the physiological effects of participating in contests may also be responsible for the observed variation in aggressive-call frequency with contest escalation. Our results demonstrate that detailed experimental studies of aggressive calling behavior in anurans, which to this point have rarely been performed, are feasible and generate important insights relating to general problems in animal contest behavior and animal communication.

Limits to reliability in avian aggressive signals

Early game theory models of aggressive signaling predicted that aggressive signals would be unreliable because all signalers would be selected to exaggerate their signals regardless of their aggressive intentions. Recently some signals have been shown to convey reliable information about aggressive intentions, but in all cases reliability is limited. Here we test whether limits to reliability are due to exaggeration of aggressive intentions, or ‘bluffing’, as originally envisioned. Our earlier work on two related songbird species, song sparrows (Melospiza melodia) and swamp sparrows (M. georgiana), has shown that low intensity ‘soft song’ is the signal that best predicts attack in both cases. Here we test two predictions of the bluffing hypothesis: (1) that the distribution of the number of soft songs given per signaler should be skewed towards high values because of widespread exaggeration and (2) that unreliability should arise from over-signaling (giving many displays and not attacking) rather than from under-signaling (giving few or no displays and attacking). Neither prediction is upheld in either species. We propose that the surprising prevalence of under-signaling can be explained by the opportunity costs of aggressive signaling.

Fiery frills: carotenoid-based coloration predicts contest success in frillneck lizards

Conspicuous color signals are commonly used by a wide diversity of animals to advertise some specific aspect of their competitive ability. In particular, orange and red colors are often used as an aggressive signal to potential rivals. The iconic Australian frillneck lizard (Chlamydosaurus kingii) has a large and extensible frill that varies geographically in the extent of yellow, orange, and red present. Focusing on the red-orange lizards found in Western Australia, we first established that, in contrast to most lizards studied to date, the red-orange–colored frill is carotenoid rather than pterin based. Second, we measured chromatic and achromatic aspects of male frills before staging dyadic contests between males to determine whether color is used to signal fighting ability. Both chromatic and achromatic measures of the red-orange patches on the lizard’s frills, as well as similarly colored patches on their throats, were reliable predictors of competitive ability; males possessing more colorful displays dominated sizematched competitors in dyadic contests. Apart from large differences in body size and mass (>10%), no other colorimetrics (e.g., white patches), frill traits (e.g., size, relative and absolute patch sizes), or morphological measures (e.g., head size, bite force) were found to predict the outcome of contests. To the best of our knowledge, this is the first example of carotenoid-based patches signaling male fighting ability in a lizard.

Socially mediated plasticity in call timing in the gray tree frog, Hyla versicolor

Call timing is an important component of the behavioral repertoire of many chorusing species that compete acoustically for mates. The costs and benefits of particular call-timing patterns may vary with factors in the social environment, yet few studies have examined the possibility of socially mediated plasticity in call timing. We studied the effects of competition on advertisement-call timing in the gray tree frog, Hyla versicolor, by manipulating intermale distances in staged interactions. We measured phase angles and the proportion of calls that overlapped as interacting pairs of males were moved closer to one another, simulating an increase in acoustic competition. We observed a shift in phase angle resulting in substantially higher levels of overlap as males were moved so that they were calling at extremely close range. At close range, males often engage in intense agonistic contests over the calling space and we found that call timing may play a role in these interactions. Although there were not major differences in the call-timing behavior of winners and losers of contests, we suggest that call overlap may be a general signal of aggressive motivation. Changes in call timing with changes in intermale distance may, therefore, be related to changes in the intended audience as competition escalates. Key wor ds: aggressive signal, anuran, call timing, competition, overlap. [Behav Ecol]

Canyon wrens alter their songs in response to territorial challenges

Many animals use low frequency and harsh sounds in aggressive contexts. Low frequencies are correlated with body size in some species, and thus can provide an indication of fighting ability. Because of this, low frequencies and harsh sounds may also indicate aggressive intent and motivation to attack. We tested for facultative adjustment of vocal behaviour in canyon wrens, Catherpes mexicanus , by using playbacks to simulate territorial intrusions. Thirteen territory-holding wrens were exposed to playback of typical conspecific songs which did not include harsh elements. Territory holders increased song rate in response to simulated intrusions, altered song type usage and altered some song sound frequency parameters. Birds singing in response to playbacks did not adjust the highest frequencies or the frequencies with maximum power in their songs, but they significantly lowered the lowest song frequencies and were more likely to append harsh notes to the ends of their songs. Results match predictions of the motivation-structural hypothesis, and suggest that canyon wrens alter their songs in multiple ways when faced with conspecific territorial intrusion. ► We tested predictions of the motivation-structural hypothesis in canyon wrens. ► We also tested whether wrens use low-frequency sounds as signals of aggression. ► Territory-holding canyon wrens changed their songs in response to playback of conspecific song. ► They used lower song frequencies and harsh notes more often following simulated territorial intrusions. ► Ours is the first study to experimentally demonstrate facultative song adjustment in a passerine.

Signalling by banks using loan loss provisions: the case of the European Union

PurposeThe purpose of this paper is to investigate whether bank managers of countries within the European Union (EU) engage in signalling, especially after implementation of international financial reporting standards (IFRS) commencing 2005.Design/methodology/approach“Signaling” is the use of loan loss provisions (LLPs) to convey signals of fiscal prudence and future profitability to investors. The authors use data from 18 countries across the EU covering the pre and post IFRS regimes and apply univariate and multivariate tests in order to test signaling behavior under both accounting regimes.FindingsThe findings indicate insufficient evidence that financially healthy banks engage in signaling behavior. However, banks facing financial distress appear to engage in aggressive signaling relative to healthy banks. Finally, the propensity to engage in signaling behavior is more pronounced for financially distressed banks in the post IFRS regime. While IFRS, under IAS 39 sort to mitigate the discretionary component of LLPs, our finding may be attributable to lax enforcement of IFRS.Practical implicationsThe findings have implications for both investors and regulators. Investors should be aware that troubled banks engage in signaling to convey positive information about their future prospects. Regulators should be aware that financially stressed banks have a greater propensity to engage in signaling and need to ensure that the provisions of IFRS (which attempts to limit discretion in estimating LLPs) are enforced more stringently.Originality/valueThe paper contributes to the growing literature on bank signaling in a number of ways. First, the authors use a sample from 18 countries within the EU which has not been done before. Second, unlike prior studies which only examined healthy banks, the authors also include financially distressed banks in the sample. Third, the authors examine signaling behavior in the pre and post IFRS regimes to understand the influence of IFRS on the propensity to engage in signaling by bank managers.

Soft song is a reliable signal of aggressive intent in song sparrows

Animals frequently use signals to modulate aggressive interactions. Establishing that a signal is aggressive or threatening requires demonstrating that it is more commonly used in agonistic contexts, that it predicts subsequent aggressive behaviors by the sender, and that receivers respond differently to this signal. Like many birds, song sparrows (Melospiza melodia) produce a low-amplitude “soft song” vocalization that has been hypothesized to be an aggressive signal. Soft song meets the first two criteria, but previous research has failed to demonstrate that soft song provokes aggression or that receivers even perceive soft song differently from normal loud song. We used a playback experiment with taxidermic mount presentation to test whether territorial male song sparrows respond differently to loud and soft song playbacks. Subjects reacted more strongly to the soft song playback by approaching the mount more closely, increasing wing wave displays, and increasing the proportion of their own songs that were soft songs, with further trends toward increasing the number of flights and attacks. These results confirm that soft song is a conventional signal of aggression in song sparrows and that increased receiver retaliation maintains its reliability.

Vocal signals predict attack during aggressive interactions in black-capped chickadees

Animals use a variety of aggressive signals to mediate territorial interactions. Often these signals can be sufficient to ward off potential rivals, thus minimizing the chance of injury due to physical encounters. Yet not all behaviours produced during territorial interactions are aggressive signals. In this investigation, we examined the aggressive signals of black-capped chickadees, Poecile atricapillus, by determining which signals predict attack on a competitor. We used a recently developed playback protocol involving a loudspeaker and a taxidermic mount to simulate an intruder on males' breeding territories. We examined males' behaviours prior to any physical attack on the mount, both in the minute before attack and the time period preceding this minute. In the minute before attack, we found that gargle calls consistently predicted attack. In the preceding time period, we found that high song rate predicted attack. Surprisingly, we found that attack and the behaviours associated with attack were not significantly correlated with male dominance status. We conclude that song rate and gargle calling behaviour communicate intent to attack during territorial interactions in black-capped chickadees. These results expand our knowledge of aggressive signals during territorial encounters by revealing novel vocalizations used to communicate an animal’s probability to attack an opponent. ► We presented territorial male chickadees with playback and a taxidermic mount. ► Birds that sang at a high rate early in playback attacked the mount. ► Birds produced gargle calls immediately prior to attacking the mount. ► Dominance rank did not predict attack but was associated with overlapping behaviour. ► We conclude that gargle calls and song rate are aggressive signals.

A Lack of Sexual Dimorphism in Width-to-Height Ratio in White European Faces Using 2D Photographs, 3D Scans, and Anthropometry

Facial width-to-height ratio has received a great deal of attention in recent research. Evidence from human skulls suggests that males have a larger relative facial width than females, and that this sexual dimorphism is an honest signal of masculinity, aggression, and related traits. However, evidence that this measure is sexually dimorphic in faces, rather than skulls, is surprisingly weak. We therefore investigated facial width-to-height ratio in three White European samples using three different methods of measurement: 2D photographs, 3D scans, and anthropometry. By measuring the same individuals with multiple methods, we demonstrated high agreement across all measures. However, we found no evidence of sexual dimorphism in the face. In our third study, we also found a link between facial width-to-height ratio and body mass index for both males and females, although this relationship did not account for the lack of dimorphism in our sample. While we showed sufficient power to detect differences between male and female width-to-height ratio, our results failed to support the general hypothesis of sexual dimorphism in the face.

Development of Badges of Status in Captive Male House Sparrows (Passer domesticus) in Relation to the Relative Ornamentation of Flock‐Mates

AbstractThe development of quality signals is expected to be largely influenced by environmental conditions among which the social environment could be an important factor, particularly in gregarious animals. Here, we investigated the role of the presumed social environment on the development of badge size in moulting male House Sparrows (Passer domesticus) over a 3‐yr time span. House Sparrow badges are testosterone‐dependent ornaments that are typically argued to be ‘badges of status’ –that is, signals of competitive ability and aggression. Immediately prior to moult, we created several groups of males, either with similarly sized badges or with badges of different sizes, and we measured each male's badge size before and after moult. The former groups were presumed to have a less clear social hierarchy than the latter groups because of group members advertising relatively similar competitive ability (and thus were expected to more frequently challenge one another in order to establish dominance). Conversely, the latter groups were presumed to have relatively higher social stability because birds would less frequently encounter rivals advertising similar agonistic threat. We predicted in groups of birds with similarly sized badges, both (1) larger changes (i.e. increases or decreases) in badge sizes and (2) higher increases in between‐individual variation within groups. The first prediction was supported in one of the 3 yrs, but there was no support for the second prediction in any year. We conclude that social environment has an effect on the development of badges of status, but that it is highly contingent on the specific nature and timing of group interactions.

Is honesty the best policy? Testing signal reliability in fiddler crabs when receiver‐dependent costs are high

Summary1.Contests between conspecific males are an important method of establishing mating rights or territories, yet the potential costs of injuries are high. To reduce potential risks, males can use signals to convey information about their underlying strength to competitors, and although individuals could signal deceptively to gain an advantage, most signals seem to be reliable.2.Theory suggests that signal reliability is maintained because individuals that signal unreliably may be punished (i.e. receiver‐imposed costs). Manipulative studies support this idea, showing that low‐quality individuals that are given high‐quality signals bear substantial costs.3.Here, we explore the importance of receiver‐imposed costs in natural, un‐manipulated populations. Specifically, we show how the likelihood of being exposed (i.e. potential receiver density) and the potential severity of punishment (i.e. average receiver size) affect the predominance of unreliable signalling among populations.4.Male fiddler crabs, Uca vomeris, use their enlarged claws as signals and weapons in combat, and the relationship between claw size (signal) and strength (quality) determines whether or not a male is a reliable signaller. We predicted that the prevalence of unreliable signalling among individual males would increase in populations where the receiver‐imposed costs of deception were low. That is, males should produce unreliable (large but weak) claws.5.We show that individual crabs produce more reliable signals of strength in populations with a high biomass, where there are both higher densities and larger average body sizes of potential receivers. Our study provides evidence that receiver‐imposed costs can maintain signal reliability in natural un‐manipulated populations and supports contemporary models of aggressive signalling.

When Less Is Best: Female Brown-Headed Cowbirds Prefer Less Intense Male Displays

Sexual selection theory predicts that females should prefer males with the most intense courtship displays. However, wing-spread song displays that male brown-headed cowbirds (Molothrus ater) direct at females are generally less intense than versions of this display that are directed at other males. Because male-directed displays are used in aggressive signaling, we hypothesized that females should prefer lower intensity performances of this display. To test this hypothesis, we played audiovisual recordings showing the same males performing both high intensity male-directed and low intensity female-directed displays to females (N = 8) and recorded the females' copulation solicitation display (CSD) responses. All eight females responded strongly to both categories of playbacks but were more sexually stimulated by the low intensity female-directed displays. Because each pair of high and low intensity playback videos had the exact same audio track, the divergent responses of females must have been based on differences in the visual content of the displays shown in the videos. Preferences female cowbirds show in acoustic CSD studies are correlated with mate choice in field and captivity studies and this is also likely to be true for preferences elucidated by playback of audiovisual displays. Female preferences for low intensity female-directed displays may explain why male cowbirds rarely use high intensity displays when signaling to females. Repetitive high intensity displays may demonstrate a male's current condition and explain why these displays are used in male-male interactions which can escalate into physical fights in which males in poorer condition could be injured or killed. This is the first study in songbirds to use audiovisual playbacks to assess how female sexual behavior varies in response to variation in a male visual display.