Signal Of Aggressive Intent Research Articles

Buzzes are used as signals of aggressive intent in Darwin’s finches

Abstract Signals of aggression may potentially reduce the fitness costs of conflict during agonistic interactions if they are honest. Here we examined whether the ‘buzz’ vocalization in two species of Darwin’s finches, the small tree finch, Camarhynchus parvulus, and the critically endangered medium tree finch, C. pauper, found in Floreana Island, Galápagos Archipelago, is a signal of aggression. Specifically, we assessed three criteria for aggressive signalling (context, predictive, and response criteria) in an observational study and a playback experiment. In the observational study, buzzes by the resident male were more common when an intruder was present on the territory in medium tree finches but not small tree finches (context criterion). In the playback experiment, buzzes increased during and after a simulated intrusion for both species (context criterion). Buzzes before the playback period predicted aggressive responses by males (predictive criterion) but buzzes during playback did not. Finally, both species responded more strongly to playbacks of conspecific buzzes compared to conspecific songs and heterospecific buzzes (response criterion). Together the results support the aggressive signal hypothesis for buzz vocalizations, although future studies are needed to understand the evolution and development of this interesting signal.

Hearing the physical condition: The relationship between sexually dimorphic vocal traits and underlying physiology.

A growing amount of research has shown associations between sexually dimorphic vocal traits and physiological conditions related to reproductive advantage. This paper presented a review of the literature on the relationship between sexually dimorphic vocal traits and sex hormones, body size, and physique. Those physiological conditions are important in reproductive success and mate selection. Regarding sex hormones, there are associations between sex-specific hormones and sexually dimorphic vocal traits; about body size, formant frequencies are more reliable predictors of human body size than pitch/fundamental frequency; with regard to the physique, there is a possible but still controversial association between human voice and strength and combat power, while pitch is more often used as a signal of aggressive intent in conflict. Future research should consider demographic, cross-cultural, cognitive interaction, and emotional motivation influences, in order to more accurately assess the relationship between voice and physiology. Moreover, neurological studies were recommended to gain a deeper understanding of the evolutionary origins and adaptive functions of voice modulation.

Song overlapping, noise, and territorial aggression in great tits

Abstract Communication often happens in noisy environments where interference from the ambient noise and other signalers may reduce the effectiveness of signals which may lead to more conflict between interacting individuals. Signalers may also evolve behaviors to interfere with signals of opponents, for example, by temporally overlapping them with their own, such as the song overlapping behavior that is seen in some songbirds during aggressive interactions. Song overlapping has been proposed to be a signal of aggressive intent, but few studies directly examined the association between song overlapping and aggressive behaviors of the sender. In the present paper, we examined whether song overlapping and ambient noise are associated positively with aggressive behaviors. We carried out simulated territorial intrusions in a population of great tits (Parus major) living in an urban–rural gradient to assess signaling and aggressive behaviors. Song overlapping was associated negatively with aggressive behaviors males displayed against a simulated intruder. This result is inconsistent with the hypothesis that song overlapping is an aggressive signal in this species. Ambient noise levels were associated positively with aggressive behaviors but did not correlate with song rate, song duration, or song overlapping. Great tits in noisy urban habitats may display higher levels of aggressive behaviors due to either interference of noise in aggressive communication or another indirect effect of noise.

Are signals of aggressive intent less honest in urban habitats?

AbstractHow anthropogenic change affects animal social behavior, including communication is an important question. Urban noise often drives shifts in acoustic properties of signals but the consequences of noise for the honesty of signals—that is, how well they predict signaler behavior—is unclear. Here we examine whether honesty of aggressive signaling is compromised in male urban song sparrows (Melospiza melodia). Song sparrows have two honest close-range signals: the low amplitude soft songs (an acoustic signal) and wing waves (a visual signal), but whether the honesty of these signals is affected by urbanization has not been examined. If soft songs are less effective in urban noise, we predict that they should predict attacks less reliably in urban habitats compared to rural habitats. We confirmed earlier findings that urban birds were more aggressive than rural birds and found that acoustic noise was higher in urban habitats. Urban birds still sang more soft songs than rural birds. High rates of soft songs and low rates of loud songs predicted attacks in both habitats. Thus, while urbanization has a significant effect on aggressive behaviors, it might have a limited effect on the overall honesty of aggressive signals in song sparrows. We also found evidence for a multimodal shift: urban birds tended to give proportionally more wing waves than soft songs than rural birds, although whether that shift is due to noise-dependent plasticity is unclear. These findings encourage further experimental study of the specific variables that are responsible for behavioral change due to urbanization.Soft song, the low amplitude songs given in close range interactions, is an honest threat signal in urban song sparrows. Given its low amplitude, soft songs may be a less effective signal in noisy urban habitats. However, we found that soft song remained an honest signal predicting attack in urban habitats. We also found that birds may use more visual signals (rapid fluttering of wings) in urban habitats to avoid masking from acoustic noise.

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

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

Songbird chemical signals reflect uropygial gland androgen sensitivity and predict aggression: implications for the role of the periphery in chemosignaling.

Chemical signals can provide useful information to potential mates and rivals. The production mechanisms of these signals are poorly understood in birds, despite emerging evidence that volatile compounds from preen oil may serve as chemosignals. Steroid hormones, including testosterone (T), may influence the production of these signals, yet variation in circulating T only partly accounts for this variation. We hypothesized that odor is a T-mediated signal of an individual's phenotype, regulated in part by androgen sensitivity in the uropygial gland. We quantified natural variation in chemosignals, T, uropygial gland androgen sensitivity, and aggressive behavior in dark-eyed juncos (Junco hyemalis). The interaction between circulating T and androgen receptor transcript abundance significantly correlated with volatile concentrations in male, but not female, preen oil. In both sexes, odorant variables correlated with aggressive response to an intruder. Our results suggest that preen oil volatiles could function as signals of aggressive intent, and, at least in males, may be regulated by local androgen receptor signaling in the uropygial gland. Because these behavioral and chemical traits have been linked with reproductive success, local regulation of androgen sensitivity in the periphery has the potential to be a target of selection in the evolution of avian olfactory signaling.

Vocal repertoire of cooperatively breeding Smooth-billed Anis

Calls are functionally diverse signals that mediate behavior in a wide variety of contexts in both passerines and non-passerines. However, the call-based acoustic communication systems of non-passerines have received less attention from investigators than those of passerines. We examined the vocal repertoire of Smooth-billed Anis (Crotophaga ani), cooperatively breeding cuckoos that live in groups with multiple breeding pairs. We recorded calls from 22 groups over two breeding seasons at the Cabo Rojo National Wildlife Refuge in Puerto Rico. We identified 11 call types and one group vocalization, and used an automated sound measurement program to quantify their acoustic features. Discriminant function analysis (DFA) correctly classified 74.2% of calls based on these features. The vocal repertoire of Smooth-billed Anis is larger than that reported for the three other species in the subfamily Crotophaginae. Smooth-billed Anis have at least two alarm calls, two nest-specific calls, and one nest defense call. We also identified one possible signal of aggressive intent, one possible appeasement signal, and two calls that may communicate identity. The relatively large vocal repertoire of Smooth-billed Anis and association of distinct call types with different functions and contexts supports the main prediction of the social complexity hypothesis, i.e., species with more complex social systems will have more complex communication systems.

It Takes All Kinds in Acoustic Communication: A New Perspective on the Song Overlapping Phenomenon

AbstractOver the last few decades, research into song overlapping produced many – often conflicting – interpretations of its function and culminated in the current debate about the usefulness of this concept. To avoid a deadlock in song overlapping research, we present a new approach to existing evidence and offer several novel hypotheses that might help enhance future experiments. Our analysis offers both a theoretical perspective and specific predictions of each testable hypothesis. We present a detailed analysis of important questions. First, what information does song overlapping convey (is it a signal of aggressive intent or of male quality)? Second, what evolutionary mechanism stabilizes honesty of song overlapping as a signal (is it an index signal, handicap, proximity risk, conventional signal or a modifier)? Additionally, we offer some alternative explanations of the phenomenon (song overlapping as a mask or an incidental phenomenon). We hope to encourage future researchers not only to gather high‐quality experimental data, but also to make more careful interpretations, as we believe that no all‐encompassing explanation of song overlapping will be formulated any time soon. Focused comparative approaches will be necessary, as song overlapping might have different functions in different species.

The role of signature whistle matching in bottlenose dolphins, Tursiops truncatus

The addressing of individuals with learned signals is inherent to human social interactions. It allows individuals to solicit the attention of a particular social companion or to direct information towards an intended recipient. The ability to address individual conspecifics with learned signals is not limited to humans, however. In songbirds, the selective addressing of individuals is facilitated by song type matching but is very much a signal of aggressive intent. The matching of learned signals is also observed in bottlenose dolphins, which will match one another's highly individualized signature whistle. Copying in dolphins occurs between close associates, which suggests that it is an affiliative signal. It could, however, also serve to manage aggression. We investigated the valence of signature whistle matching by performing interactive playback experiments. We waited until an animal produced its signature whistle and then either played back a synthetic version of its own whistle (match) or a different signature whistle (control). A total of 110 playback experiments were conducted with seven different animals from two managed groups of dolphins. The responses to the playback treatments were significantly different. Animals produced a consistent vocal response to being vocally matched, by returning the match, with no associated signal of aggression and did not respond to control playbacks in the same way. There was also an optimum time interval (<1 s) in which a match was most successful in eliciting a vocal response. Our results show that signature whistle matching is an affiliative signal that allows bottlenose dolphins to address social companions. Furthermore, these matching exchanges are driven by temporal associations, which appear to be essential in allowing animals to direct signals to particular individuals in large communication networks.

Male response to an aggressive visual signal, the wing wave display, in swamp sparrows

Signaling often involves complex suites of behaviors that incorporate different sensory modalities. Whatever modality is used to establish that a signal functions in communication researchers must demonstrate that receivers respond to it. The territory defense response of male swamp sparrows involves a variety of behaviors that includes both vocal and visual displays. One of these, the “wing wave” display, is a distinctive movement that predicts physical attack. Here, we use robotic taxidermic mounts paired with song to test the hypothesis that wing waving is a signal and, specifically, that male receivers respond to wing waving as a signal of aggressive intent. As predicted, subjects responded more aggressively to the mount during wing waving trials than during stationary trials. A second experiment demonstrated that this effect cannot be attributed simply to increased attention to movement. Less expectedly, subjects did not alter their own display behavior in response to wing waving as compared to a static mount. We conclude that the wing wave display in the context of singing is a signal that functions in male–male aggressive communication. Questions remain, including whether wing waving functions as a signal in the absence of singing and whether wing waving and song are redundant signals or communicate different information.

Low-amplitude songs produced by male dark-eyed juncos (Junco hyemalis) differ when sung during intra- and inter-sexual interactions

The function and relative importance of low-amplitude (quiet) acoustic signals remains a poorly understood aspect of animal communication. Research in songbirds has predominantly focused on the role of low-amplitude songs as reliable predictors of attack and signals of aggressive intent. However, many species of songbirds produce low-amplitude song in the presence of both male and female conspecifics, suggesting a broader range of functions beyond aggressive behavior. Male dark-eyed juncos (Junco hyemalis) produce two distinct types of low-amplitude song: (1) soft long-range song (LRS), which is a lower amplitude version of junco LRS, and (2) short-range song (SRS), which is substantially more complex and structurally distinct from LRS. SRS is also sung at two distinct tempos (slow and fast) and has been observed in both male- and female-directed contexts. In this study, we presented free-living male juncos with a caged male or female conspecific accompanied by playback of LRS, a female precopulatory trill, or slow SRS and quantified the songs elicited by each social condition. Males produced slow SRS only in response to a female conspecific paired with a precopulatory trill or slow SRS, never to a male, even when males were paired with slow SRS. Together these observations indicate that slow SRS is a female-directed song. In contrast, males produced both LRS and soft LRS in the presence of conspecifics of both sexes, which suggests that LRS may serve a similar function with respect to male and female receivers regardless of amplitude. These results highlight the importance of investigating the production of low-amplitude songs in both intra- and inter-sexual contexts and provide further evidence that in juncos, slow SRS functions in courtship.

The receiver-dependent cost of soft song: a signal of aggressive intent in songbirds

Social costs are one mechanism whereby reliability in signalling systems can be maintained. We measured the strength of aggressive response to territorial playback to ask whether the reliability of ‘soft song’, a strongly aggressive signal in the song sparrow, Melospiza melodia, is enforced by a social cost in the form of the receiver's aggressive response. We also asked whether this cost is imposed by all receivers, or whether a differential response is found primarily or exclusively among the most aggressive subjects. We first measured the strength of each male's aggressive response to playback on his territory, and then tested his responses to ‘warbled’ soft song and to broadcast song playbacks. While we found substantial variation in individual aggressiveness, nearly all males responded more strongly to warbled soft songs. Thus we provide evidence that warbled soft song imposes a social cost in the form of receiver retaliation, and provide a possible explanation for how the signal's reliability is maintained. Questions about soft song remain, notably why selection should favour low amplitude in vocal signals of aggression in songbirds.

Sing softly and carry a big stick: signals of aggressive intent in the song sparrow

Reliability of signalling has been the focus of much research in behavioural ecology, yet few studies have been able to show that putative aggressive signals actually predict subsequent escalation (e.g. attack). This is partly because until recently researchers have usually used stimuli (e.g. song playback) that did not permit subsequent escalation in the form of attack. We presented male song sparrows, Melospiza melodia, with a traditional simulated intrusion that involved only playback of their own song (no-mount condition), or a simulated intrusion that coupled playback of their own song with a taxidermic mount. We hypothesized that aggressive signals that are predictive of attack should increase in frequency when there is a visible intruder present (i.e. in the mount trials). Our results showed that only low-amplitude song (soft song) and wing waves (the latter effect only approached significance) increased in frequency when there was a mount present, consistent with previous research. We discuss these results in the context of how reliability of such a low-cost signal can be maintained, and propose new hypotheses for future research.

Display of an Inedible Prop as a Signal of Aggression? Adaptive Significance of Leaf‐Display by the Turquoise‐Browed Motmot, Eumomota superciliosa

AbstractThe incorporation of an inedible object (prop) into a behavioral display is rare among birds. Only four avian taxa have been reported to display with a prop, and in all studied species, prop use has been found to play a role in mate acquisition. However, little is known about the context and adaptive significance of prop use by the motmots. Both male and female motmots perform a leaf‐display, whereby a leaf is held horizontally in the tip of the bill for an extended period. I collected observational data on leaf‐display by the turquoise‐browed motmots (Eumomota superciliosa) to investigate the potential role of this display as a mate‐choice or agonistic signal. Observations of 61 leaf‐displays by 27 individuals indicate that males and females are similarly likely to perform the leaf‐display, that the leaf‐display is performed throughout the long‐breeding season, and that the display is performed by both paired and unpaired birds. These observations suggest that the leaf‐display does not play a role in mate‐attraction. However, the leaf‐display was often performed in association with a chase or fight with a same‐sex individual, and the display was sometimes performed in conjunction with an agonistic display at nest sites, suggesting that the leaf‐display may function in both sexes as a signal of aggressive intent or as a signal of status.

Soft song in song sparrows: response of males and females to an enigmatic signal

Low-amplitude “soft song” is used by a variety of songbirds; in some species during aggressive encounters, in others during courtship, and yet others in both these contexts. In song sparrows (Melospiza melodia), soft song has thus far been observed only in aggressive encounters, where its production is a more reliable predictor of attack than any other signaling behavior. We used song playback to test the response of both male and female song sparrows to soft song. The design of the playback experiments took into account the existence of two classes of soft song: crystallized soft song, which consists of song types also found in the broadcast repertoire, and warbled soft song, which consists of less-structured song types not found in the broadcast repertoire. Female song sparrows responded with significantly less courtship display to the playback of crystallized soft song than to that of normal broadcast song, and response to warbled soft song was if anything lower than to that of crystallized soft song. Male song sparrows responded equally aggressively to normal broadcast song as to crystallized soft song, and equally aggressively to warbled soft song as to crystallized soft song. The female results support the conclusion that neither form of soft song functions in courtship. The male results suggest that the reliability of soft song as a signal of aggressive intent is not maintained by a receiver retaliation rule.

Bird song as a signal of aggressive intent

A central question in animal communication research concerns the reliability of animal signals. The question is particularly relevant to aggressive communication, where there often may be advantages to signaling an exaggerated likelihood of attack. We tested whether aggressive signals are indeed reliable signals of attack in song sparrows (Melospiza melodia). We elicited aggressive signaling using a 1-min playback on a male’s territory, recorded the behavior of the male for 5 min, and then gave him the opportunity to attack a taxidermic mount of a song sparrow associated with further playback. Twenty subjects attacked the mount and 75 did not. Distance to the speaker was a significant predictor of attack for both the initial recording period and the 1 min before attack. For the initial recording period, none of the measures of singing behavior that we made was a significant predictor of attack, including song-type matching, type-switching frequency, and song rate. For the 1-min period immediately before attack, only the number of low amplitude “soft songs” was a significant predictor of attack. Although most aggressive signals contained little information on attack likelihood, as some models suggest should be the case, the unreliability of these signals was not caused by convergence of individuals on a single signaling strategy, as those models argue should occur.

Plumage color as a status signal in male–male interaction in the red-flanked bushrobin, Tarsiger cyanurus

Recent studies have suggested that structural-based coloration is an honest signal of male genetic and/or conditional quality in sexual selection. However, whether structural coloration functions in intrasexual competition is unknown. We examined whether plumage color functions as a status signal during intrasexual interactions in the red-flanked bushrobin Tarsiger cyanurus; adult males have many blue plumes as structural coloration whereas yearling males and females are olive brown with few blue plumages. Blue males did not always dominate olive-brown males. The number of interactions did not differ with the colors of the two birds involved. The interactions of a blue male and an olive-brown male were less aggressive than those of two blue or of two olive-brown males. In this study, we found that structural plumage coloration may serve as a signal of aggressive intent and lower the escalation level of an aggressive interaction in a manner consistent with hypotheses regarding the evolution of delayed plumage maturation.

Effects of different levels of song overlapping on singing behaviour in male territorial nightingales (Luscinia megarhynchos)

In signalling interactions, animals can directly address information to a specific individual. Vocal overlapping is such a signalling strategy used in songbirds, anurans, and insects. In songbirds, numerous studies using high rates of song overlap to simulate an escalating situation have shown that song overlapping is perceived as a threatening signal by interacting and by listening (eavesdropping) individuals, indicating a high social relevance of song overlapping. Here we present a playback experiment on nocturnally singing male territorial nightingales (Luscinia megarhynchos). Using three different rates of song overlap (1, 25, or 50%), we tested whether or not lower levels of song overlapping act as a signal of aggressive intent and if birds would increase the intensity of their response with increasing level of song overlapping. Subjects did not vary song duration in response to the different playback treatments but increasingly interrupted their singing with increasing overlap by the three playback treatments. The effects persisted even after the playback ceased to overlap and switched to an alternating singing mode. These results expand on previous studies by showing that song overlapping is interpreted as an aggressive signal even when it is used at low or moderate levels. They suggest that, within the range tested here, increasing levels of song overlapping are perceived to be increasingly aggressive.

Excess Capacity and Entry: Some Empirical Evidence

RECENT articles by Spence [I977], Dixit [I979], and Schmalensee [I98I] have elaborated on Wenders' [I97I1 proposition that excess capacity can act as a barrier to entry. The intent of all of these treatments is to suggest that firms may rationally invest in excess capacity with the expectation that excess capacity will be seen by potential entrants either as a signal of aggressive intent' or as a credible threat.2 When excess capacity does not lead to creation of a credible threat, it may still act as a barrier to entry by shifting the risk-return perceptions of potential entrants enough to divert the potential entrants' investments into other industries. 3 Alternatively, investment in excess capacity by incumbent firms may reduce the prospects of an active market for the used equipment of a defunct entrant and thereby help to convert fixed costs into sunk costs, consequently raising entry barriers.4 This paper represents an exploratory effort to investigate empirically the prevalence and efficiency of excess capacity in deterring entry. First, a simple and traditional model of entry is presented. This is followed by the description of the data used to explore the excess capacity-investment concept. A combined results and comments section follows the description of the data.