Acoustic signalling in a wolf spider: can signal characteristics predict male quality?

Size matters: Antagonistic effects of body size on courtship and digging in a wolf spider with non-traditional sex roles

Body size, nuptial gift characteristics and courtship behaviour, among other traits, can reflect the quality of a potential mate and, thus, might be under sexual selection. To maximize their mating success, individuals can show behavioural plasticity in sexual context. Allocosa senex is a burrow-digging wolf spider that exhibits reversal in courtship roles and in sexual size-dimorphism expected for spiders. Males construct the mating refuge and females prefer males that build longer burrows, which are considered as nuptial gifts because they are delivered to them after mating. This study aims to determine whether male body size and female reproductive status influence burrow dimensions, courtship displays, female preferences and cannibalism rate in A. senex. For that purpose, we allowed males to construct burrows and performed sexual trials under laboratory conditions. Larger males were more courted by females, and in turn, they expressed more vibratory behaviours during courtship. However, and contrary to our expectations, smaller males constructed longer burrows. We suggest that males of A. senex exhibit size-dependent behavioural plasticity, and when they are smaller, they invest more in burrow construction to compensate their lower opportunities of courting intensively as larger males. In addition, females courted differentially according to their reproductive status, overriding male preferences for virgin females. This study opens several doors to future research regarding mutual choice in A. senex and the traits assessed by males and females during courtship, as well as about the influence of intrinsic and extrinsic factors shaping reproductive decision-making in this and other wandering spider species.

Condition-dependent differences in male vibratory pre-copulatory and copulatory courtship in a nuptial gift-giving spider

Abstract

Condition-dependent secondary sexual traits and signals are often crucial for mate choice decisions. Nuptial gifts, provided by the male to the female during mating, may represent an indicator of male condition, especially if production of the gift is energetically costly. Additionally, other signalling modalities may well play a role in mate choice in such systems. Females of the nursery web spider Pisaura mirabilis preferably mate with males that provide a prey item wrapped in silk. Apart from the nuptial gift, vibrational signals employed during courtship and mating may reveal additional information about male condition. We tested condition-dependence of male vibrational signals of well-fed versus starved males, when in contact with female dragline silk and during mating trials. Our results show that vibrational signals are produced in P. mirabilis, both during pre-copulatory courtship and during copulation. Male courtship signals were condition-dependent: males in good condition initiated signalling earlier and emitted more vibrational pulses than poor-condition males. They were also more likely to be accepted by the female for copulation. We additionally identified vibrational signals during copulation. These signals were different from pre-copulatory courtship vibrations but did not differ between the treatment groups. This study shows that vibrational communication plays an important role before and during copulation in P. mirabilis. It sets the stage for further experiments on spider biotremology associated with nuptial gift giving behaviour.

Significance statement

Male courtship behaviour can indicate a male’s condition and quality and be subject to female mate choice. Vibrational communication during mating plays a crucial role in many animal species. Spiders are known to be extremely sensitive towards vibrations, and there is evidence that vibratory signals are also used during courtship. Here, we study the nuptial gift-giving spider Pisaura mirabilis in which courtship entails providing a nuptial gift by the male to the female. The gift quality determines on the probability and duration of mating. We investigated the role of vibrational behaviour in this species by standardizing nuptial gifts. Our study demonstrates that vibratory signals comprise information about the male’s condition, that signals are also produced during mating and that courtship and copulatory signals are strikingly different. We suggest that vibrational communication provides important condition-dependent traits for female mate choice in addition to the nuptial gift.

Multimodal communication in courting fiddler crabs reveals male performance capacities

Courting males often perform different behavioural displays that demonstrate aspects of their quality. Male fiddler crabs, Uca sp., are well known for their repetitive claw-waving display during courtship. However, in some species, males produce an additional signal by rapidly stridulating their claw, creating a 'drumming' vibrational signal through the substrate as a female approaches, and even continue to drum once inside their burrow. Here, we show that the switch from waving to drumming might provide additional information to the female about the quality of a male, and the properties of his burrow (multiple message hypothesis). Across males there was, however, a strong positive relationship between aspects of their waving and drumming displays, suggesting that drumming adheres to some predictions of the redundant signal hypothesis for multimodal signalling. In field experiments, we show that recent courtship is associated with a significant reduction in male sprint speed, which is commensurate with an oxygen debt. Even so, males that wave and drum more vigorously than their counterparts have a higher sprint speed. Drumming appears to be an energetically costly multimodal display of quality that females should attend to when making their mate choice decisions.

Attention-Seeking Displays

Animal communication abounds with extravagant displays. These signals are usually interpreted as costly signals of quality. However, there is another important function for these signals: to call the attention of the receiver to the signaller. While there is abundant empirical evidence to show the importance of this stage, it is not yet incorporated into standard signalling theory. Here I investigate a general model of signalling - based on a basic action-response game - that incorporates this searching stage. I show that giving attention-seeking displays and searching for them can be an ESS. This is a very general result and holds regardless whether only the high quality signallers or both high and low types give them. These signals need not be costly at the equilibrium and they need not be honest signals of any quality, as their function is not to signal quality but simply to call the attention of the potential receivers. These kind of displays are probably more common than their current weight in the literature would suggest.

Metabolic efficiency in courtship favors males with intermediate mass in the Australian redback spider, Latrodectus hasselti

Recent studies have suggested that metabolic efficiency may be an important factor in male mating success when females require vigorous and/or prolonged courtship. In capital breeding animals in which a male's resource pool is fixed at adulthood the relationship between energy expenditure and courtship performance may be especially important, as males are expected to utilize their finite resources efficiently when soliciting mates. Males may benefit from being efficient, i.e., achieving a sufficiently high level of courtship signaling at low energetic cost, if it enables them to acquire mates before their limited energy reserves are depleted. We investigated the relationship between metabolic efficiency and courtship vibrational signaling in the Australian redback spider, Latrodectus hasselti, a semelparous capital breeder where males invest heavily in courtship to secure a mating. We assessed metabolic rate in a sample of males and measured two courtship components (duty cycle and amplitude) that reflected the energy content of web-borne vibrations. We then calculated two indices of metabolic efficiency for these courtship properties. There was a quadratic relationship between mass and duty cycle such that the highest duty cycle signals were performed by males having intermediate mass. Furthermore, intermediate-mass males were also the most metabolically efficient. Prolonged courtship is necessary in L. hasselti for successful mating, and the results of this study suggest that intermediate-mass males are superior courters because they utilize their finite resource pool most efficiently to produce high energy vibrational signals.

Why not lie? Costs enforce honesty in an experimental signalling game

Communication depends on reliability. Yet, the existence of stable honest signalling presents an evolutionary puzzle. Why should animals signal honestly in the face of a conflict of interest? While students of animal signalling have offered several theoretical answers to this puzzle, the most widely studied model, commonly called the 'handicap principle', postulates that the costs of signals stabilize honesty. This model is the motivating force behind an enormous research enterprise that explores signal costs--whether they are physiological, immunological, neural, developmental or caloric. While there can be no question that many signals are costly, we lack definitive experimental evidence demonstrating that costs stabilize honesty. This study presents a laboratory signalling game using blue jays (Cyanocitta cristata) that provides, to our knowledge, the first experimental evidence showing honesty persists when costs are high and disappears when costs are low.

Fitness cost of pheromone production in signaling female moths

A secondary sexual character may act as an honest signal of the quality of the individual if the trait bears a cost and if its expression is phenotypically condition dependent. The cost of increasing the trait should be tolerable for individuals in good condition but not for those in a poor condition. The trait thus provides an honest signal of quality that enables the receiver to choose higher quality mates. Evidence for sex pheromones, which play a major role in shaping sexual evolution, inflicting a signaling cost is scarce. Here, we demonstrate that the amount of the major component of the pheromone in glands of Lobesia botrana (Lepidoptera) females at signaling time was significantly greater in large than in small females, that male moths preferred larger females as mates when responding to volatile signals, and small virgin females, but not large ones, exposed to conspecific pheromone, produced, when mated, significantly fewer eggs than nonexposed females. The latter indicates a condition-dependent cost of signaling. These results are in accordance with the predictions of condition-dependent honest signals. We therefore suggest that female signaling for males using sex pheromones bears a cost and thus calling may serve as honest advertisement for female quality.

The design of complex sexual traits in male barn swallows: associations between signal attributes

Variation in the expression of sexually selected traits among individuals is widely investigated on the premise that these traits evolved to signal male quality. Significant repeatabilities of sexual signals and their associations with condition, mating success, survivorship and age may be the signatures of sexual selection. However, little is known about the relationship between these sexual attributes. Here we studied 28 acoustic and visual traits in the barn swallow, Hirundo rustica, that may potentially function in sexual selection. Based on effect sizes calculated at the between-individual level, we assessed the relationship between repeatability, condition-dependence, attractiveness, age-dependence and viability indicator value of sexual traits using sexual signals as the units of analyses. Those traits that showed high within-year repeatability also showed high between-year repeatability, indicating that between-individual variation is consistent within and among seasons. In addition, age-dependence of traits, probably causing between-year variation, was negatively related to between-year repeatability. Condition-dependence was negatively correlated with effect sizes for the extent to which traits predicted viability. Therefore, traits that are positively related to immediate condition are those that are negatively related to survival, which may be the signature of a trade-off between current and future reproductive success ultimately reflecting signal reliability. No other significant relationship was found between trait attributes. We conclude that multiple sexual signals reflect different aspects of male quality in the barn swallow.

Seismic signal production in a wolf spider: parallel versusserial multi-component signals

Animal signals can consist of multiple parts within or across sensory modalities (multi-component signals or multimodal signals). While recent work has focused on multimodal signals, the production, processing and evolution of multi-component signals has received considerably less attention. Here, using synchronous high-speed video and laser vibrometer recordings followed by experimental manipulations of putative sound-producing structures, we explored the mechanisms of seismic signal production in the courtship display of Schizocosa stridulans Stratton. Two types of seismic courtship signals were observed: `rev' and `idle' signals. Revs consist of a high-frequency component produced by flexions of the male pedipalp(stridulation) simultaneous with a low-frequency component produced by movements of the abdomen (tremulation). This multi-component signal is produced by independent structures and represents a parallel multi-component display. By contrast, idle displays consist of a high-intensity component produced by drumming of the forelegs on the substrate (percussion) followed by a high-frequency component produced by flexions of the male pedipalp(stridulation). While the components of the idle display are also produced by independent structures, the leg drumming and palp flexions occur serially and do not overlap in time. We discuss the selective pressures that may drive the evolution of multiple sound-producing structures as well as the selective pressures that drive the evolution of parallel versus serial multi-component signals.

Negatively condition dependent predation cost of a positively condition dependent sexual signalling

Predation is considered as an important factor constraining the expression of sexual signals. Nevertheless, direct quantitative evidence for predation provoking significant viability costs on individuals signalling at high rates is scarce. Moreover, it is unclear whether high rate signallers are able to balance presumably increased predation costs. We examined whether a condition dependent audible sexual signal, drumming, makes Hygrolycosa rubrofasciata male spiders more prone to predation by pied flycatchers (Ficedula hypoleuca), and whether sexual signalling rate is related to escaping ability once attacked. When birds were given a choice between two spider males manipulated to drum either one or three bouts per minute using playbacks, naïve birds attacked the males randomly regardless of the drumming rate. However, experienced birds chose significantly more often the males with high signalling rate. When spiders were allowed to escape, males with high sexual signalling rate tended to be better at escaping attacks than males with low sexual signalling rate. This study provides evidence that high signalling rate increases the risk of predation, but simultaneously males with high mobility, which correlates positively with signalling rate seemed to be better at compensating this cost.

A trade-off between sexual signalling and immune function in a natural population of the drumming wolf spider Hygrolycosa rubrofasciata

The field of ecological immunology is ultimately seeking to address the question 'Why is there variation in immune function?' Here, we provide experimental evidence that costs of ubiquitous sexual signals are a significant source of variation in immune function. In the mating season, males of the wolf spider Hygrolycosa rubrofasciata drum against dry leaves while wandering around the habitat searching for receptive females. According to a previous study, the male metabolic rate during the drumming increases 22-fold compared to the resting metabolic rate. In the present study, we examined whether investment in costly courtship drumming decreases male immune function in a wild population of H. rubrofasciata. We induced males to increase their drumming rate by introducing females in proximity. As estimates of male immune function, we used lytic activity and encapsulation rate. Lytic activity estimates the concentration of antimicrobial peptides in haemolymph, which have been shown to play an important role in defence against bacteria, viruses and fungi. Encapsulation is an important defence mechanism against nematodes and insect parasitoids, but it also plays a role in defence against viruses. Our results show that males with nonarbitrarily increased investment in drumming rate had considerably lower lytic activities than control males. Also, there was a tendency for males with nonarbitrarily increased investment in drumming rate to have lower encapsulation rates than control males. This study provides experimental evidence for the first time, to our knowledge, that there are direct immunological costs of sexual signalling in natural populations. Therefore, immunological costs of sexual signals may provide significant phenotypic variation to parasite-mediated sexual selection.

Sexual selection research on spiders: progress and biases

The renaissance of interest in sexual selection during the last decades has fuelled an extraordinary increase of scientific papers on the subject in spiders. Research has focused both on the process of sexual selection itself, for example on the signals and various modalities involved, and on the patterns, that is the outcome of mate choice and competition depending on certain parameters. Sexual selection has most clearly been demonstrated in cases involving visual and acoustical signals but most spiders are myopic and mute, relying rather on vibrations, chemical and tactile stimuli. This review argues that research has been biased towards modalities that are relatively easily accessible to the human observer. Circumstantial and comparative evidence indicates that sexual selection working via substrate-borne vibrations and tactile as well as chemical stimuli may be common and widespread in spiders. Pattern-oriented research has focused on several phenomena for which spiders offer excellent model objects, like sexual size dimorphism, nuptial feeding, sexual cannibalism, and sperm competition. The accumulating evidence argues for a highly complex set of explanations for seemingly uniform patterns like size dimorphism and sexual cannibalism. Sexual selection appears involved as well as natural selection and mechanisms that are adaptive in other contexts only. Sperm competition has resulted in a plethora of morphological and behavioural adaptations, and simplistic models like those linking reproductive morphology with behaviour and sperm priority patterns in a straightforward way are being replaced by complex models involving an array of parameters. Male mating costs are increasingly being documented in spiders, and sexual selection by male mate choice is discussed as a potential result. Research on sexual selection in spiders has come a long way since Darwin, whose spider examples are reanalysed in the context of contemporary knowledge, but the same biases and methodological constraints have persisted almost unchanged through the current boom of research.

Adaptive significance of synchronous chorusing in an acoustically signalling wolf spider

Synchronous sexual signalling is a behavioural phenomenon that has received considerable theoretical interest, but surprisingly few empirical tests have been conducted. Here, we present a set of experiments designed to determine (i) whether the sexual signalling of the drumming wolf spider Hygrolycosa rubrofasciata is synchronous, and (ii) whether the synchrony may have evolved through female preference. Using controlled playback experiments, we found that males actively synchronized their drumming bouts with other males and females significantly preferred closely synchronized drumming clusters compared with loose clusters. In loose clusters, the first drumming signals attracted the most female responses, whereas in close clusters, the last drumming signals were the most heeded. We suggest that this female preference for the last drummer can maintain male synchronous signalling in H. rubrofasciata.

Decreased sexual signalling reveals reduced viability in small populations of the drumming wolf spider Hygrolycosa rubrofasciata

One of the important goals in conservation biology is to determine reliable indicators of population viability. Sexual traits have been suggested to indicate population extinction risk, because they may be related to viability through condition dependence. Moreover, condition-dependent sexual traits may be more sensitive indicators of population viability than early life-history traits, because deleterious fitness effects of inbreeding tend to be expressed mainly at the end of the species' life history. However, empirical evidence of the significance of sexual behaviour for population viability is missing. In this study, we examined two male sexual traits and survival in 39 different-sized and isolated natural populations of the wolf spider, Hygrolycosa rubrofasciata. We also used several traits to estimate female reproductive success in 25 populations of H. rubrofasciata. According to previous studies, H. rubrofasciata males have a costly and condition-dependent acoustic signal, courtship drumming, which is the target of female choice. Males with a high drumming rate have considerably higher viability than males with a low drumming rate, and females that mate with the more actively drumming males gain genetic benefits in terms of increased offspring viability. Our results show that males in small populations had both lower survival and lower drumming rate than males in larger populations. However, we did not find any evidence for a decline in important early life-history traits (offspring number, hatching success or offspring body mass) or female body mass in small populations. Our results have two important messages for conservation biology. First, they show that sexual traits can be used as sensitive indicators of population viability. Second, the indirect benefits of female choice in terms of good genes might partially compensate for the reduction of viability in declining populations. Also, our results support the view that deleterious effects of small population size are expressed at the end of the species' life history.