Aggressive Signal Design in the Jacky Dragon (Amphibolurus muricatus): Display Duration Affects Efficiency

Design characteristics of signals, such as their duration, may have evolved to maximize signal efficiency. It is commonly assumed that constraints on signal design have usually shaped the most optimal display characteristics to improve signal transmission and information transfer of the signaller, and detection by intended receivers. In this study, we tested whether the characteristics (duration, speed and frequency) of an aggressive display, the push‐up body rock, exhibited by the Jacky dragon (Amphibolurus muricatus) have likely evolved for optimal signal efficiency, as it is able to draw attention to the signaller. We performed two video playback experiments using high‐resolution 3D animations testing the effect of variation in push‐up body rock structure. In experiment 1, we manipulated push‐up body rock display structure. We gradually increased the number of push‐ups exhibited by a digitally animated Jacky dragon increasing the overall display duration. In experiment 2, we developed four stimuli based on population‐typical push‐up body rock display for duration (short and long), and frequency of push‐ups (1 or 5 consecutive push‐ups) by manipulating push‐ups’ speed. In both experiments, we measured the probability of an orienting response and response latency of focal lizards when being exposed to the different stimuli. Our results showed that display duration is critically important for signal efficiency in the aggressive push‐up body rock display. If we are to understand the design characteristics of signals used in animal communication, then it appears important to consider the possible trade‐off between signal efficiency and costs.     

Seismic Signaling is Crucial for Female Mate Choice in a Multimodal Signaling Wolf Spider

Complex courtship signals can be dissected into distinct components that can either function independently or via interactions with one another. Male Rabidosa rabida wolf spiders use courtship displays that couple a seismic signal with the waving of an ornamented foreleg. While previous studies suggest that female R. rabida exhibit mate choice and that both the seismic and visual modalities are important in mating interactions, it remains unclear how variation in each component influences female mating decisions. To investigate this, we ran two separate experiments in which we manipulated (1) male diets, to induce variation in the seismic courtship signal, and (2) male foreleg color, to artificially induce variation in visual foreleg ornamentation. To determine the influence of variation in each component independently, females were paired with males in environments that allowed the detection of only the manipulated signal component (e.g. seismic signal only and visual signal only). Variability in the seismic signal alone influenced female mate choice, but variability in visual ornamentation alone did not. In a third experiment, we manipulated foreleg color and allowed it to interact with the seismic signal to determine whether inter‐signal interactions influence female mating decisions. When females were able to detect both signal components, variation in visual ornamentation did influence mate choice – females preferred ornamented males. Together, these results suggest that the seismic signal of male R. rabida is integral for female mate choice and that the components of the courtship display interact to influence female mating decisions.     

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.     

Large-scale patterns of signal evolution: an interspecific study of Liolaemus lizard headbob displays

Although many factors have been shown to influence the evolution of species recognition signals in a wide variety of taxa, it is difficult to draw general conclusions because of fundamental differences in the morphologies and ecologies of the animals considered. In this study, two morphologically and ecologically similar lizard genera (Sceloporus and Liolaemus) are used to provide replicate examples of the evolution of a complex visual display. New data on the headbob displays of 16 Liolaemus species are presented. As in other taxa, phylogenetic analyses show that evolutionary changes in display structure have been rapid, leaving little, if any, phylogenetic information in the display structure. Evolutionary changes in display structure also do not appear to be closely associated with any major habitat characteristics. Despite this rapid evolution, Liolaemus lizards produce headbob displays that are remarkably simple in structure in comparison to those produced by Sceloporus, perhaps compensating for lower complexity by frequent use of other visual displays such as forelimb and tail waves.     

Display Plasticity in Response to a Robotic Lizard: Signal Matching or Song Sharing in Lizards?

Many territorial songbirds alter the structure of their songs after listening to and interacting repeatedly with the same neighbors. Here, we use a robotic lizard to test for similar learned changes in signal structure in male Sagebrush lizards, Sceloporus graciosus. Subjects were exposed to two types of headbob displays (species‐typical and unusual) both in short‐term tests and in repeated exposures for 10 d. We found no evidence for immediate changes in signal structure to match a particular opponent (signal matching) or long‐term changes after repeated exposure (‘song’ sharing). If anything, the lizards’ displays became less like that of the robotic stimulus over time. Further tests of other taxa are needed to identify the evolutionary forces that lead to these forms of behavioral plasticity and to determine whether song sharing and signal matching are unique characteristic of songbirds. Lizards also became more agitated and produced more highly aggressive displays of their own when confronted with headbob displays that violated the basic syntactic structure of their display system, confirming that they were paying attention to subtle differences in display structure despite the artificial nature of the treatments. Thus, our study also adds to the growing evidence supporting the use of robotic playbacks to study animal communication.     

Why do curly tail lizards (genus Leiocephalus) curl their tails? An assessment of displays toward conspecifics and predators

Animal display behaviors are used to convey specific messages to other animals, including potential mates, rivals, and predators. However, because these different types of interactions can be mediated by a single behavioral display, or conversely, multiple signals can be used to convey one specific message, interpretation of any particular behavioral display can be difficult. Leiocephalus lizards (i.e., curly tails) provide an excellent opportunity to study the use of display behaviors across multiple contexts. Previous research has demonstrated that the use of tail curling in these lizards is associated with predation risk, but less is known regarding the use of this behavior in social interactions with conspecifics. The goal of this study was to determine the extent to which tail curling display behavior is used to mediate both social and predatory interactions in two species, Leiocephalus barahonensis and L. carinatus. We found that in lizards of both species, tail curling was used in interactions with both conspecifics and potential (human) predators. However, tail curl intensity did not differ between lizards involved in social encounters and solitary lizards, although L. barahonensis lizards performed more headbobs during social than non‐social observations. Further, L. carinatus lizards exhibited greater intensity of tail curling upon fleeing from a human predator than during observations in which individuals interacted with conspecifics, and lizards that exhibited tighter tail curls fled from predators for a longer distance. Finally, tail curl intensity was not correlated with headbob displays in either species, suggesting that these two components of display communicate different information. Our results suggest that tail curling displays, while consistently a component of interactions with potential predators, are not a necessary component of social interactions. These data contribute to a more complete understanding of how and why visual signals evolve for use in communication across multiple contexts.     

The role of colour in signalling and male choice in the agamid lizard Ctenophorus ornatus

Bright coloration and complex visual displays are frequent and well described in many lizard families. Reflectance spectrometry which extends into the ultraviolet (UV) allows measurement of such coloration independent of our visual system. We examined the role of colour in signalling and mate choice in the agamid lizard Ctenophorus ornatus. We found that throat reflectance strongly contrasted against the granite background of the lizards' habitat. The throat may act as a signal via the head-bobbing and push-up displays of C. ornatus. Dorsal coloration provided camouflage against the granite background, particularly in females. C. ornatus was sexually dichromatic for all traits examined including throat UV reflectance which is beyond human visual perception. Female throats were highly variable in spectral reflectance and males preferred females with higher throat chroma between 370 and 400 nm. However, female throat UV chroma is strongly correlated to both throat brightness and chest UV chroma and males may choose females on a combination of these colour variables. There was no evidence that female throat or chest coloration was an indicator of female quality. However, female brightness significantly predicted a female's laying date and, thus, may signal receptivity. One function of visual display in this species appears to be intersexual signalling, resulting in male choice of females.     

Sex-specific visual performance: female lizards outperform males in motion detection

In animal communication, complex displays usually have multiple functions and, male and female receivers often differ in their utilization and response to different aspects of these displays. The perceptual variability hypothesis suggests that different aspects of complex signals differ in their ability to be detected and processed by different receivers. Here, we tested whether receiver male and female Sceloporus graciosus lizards differ in visual motion detection by measuring the latency to the visual grasp response to a motion stimulus. We demonstrate that in lizards that largely exhibit complex motions as courtship signals, female lizards are faster than males at visually detecting motion. These results highlight that differential signal utilization by the sexes may be driven by variability in the capacity to detect different display properties.     

Chemoreception and the Assessment of Fighting Abilities in the Lizard Liolaemus monticola

When an individual faces the risk of a conflict, its ability to make ‘correct’ decisions is crucial to its fitness. Research on decision making has focused mainly on visual and acoustic signals, while chemical signals have received much less attention, despite their relevance for many species. Chemosignals can be detected in the absence of the signaller and, in the context of fighting risk, this property confers the advantage that the receiver can avoid agonistic interactions or, if they are unavoidable, that it can prepare itself for the conflict. I studied the behaviour of males of the lizard Liolaemus monticola in the laboratory when they were confronted with chemosignals of a potential opponent. During this ‘pre‐confrontation’ stage, I tested the following predictions: (1) lizards can derive precise information from chemosignals of conspecifics, and use this to respond with precision to the perceived risk and (2) the best predictor of the receiver behaviour, and therefore the best predictor of the risk involved in the fight, is the relative fighting ability of opponents. As a measure of fighting ability, I used body size. ‘Intruders’ were placed in the terrarium of unfamiliar ‘residents’ during the absence of the latter, and their behaviours were recorded. Simple regressions were performed between the different behavioural variables and with the body sizes of intruder and resident, and with the relative difference in body sizes of opponents. The latter was the best predictor of intruder behaviour: it was negatively correlated with behaviours associated with activity (i.e. motion time), chemoexploration (i.e. number of tongue flicks) and behaviours associated with social interactions (i.e. head bobs). These results suggest that males can process information from chemosignals and decisions made during the ‘pre‐confrontation’ stage are based on the assessment of the relative fighting abilities (i.e. relative body size) of opponents.     

Patch Defence in the Parasitoid Wasp Trissolcus basalis (Insecta: Scelionidae): The Time Structure of Pairwise Contests,and the ‘Waiting Game’

Aggressive defence of host patches has been reported in many parasitoid wasps, but rarely examined in quantitative detail. One aspect of interest is that foraging female parasitoids do not simply consume resource patches, they invest offspring in them. Therefore, patch defence in parasitoids can involve not only resource defence prior to oviposition, but also postoviposition defence of offspring (maternal care). In this paper, the time-structure and sequence of pairwise agonistic contests between females of the parasitoid Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae) are analysed. Three main periods were evident in contests. In the first period, both females exploited the patch with no aggression. After the initiation of fighting, they entered a ‘contest period’, during which resident and intruder roles became clearly resolved. The resident then usually guarded the patch for up to several hours before leaving. This signalled the beginning of the third period, in which the intruder returned to superparasitise the patch. During the contest period, resident behaviour initially reflected the trade-off between exploiting fresh hosts, and defending those it had already parasitised from the intruder, which persistently returned to the patch to try and oviposit, with some success. However, when the patch became fully parasitised, both resident and intruder switched to a ‘waiting game’, in which they sat motionless for extended periods, the resident on the patch and the intruder at a distance. These stand-offs were punctuated by occasional aggressive patrolling by the resident, and cryptic returns to the palch by the intruder. This waiting game appears to be an informational war of attrition, suggesting a conceptual basis for modelling patch-leaving decisions using evolutionary game theory.     

Ready to Fight: Reliable Predictors of Attack in a Cooperatively Breeding,Non‐Passerine Bird

Signal reliability is a major focus of animal communication research. Aggressive signals are ideal for measuring signal reliability because the signal referent – attack or no attack – can be measured unambiguously. Signals of aggressive intent occur at elevated rates in aggressive contexts, predict subsequent aggression by the signaler, and elicit appropriate responses from receivers. We tested the ‘predictive criterion’ in smooth‐billed anis, Crotophaga ani, by broadcasting one of two playback types (‘ahnee’ calls only or ‘ahnee + hoot’ calls), presenting a taxidermic mount, and observing the animals’ behavior. Based on the hypotheses that ‘hoot’ calls and ‘throat‐inflation’ displays signal aggressive intent, we predicted that they would be associated with attack, and that signaling rate would increase over the time period leading up to an attack. Indeed, both hoots and throat‐inflation displays reliably predicted attack. The second prediction, that signaling rate increases in the time leading up to attack, was strongly supported for throat‐inflation displays, which increased over the pre‐attack period in both treatments. Hoots increased over the pre‐attack period in ahnee playbacks but not in ahnee + hoot playbacks. Hierarchical signaling systems are characterized by early, less‐reliable predictors of attack, and later, more reliable predictors of attack. During both natural and simulated interactions, the more‐reliable throat‐inflation display tended to precede the less‐reliable hoot call, suggesting that this signaling system is not hierarchical. In a comparison of 11 putative signals of aggressive intent in birds, the throat‐inflation display had the second highest mutual information (reduction in uncertainty) among visual signals and non‐passerine signals while hoots had below‐average mutual information. Natural observations indicate that both hoots and throat‐inflation displays occur in the context of aggressive between‐group encounters, and hoots also occur during within‐group interactions. Throat‐inflation displays appear to be reliable indicators of aggressive intent, but the function of hoot calls is less clear.     

Negative Association Between Conspicuous Visual Display and Chemosensory Behavior in Two Phrynosomatid Lizards

Communication in one sensory modality can influence communication in others. Lizards in many phrynosomatid species use primarily visual but also chemical signals. The striped plateau lizard, Sceloporus virgatus , exhibits evolutionary loss of a male color signal that in many species is used during aggressive postural displays towards conspecific males. These patches are used similarly in Urosaurus , the sister genus to Sceloporus . We compared a species in which a color signal has been lost, S. virgatus , to a species retaining the ancestral character state of blue abdominal display patches, Urosaurus ornatus , the common tree lizard, to test two hypotheses: (i) conspicuous postural displays that reveal the abdominal patch location are used less in the species that has lost the color patches; and (ii) potential chemical signals are used more in the species with the color loss. We analyzed both visual display behavior (push-up, full-show) and chemosensory behavior (tongue flick and nose tap) of male lizards following their introduction to a resident conspecific male in his home terrarium. Resident males performed very low rates of all behaviors, but intruders exhibited sufficient behavior for analysis.
Supporting the first hypothesis, S. virgatus were less likely than U. ornatus to perform full-show, a display that reveals abdominal skin. Male S. virgatus were more likely to perform push-up than U. ornatus , although S. virgatus performed push-up infrequently. Push-up is a postural display that does not specifically reveal the abdominal patch location. Supporting the second hypothesis, S . virgatus were more likely to perform chemosensory behaviors and performed them at a greater rate than did U. ornatus . Work comparing more closely related species is warranted to determine whether a negative association between conspicuous visual displays and chemosensory behavior is a general pattern.     

The function of agonistic display behaviours in Gnathonemus petersii

In pairs of interacting Gnathonemus petersii , a mormyrid electric fish, dominant individuals were larger (longer body). Pairs of interacting fish of similar body size emitted more parallel displays at the onset of an interaction. Over the course of 10 min interactions, head butting increased and parallel display decreased. This decrease occurred primarily in pairs that contained a prior resident and intruder, as compared with two intruders. The patterns of electric organ discharge during parallel display, and the possible sensory modalities mediating this behaviour are discussed.     

Risky Courtship: Background Contrast,Ornamentation, and Display Behavior of Wolf Spiders Affect Visual Detection by Toad Predators

Males that search widely for females and perform conspicuous courtship displays run a high risk of being detected by their predators. Therefore, gains in reproductive success might be offset by increased mortality due to predation. Male brush‐legged wolf spiders (Schizocosa ocreata) with larger decorative traits (foreleg tufts) are preferred by females as mates, but are more readily detected by predators. However, predation risk may also be influenced by the interaction between components of signals and the environment in which signaling occurs. Courting male spiders were readily accepted as prey by a sympatric predator, the American toad (Anaxyrus americanus). We used video playback to tease apart the interactive effect between visual signals and the signaling environment on the ability of toads to detect courting spiders as a function of distance, background contrast, the presence or absence of male foreleg tufts, and behavioral activity. The response of toads to video sequences of male spiders was similar to their response to live male spiders. Toad response varied over distance toward spiders displayed against high contrast (sunny) vs. low contrast (shaded) backgrounds. Beyond 30 cm, more toads detected courting male spiders against light, ‘sunny’ backgrounds and detected them faster when compared to the same spider stimulus against darker, ‘shady’ backgrounds. In choice tests, toads oriented more often toward courting males with leg tufts than those without. Toad responses also varied with male spider behavior in that only videos of moving males were attacked. Latency to orient and detection by toads was significantly greater for walking males than courting males, and this effect was most evident at distances between 30 cm and 50 cm. Results supported that courting wolf spiders are at significant risk of predation by visually acute predators. Distance, background contrast, and the presence of foreleg decorations influence detection probability. Thus, the same complex visual signals that make males conspicuous and are preferred by females can make males more vulnerable as prey to toads.     

Everybody Needs Good Neighbours: Coalition Formation Influences Floater Fight Choice

In territorial species, it is sometimes less costly to help a neighbour fight off an intruder than to re‐establish territory boundaries with a new, potentially stronger neighbour. In fiddler crabs, a male resident will only help his neighbour if he is larger than the intruder who, in turn, is larger than the challenged neighbour. Does this influence with whom a territory‐seeking male decides to fight? I show that territory‐seeking males appear to choose opponents based partly on the size of the resident’s nearest neighbour. By avoiding challenging resident males with larger neighbours, territory‐seeking males can reduce the likelihood of initiating a fight with a resident who might gain help from his neighbour that decreases the likelihood that the intruder will win the fight.     

Function and evolution of the frill of the frillneck lizard, Chlamydosaurus kingii (Sauria: Agamidae)

Relative to body size, the frill of the Australian agamid lizard Chlamydosaurus kingii is one of the largest and most spectacular display structures seen in any animal species. More than 300 hours observation of free-ranging lizards, combined with data on museum specimens, revealed that the frill is used primarily for intraspecific communication and predator deterrence. Earlier hypotheses on alternative uses for the frill (gliding, food storage, thermoregulation or auditory enhancement) are not supported. The folded frill may also enhance camouflage, but this is probably a fortuitous effect rather than an adaptation.
Male frillnecks frequently display and fight during the mating season. Male displays are highly stereotyped, and involve repeated partial erection of the frill, head-bobbing, tail-lashing, and waving of forelimbs. Both males and females erect the frill during social encounters, and in response to potential predators. Males grow larger than females and have larger heads than do females at the same body size, but no dimorphism is apparent in the relative size of the frill. The extreme development of the display structure in this species may be due to general allometric relationships, as well as to ecological features that have intensified the action of sexual selection in Chlamydosaurus.     

ADAPTATION AND PLASTICITY OF ANIMAL COMMUNICATION IN FLUCTUATING ENVIRONMENTS

Adaptations that facilitate the reception of long‐range signals under challenging conditions are expected to generate signal diversity when species communicate in different habitats. Although we have a general understanding of how individual communicating animals cope with conditions influencing signal detection, the extent to which plasticity and evolutionary changes in signal characteristics contribute to interspecific differences in signaling behavior is unclear. We quantified the visual displays of free‐living lizards and environmental variables known to influence display detection for multiple species from two separate island radiations. We found evidence of both adaptive evolution and adaptive plasticity in display characteristics as a function of environmental conditions, but plasticity accounted for most of the observed differences in display behavior across species. At the same time, prominent differences between the two island radiations existed in aspects of signaling behavior, unrelated to the environment. Past evolutionary events have therefore played an important role in shaping the way lizards adjust their signals to challenges in present‐day environments. In addition to showing how plasticity contributes to interspecific differences in communication signals, our findings suggest the vagaries of evolution can in itself lead to signal variation between species.     

Display behaviour and dewlap colour as predictors of contest success in brown anoles

Many animals display visual signals in male contests for access to females and territories. These visual signals can be multimodal and stimulate different aspects of a signal receiver's visual system. Over two summers, we tested whether aspects of behaviour and dewlap colour might function as signals that predict contest success when males compete for access to either mates or territories in male brown anole lizards. We found that behaviour (PC1, a correlated composite of head‐bob, push‐up, and dewlap extension frequency) and an aspect of dewlap colour (PC3, the relative amounts of ultraviolet, yellow, orange, and red of the dewlap margin) were retained in the minimum adequate model predicting contest success across years and social contexts. Winners showed significant differences in behaviour (winners displayed more) and dewlap margin PC3 (winners had lower PC3 scores) compared to contest losers. These findings suggest that display behaviour and dewlap colour might serve as signals indicating a male's ability to win contests for access to females and territories. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 646–655.     

Behavioural responses of reptile predators to invasive cane toads in tropical Australia

The ecological impact of an invasive species can depend on the behavioural responses of native fauna to the invader. For example, the greatest risk posed by invasive cane toads (Rhinella marina Bufonidae) in tropical Australia is lethal poisoning of predators that attempt to eat a toad; and thus, a predator's response to a toad determines its vulnerability. We conducted standardized laboratory trials on recently captured (toad‐naïve) predatory snakes and lizards, in advance of the toad invasion front as it progressed through tropical Australia. Responses to a live edible‐sized toad differed strongly among squamate species. We recorded attacks (and hence, predator mortality) in scincid, agamid and varanid lizards, and in elapid, colubrid and pythonid snakes. Larger‐bodied predators were at greater risk, and some groups (elapid snakes and varanid lizards) were especially vulnerable. However, feeding responses differed among species within families and within genera. Some taxa (notably, many scincid and agamid lizards) do not attack toads; and many colubrid snakes either do not consume toads, or are physiologically resistant to the toad's toxins. Intraspecific variation in responses means that even in taxa that apparently are unaffected by toad invasion at the population level, some individual predators nonetheless may be fatally poisoned by invasive cane toads.