Honesty of agonistic signalling and effects of size and motivation asymmetry in contests

Game theoretical models predict that the main function of fighting behaviour is to assess the relative fighting ability of opponents. The sequential assessment game has often been used to investigate contests, while honest signalling theory has received much less attention. With the wolf spider Hygrolycosa rubrofasciata we investigated whether male agonistic signalling can reveal honest information about fighting ability, and how size and motivation asymmetries affect male fighting behaviour. We also determined whether male–male competition affects the courtship behaviour of the males. We found that agonistic drumming activity is an honest indicator of male fighting ability, and that relative size asymmetry and motivation to fight both contribute to the fighting ability. We also found that male–male competition decreases the courtship drumming rate of subdominant males, suggesting that male–male competition limits the opportunities for female choice. We conclude that sequential assessment and honest signalling may both be used in settling contests, and that they may be used simultaneously. Received: 10 December 1998 / Received in revised form: 23 June 1999 / Accepted: 5 July 1999     

Mutual assessment via visual status signals in Polistes dominulus wasps

Many animals use signals to assess the fighting ability of rivals and reduce the cost of aggressive competition. However, little is known about how an individual''s own quality influences their signal assessment decisions. Polistes dominulus wasps have visual signals of fighting ability that provide a good model for testing the dynamics of rival choice. We found that rival assessment behaviour was influenced by the advertised quality of the individual, their rivals, and the interaction between individual and rival quality. Individuals of high advertised quality were more likely to challenge rivals and individuals of low advertised quality were more likely to be challenged. However, when choosing among two rivals with different advertised quality, individuals did not simply choose the lower quality rival. Instead, they only preferred the lower quality rival when there was a small difference between their own advertised quality and that of their rivals. Individuals were not choosy when both rivals advertised relatively high or relatively low quality. Therefore, although P. dominulus facial patterns function as conventional signals of fighting ability that provide valuable information about their bearer''s behavioural strategy, there is substantial variation in signal responses based on the relative intensity of the senders'' and receivers'' signals.     

Continuum limits of pattern formation in hexagonal-cell monolayers

Intercellular signalling is key in determining cell fate. In closely packed tissues such as epithelia, juxtacrine signalling is thought to be a mechanism for the generation of fine-grained spatial patterns in cell differentiation commonly observed in early development. Theoretical studies of such signalling processes have shown that negative feedback between receptor activation and ligand production is a robust mechanism for fine-grained pattern generation and that cell shape is an important factor in the resulting pattern type. It has previously been assumed that such patterns can be analysed only with discrete models since significant variation occurs over a lengthscale concomitant with an individual cell; however, considering a generic juxtacrine signalling model in square cells, in O’Dea and King (Math Biosci 231(2):172–185 2011), a systematic method for the derivation of a continuum model capturing such phenomena due to variations in a model parameter associated with signalling feedback strength was presented. Here, we extend this work to derive continuum models of the more complex fine-grained patterning in hexagonal cells, constructing individual models for the generation of patterns from the homogeneous state and for the transition between patterning modes. In addition, by considering patterning behaviour under the influence of simultaneous variation of feedback parameters, we construct a more general continuum representation, capturing the emergence of the patterning bifurcation structure. Comparison with the steady-state and dynamic behaviour of the underlying discrete system is made; in particular, we consider pattern-generating travelling waves and the competition between various stable patterning modes, through which we highlight an important deficiency in the ability of continuum representations to accommodate certain dynamics associated with discrete systems.     

Ageing, reproductive value, and the evolution of lifetime fighting behaviour

The study of sexual selection has recently been enriched by an explicit life history focus. This lifetime perspective has much to offer the field; however, most existing interdisciplinary exchanges have restricted themselves to the context of mate choice and associated phenomena. The present study explores the consequences of an explicit life history view upon the evolution of male–male contest competition. Key to this view is the fact that fighting typically has lifetime consequences, and thus the costs involved with contests are best considered in lifetime currencies such as residual reproductive value. The rate of contemporary contest cost accrual may vary among contestants due to differences between them in what they 'stand to lose', in terms of future reproductive opportunities. It is also suggested that it may be fruitful to partition key life history parameters into components that are either dependent or independent of future choice of mating strategy. Using a simple simulation, it is demonstrated that the optimum pattern of lifetime contest participation may vary depending upon lifetime variation in 'strategy-independent' components of reproductive value (such as the rate of mortality experienced in contexts unrelated to mating behaviour). In line with previous models of age-specific sexual advertisement, increasing lifetime aggression appears as the most favoured strategy; however, young age aggression is predicted if resource holding potential is expected to decline with age irrespective of prior contest participation. The model is discussed, along with key concepts surrounding the life history viewpoint, in light of prior life history/sexual selection models and the existing empirical data regarding lifetime fighting strategies in the wild.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 565–578.     

Social Eavesdropping: A Game-Theoretic Analysis

Here we extend the classic Hawk-Dove model of animal conflict to allow for continuous variation in fighting strengths. Whereas the winner of a fight is chosen at random in the discrete game, in our continuous game, the winner of any fight is the stronger individual, and costs are higher for more evenly matched opponents. We identify the evolutionary stable strength threshold beyond which an animal should be prepared to engage in aggressive behaviour and show that this threshold increases with variance in fighting strength when the costs of aggression are insensitive to the level of strength asymmetry, but decreases with variance when the costs are sensitive to the level of asymmetry. In contrast to the classic discrete game, population-wide aggressive behaviour occurs only when the costs of fighting are zero. It is now known that animals can eavesdrop on the outcome of contests between neighbours and modify their behaviour towards observed winners and losers. We therefore further extend our model to allow for social eavesdropping within networks comprising three individuals. Whereas earlier work showed that eavesdropping increases the frequency of mutually aggressive contests in the discrete game by enhancing the value of victory, here we show that aggression thresholds in the continuous game are always higher with eavesdropping than without it: for sufficiently weak animals, avoiding the costs of challenging an observed winner over-rides the potential benefit of winning, so that eavesdropping reduces the frequency of aggressive encounters. Thus, even though strength is not directly observable, information is extracted from the variation in fighting ability that the classic Hawk-Dove game ignores.     

Effects of female body size and age and male mating status on male combat in Anastatus disparis (Hymenoptera: Eupelmidae)

1. Aggressive behaviour is widely exhibited by animals to acquire important resources and usually shows a dangerous or nondangerous pattern. Dangerous fighting patterns are usually characterized by fights ending with contestants being severely injured or killed. Resource value is an important nonstrategic factor influencing fighting behaviour. Studies of many species addressing nondangerous fighting behaviour have shown that when resource values change, organisms usually adjust their fighting behaviour accordingly. Only a few species show dangerous fighting patterns. Thus, few relevant studies have addressed how variation in resource value affects aggression with a dangerous fighting pattern. 2. Here, an egg parasitoid wasp, Anastatus disparis, which exhibits a dangerous fighting pattern to acquire mating opportunities, was used as an experimental model to study the adjustment of fighting behaviour resulting from a change in resource value. 3. Our results show that the female properties of body size and age affect their objective resource value and that males increased their fighting intensity for relatively large and young females. However, male mating status in A. disparis may not influence the subjective value of mate resources, and fighting intensity did not significantly differ between mated and virgin males. In addition, the number of times a male had previously mated had no significant effect. These results suggest that mating opportunities are important for both virgin and mated males, resulting in neither of them showing any adjustment in fighting for mating opportunities. 4. Generally, A. disparis males with extreme fighting patterns adjust their fighting behaviour according to the variation in resource value, which avoids the meaningless costs of injury and death.     

Dragon wars: Movement‐based signalling by Australian agamid lizards in relation to species ecology

It is evident that the environment has the potential to affect animal communication strategies. Species from diverse taxonomic groups using signals from different modalities are known to generate signals that suit the structure of their habitat in order to maximize efficiency. Studies of acoustically communicating species dominate the literature, but visual signals are also tailored to local conditions. There is now increasing evidence that dynamic visual signals, in the form of movement‐based displays, are also influenced by habitat characteristics. Australia's dragon lizards (Family: Agamidae) employ such dynamic signals in a variety of contexts but are particularly common in territory defence. With a few notable exceptions, the signalling behaviour of this group has been relatively overlooked, and the knowledge that does exist is contained in scientific papers focused on other topics or unpublished accounts from herpetologists. In this review, we collated information on the signalling behaviour of these animals and determined that 34 of the 78 species use movement‐based signalling. This number is likely to be an underestimate, as knowledge of the signalling behaviour of many species is lacking. The richly contrasting environments of Australia inhabited by these lizards provide considerable variation in ecological context, so our second objective was to place known signalling behaviour in the context of species ecology. After controlling for phylogeny, we found that broad habitat classifications do not strongly influence the likelihood of motion signalling, and specific motor patterns are not more likely to occur in particular microhabitats. We conclude by suggesting that fully understanding the motion signalling behaviour of Australia's agamids will require documenting the displays of species for which there are no data, while taking into account the high variability existing within motor patterns and considering in detail the environmental context under which signalling takes place.     

Proportional processing of a visual mate choice signal in the green swordtail,Xiphophorus hellerii

During mate choice, receivers often assess the magnitude (duration, size, etc.) of signals that vary along a continuum and reflect variation in signaller quality. It is generally assumed that receivers assess this variation linearly, meaning each difference in signalling trait between signallers results in a commensurate change in receiver response. However, increasing evidence shows receivers can respond to signals non-linearly, for example through Weber's Law of proportional processing, where discrimination between stimuli is based on proportional, rather than absolute, differences in magnitude. We quantified mate preferences of female green swordtail fish, Xiphophorus hellerii, for pairs of males differing in body size. Preferences for larger males were better predicted by the proportional difference between males (proportional processing) than the absolute difference (linear processing). This demonstration of proportional processing of a visual signal implies that receiver perception may be an important mechanism selecting against the evolution of ever-larger signalling traits.     

Plasticity of aggressive signalling and its evolution in male spring peepers, Pseudacris crucifer

Acoustic communication is important for determining and maintaining intermale spacing in breeding aggregations of anurans and insects. Because the number and proximity of signalling males can show extensive temporal and spatial variation, we should expect to find mechanisms that permit males to modify their signalling behaviour in ways that balance the needs to attract females and defend their calling space. We conducted two field playback experiments to investigate the role of plasticity in male aggressive signalling in intermale spacing of spring peepers (Anura, Hylidae). In the first experiment, we found a positive correlation between the amplitude of the advertisement calls of a male's nearest neighbour and the stimulus amplitude at which the male first produced aggressive calls. In the second experiment, repeated presentations of advertisement calls above a male's aggressive threshold resulted in rapid decreases in aggressive signalling and significant, but temporary, elevations of aggressive thresholds. We suggest that short-term habituation to a neighbour's calls could function as a proximate mechanism for plasticity in aggression that would allow males to accommodate nearby callers while also tracking fluctuations in the local density of calling individuals. In a third experiment, we examined female choice as an ultimate-level explanation for plasticity in male aggression. Females preferred advertisement calls to aggressive calls, but this preference was weak and was abolished by a 6-dB reduction in the amplitude of the advertisement call. We suggest that female preferences probably function as only one possible source of selection on plasticity in male aggressive signalling and propose energetic limitations as an additional source of selection. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Does signalling mitigate the cost of agonistic interactions? A test in a cricket that has lost its song

Prevailing models of animal communication assume that signalling during aggressive conflict mitigates the costs of fighting. We tested this assumption by staging dyadic encounters between male field crickets, Teleogryllus oceanicus, under three conditions: (i) both males could sing aggressive songs, (ii) neither male could sing, and (iii) one male could sing but the other could not. We conducted experiments on males from a Hawaiian population from Kauai that has recently evolved signal loss, and males from a Hawaiian population from the Big Island that has not. Among both populations, interactions between two silent males were characterized by higher levels of aggression than interactions involving one or two singing males. Because the level of aggression is strongly related to the cost of fighting, these data demonstrate that signalling mitigates the cost of fighting. In mixed trials, we found no statistically significant differences between the behaviour of calling and non-calling males in either population. We conclude that there is no evidence that the Kauai population exhibits special adaptations to alleviate the costs of signal loss. Finally, we found that males were much more likely to signal after their opponent''s retreat than after their own retreat. Aggressive song therefore meets the definition of a ‘victory display’.     

Aggressive communication in fulmars (Fulmarus glacialis) competing for food

A simple model which considers communication of subjective resource value during aggressive encounters is developed. If different behavioural patterns differ in effectiveness (probability of winning) the model predicts that: (1) the expected cost should differ for different behaviour patterns; (2) when effectiveness increases, the associated cost should increase; and (3) the optimal choice should be influenced by the value the resource represents for the animal. These predictions were tested on field data from 1383 interactions between fulmars competing for food. In these interactions, one bird was the ‘owner’ who defended the food item and during the time of ownership ate pieces of the food item. The owner was regularly challenged and take-overs occurred in 22% of interactions. The effectiveness of a given behaviour pattern was measured as the proportion of interactions won. The cost was measured as the proportion of interactions that entailed physical fighting. Resource value was estimated indirectly by comparing time as owner with choice of behaviour. The results indicate that an animal's choice of behaviour influenced its probability of victory, and expected cost, and that effectiveness and cost correlate positively. Furthermore, the frequency with which different behaviours were observed is influenced by the value of the resource for the bird.     

Mate choice and the operational sex ratio: an experimental test with robotic crabs

The operational sex ratio (OSR: sexually active males: receptive females) predicts the intensity of competition for mates. It is less clear, however, under what circumstances, the OSR predicts the strength of sexual selection – that is, the extent to which variation in mating success is attributable to traits that increase the bearer's attractiveness and/or fighting ability. To establish causality, experiments that manipulate the OSR are required. Furthermore, if it is possible to control for any OSR‐dependent changes in the chosen sex (e.g. changes in male courtship), we can directly test whether the OSR affects the behaviour of the choosing sex (e.g. female choice decisions). We conducted female mate choice experiments in the field using robotic models of male fiddler crabs (Uca mjoebergi). We used a novel design with two females tested sequentially per trial. As in nature, the choice of the first female to mate therefore affected the mates available to the next female. In general, we detected significant sexual selection due to female choice for ‘males’ with larger claws. Importantly, the strength of sexual selection did not vary across five different OSR/density treatments. However, as the OSR decreased (hence the number of available males declined), females chose the ‘males’ with the largest claws available significantly more often than expected by chance. Possible reasons for this mismatch between the expected and observed effects of the OSR on the strength of sexual selection are discussed.     

Cannibalism in cephalopods

Cannibalism refers to the action of consuming a member of the same species and is common in many taxa. This paper reviews the available literature on cannibalism in cephalopods. All species of the class Cephalopoda are predators and cannibalism is common in most species whose diet has been studied. Cannibalism in cephalopods is density-dependent due to their aggressive predatory and in case of the octopuses territorial nature. It also depends upon local and temporal food availability and of the reproductive season. Cannibalistic behaviour is positively related to the size of both cannibal and victim. It can affect population dynamics of cephalopods in periods of low food availability and/or high population abundance. Cephalopods are generally restricted in their ability to store energy. It is thus assumed that cannibalism is part of a population energy storage strategy enabling cephalopod populations to react to favourable and adverse environmental conditions by increasing and reducing their number. Finally, we propose five orientation points for future research on cannibalism in cephalopods.     

Knee-clicks and visual traits indicate fighting ability in eland antelopes: multiple messages and back-up signals

Background  

Given the costs of signalling, why do males often advertise their fighting ability to rivals using several signals rather than just one? Multiple signalling theories have developed largely in studies of sexual signals, and less is known about their applicability to intra-sexual communication. We here investigate the evolutionary basis for the intricate agonistic signalling system in eland antelopes, paying particular attention to the evolutionary phenomenon of loud knee-clicking.     

Multiple Cues in Status Signalling: The Role of Wingbars in Aggressive Interactions of Male House Sparrows

During aggressive interactions, animals may signal their competitive ability by various ornaments referred to as badges of status. The use of a single badge predicting dominance rank occurs in many vertebrate species. However, animals often display multiple ornaments that may convey information about either different or the same aspects of the signaller's quality, or alternatively, may serve as signal amplifiers. We observed the fighting behaviour of male house sparrows in two captive flocks to investigate whether they may use multiple cues in status signalling during aggressive interactions. Beside the status‐signalling bib, male sparrows possess a conspicuous white wingbar that they often display upon aggressive encounters. We tested whether bib size and the wingbar's conspicuousness (i.e. its achromatic contrast with the neighbouring dark feathers) or its area predicted success in various aspects of fighting. We found that bib size strongly predicted overall fighting success (i.e. proportion of fights won) and defence success (i.e. proportion of successful defences out of all attacks received). Wingbar conspicuousness was positively related to defence success after controlling for the effect of bib size in multivariate analyses. Furthermore, displaying the wings also tended to improve the birds’ success in defence but not in attack. Wingbar area was unrelated to any measured aspect of fighting ability. We suggest that bib size and wingbar conspicuousness may convey multiple messages on fighting abilities, specifically on overall aggressiveness and defending potential, respectively. Alternatively, wingbars may serve as amplifiers for the wing displays of aggressive motivation. Thus, male sparrows may use multiple cues in assessing the competitive ability of opponents during social interactions.     

Signal verification can promote reliable signalling

The central question in communication theory is whether communication is reliable, and if so, which mechanisms select for reliability. The primary approach in the past has been to attribute reliability to strategic costs associated with signalling as predicted by the handicap principle. Yet, reliability can arise through other mechanisms, such as signal verification; but the theoretical understanding of such mechanisms has received relatively little attention. Here, we model whether verification can lead to reliability in repeated interactions that typically characterize mutualisms. Specifically, we model whether fruit consumers that discriminate among poor- and good-quality fruits within a population can select for reliable fruit signals. In our model, plants either signal or they do not; costs associated with signalling are fixed and independent of plant quality. We find parameter combinations where discriminating fruit consumers can select for signal reliability by abandoning unprofitable plants more quickly. This self-serving behaviour imposes costs upon plants as a by-product, rendering it unprofitable for unrewarding plants to signal. Thus, strategic costs to signalling are not a prerequisite for reliable communication. We expect verification to more generally explain signal reliability in repeated consumer–resource interactions that typify mutualisms but also in antagonistic interactions such as mimicry and aposematism.     

Female Mate Choice Can Drive the Evolution of High Frequency Echolocation in Bats: A Case Study with Rhinolophus mehelyi

Animals employ an array of signals (i.e. visual, acoustic, olfactory) for communication. Natural selection favours signals, receptors, and signalling behaviour that optimise the received signal relative to background noise. When the signal is used for more than one function, antagonisms amongst the different signalling functions may constrain the optimisation of the signal for any one function. Sexual selection through mate choice can strongly modify the effects of natural selection on signalling systems ultimately causing maladaptive signals to evolve. Echolocating bats represent a fascinating group in which to study the evolution of signalling systems as unlike bird songs or frog calls, echolocation has a dual role in foraging and communication. The function of bat echolocation is to generate echoes that the calling bat uses for orientation and food detection with call characteristics being directly related to the exploitation of particular ecological niches. Therefore, it is commonly assumed that echolocation has been shaped by ecology via natural selection. Here we demonstrate for the first time using a novel combined behavioural, ecological and genetic approach that in a bat species, Rhinolophus mehelyi: (1) echolocation peak frequency is an honest signal of body size; (2) females preferentially select males with high frequency calls during the mating season; (3) high frequency males sire more off-spring, providing evidence that echolocation calls may play a role in female mate choice. Our data refute the sole role of ecology in the evolution of echolocation and highlight the antagonistic interplay between natural and sexual selection in shaping acoustic signals.     

Action potentials as multifunctional signals in plants: a unifying hypothesis to explain apparently disparate wound responses

Abstract I suggest that action potentials, which occur in most, if not all, plants, play a major role in intercellular and intracellular communication. Long-distance communication is achieved through the transmitted changes in membrane potential, whereas local signalling is achieved through changes in the subcellular localization of ions (K⁺, Cl^?, and Ca²⁺) and of water, and perhaps by membrane depolarization and current flow. These local changes in ion concentration can lead to modified activities of enzymes in the cell wall (e.g. pectinase), the plasma membrane (e.g. cellulose synthetase, callose synthetase), and the cytoplasm (e.g. protein kinase). In particular, the elevated concentration of cytoplasmic Ca²⁺ is shown to have a major, yet complex, role, especially in the modulation of translation. I suggest further that, in addition to their role in wounding, action potentials may also be important signalling mechanisms for chilling injury, mechanical perturbation and invasion by pathogens, as well as for light and gravity.     

The locus of sexual selection: moving sexual selection studies into the post‐genomics era

Sexual selection drives fundamental evolutionary processes such as trait elaboration and speciation. Despite this importance, there are surprisingly few examples of genes unequivocally responsible for variation in sexually selected phenotypes. This lack of information inhibits our ability to predict phenotypic change due to universal behaviours, such as fighting over mates and mate choice. Here, we discuss reasons for this apparent gap and provide recommendations for how it can be overcome by adopting contemporary genomic methods, exploiting underutilized taxa that may be ideal for detecting the effects of sexual selection and adopting appropriate experimental paradigms. Identifying genes that determine variation in sexually selected traits has the potential to improve theoretical models and reveal whether the genetic changes underlying phenotypic novelty utilize common or unique molecular mechanisms. Such a genomic approach to sexual selection will help answer questions in the evolution of sexually selected phenotypes that were first asked by Darwin and can furthermore serve as a model for the application of genomics in all areas of evolutionary biology.