We have (1) characterized vibratory cues of house fly and house cricket prey and vibratory signals of L. hesperus and T. agrestis males; (2) determined that vibratory courtship signals of males differ from prey vibratory cues; and (3) ascertained the vibration parameter(s) triggering a predatory response in females.
On both L. hesperus and T. agrestis webs, cricket and fly vibrations were similar: short, sporadic, and on average with high amplitude modulation. Most vibrations of L. hesperus and T. agrestis males were continuous, lengthy, and lacking a complex temporal structure. Vibrations of L. hesperus males differed from prey in terms of duration and dominant frequency. Male vibrations were of lower amplitude than fly, but not cricket, vibrations. Vibrations of T. agrestis males differed from prey in terms of duration only. During the playback experiment, significantly fewer L. hesperus females responded aggressively to low-amplitude vibrations, irrespective of whether these stimuli were recorded vibrations of prey or male spiders, suggesting that the likelihood of a predatory response depends on the amplitude but not the waveform of incoming vibrations. Below we discuss the implications of these findings for male signal function and signalling constraints.
The absence of complex temporal patterns in most courtship vibrations of L. hesperus and T. agrestis males is in stark contrast to observations in other spiders. For example, female Cupiennius getazi use the duration and structure of male-produced syllables to identify conspecific males . Males of the wolf spider Lycosa tarentula fasciiventris produce courtship vibrations that comprise series of repeating syllables followed by pauses at regular intervals . The temporal structure of courtship vibrations produced by male Schizocosa ocreata is linked to female mate choice . Finally, the vibratory courtship displays of 11 species of jumping spiders (Salticidae) within the Habronattus coecatus clade are complex and comprise up to 20 distinct elements organized in motifs . All of the above examples refer to wandering spiders whose courtship takes place on plant stalks, leaf litter, or the ground. Similarly, courting males of the orb-weaver Argiope keyserlingi produce vibrations with repeated, pulse-like characteristics . This distinct temporal patterning may be well transmitted because A. keyserlingi males court on a single silk thread. The resulting vibrations are quite different from the ones reported in our study, but the abdominal tremulation of L. hesperus males and the shuddering of A. keyserlingi males are very similar types of behaviour. The absence of temporally complex signalling in L. hesperus, and its scant presence in T. agrestis, is curious. Based on their rate and amplitude modulation, tremulations can produce signals with a lot of information  but abdominal tremulations of male L. hesperus generated uniform waveforms that can be described solely on the basis of their amplitude and frequency. Future work is needed to reveal whether sheet and tangle webs impose constraints on the temporal complexity of signals.
The transmission properties of a medium impose constraints on the characteristics of signals [1, 34]. Contrary to orb webs, L. hesperus tangle webs and T. agrestis sheet webs are not uniform structures. The density of threads and their orientation, degree of tension, number of connections and distance to anchor points all vary greatly from one area of the web to another, and likely affect the transmission characteristics of the webs. When we explored the propagation properties of transversal vibrations on L. hesperus and T. agrestis webs (using frequency sweeps from 0 to 500 Hz), we found great variability both within and between webs in both types of webs (S. Vibert, unpublished data). Within a single web, transmission profiles obtained at different locations were sometimes very dissimilar. Similarly, playback of recorded prey vibrations of known dominant frequency revealed that frequency was not well transmitted across L. hesperus webs (S. Vibert, unpublished data).
There are several plausible explanations for the difference between male and prey vibrations. Vibrations on webs during courtship interactions might communicate species identity and help females distinguish between con- and heterospecific males or between conspecific males and potential prey. Alternatively, vibrations of a male might communicate his identity, quality, or current location.
Our results suggest that the low-amplitude vibrations produced by L. hesperus males reduce the probability of being attacked by females during courtship. Female attack rate was twice as low when prey or male vibrations were played back at the low amplitude of male abdominal tremulations than at the high amplitude of prey vibrations. We also observed females twitching their abdomen dorso-ventrally in response to three low-amplitude playbacks. In a previous study , 75% of L. hesperus females displayed “twitching” during advanced stages of courtship, whereas no female ever displayed twitching in response to live prey (S. Vibert, pers. obs.). Our results do suggest that L. hesperus males must “whisper” during courtship, but the potential information content and sexual function of these whispers are yet be studied. It would be particularly interesting to investigate whether T. agrestis females respond differently to vibrations of varying duration, the one parameter in which vibrations of males differed from those of prey. While prey vibrations were intermittent, vibrations of L. hesperus and T. agrestis males were continuous, which may be another determinant factor for the females’ predatory responses.
The function of L. hesperus male vibratory signals is not likely to advertise male quality. Whenever males deploy acoustic signals that broadcast their quality, females prefer loud (high-amplitude) signals, as has been demonstrated in gray tree frogs , túngara frogs , katydids , wax moths , and passerine birds [39, 40]. Whether the amplitude of vibratory signals produced by courting spider males is indicative of their quality as prospective mates, or whether it serves another function, has hardly been studied. Large males of the funnel-web spider Agelenopsis aperta are more likely to achieve mating success , possibly because they produce louder signals, as has been shown for airborne signals in the toad Bufo americanus. Similarly, male Schizocosa ocreata wolf spiders producing higher-amplitude signals were more successful at securing a mate . In the wandering spider Cupiennius, however, the amplitude of signals seems of no relevance to females [42–44]. The quiet songs of birds exemplify a signalling display characterized by low amplitude; quiet songs prevent eavesdropping from competitors or rivals in contexts of territorial disputes or mating interactions . The courtship display of L. hesperus might well represent a novel context in which males must signal at low amplitude to avoid triggering a female predatory response.
A reduction of female aggressiveness is often cited as one of the possible functions of male courtship in spiders but few studies have tested this experimentally. Many adaptations may function to avoid or reduce female aggressiveness. Behavioural adaptations include approaching a female while she is feeding , mate binding , or inducing a quiescent state . We suggest that courtship signals of L. hesperus and T. agrestis males that differ markedly from prey vibrations might represent another adaptation in males facing large and aggressive females. Conversely, in species where females are not aggressive towards males, it may be adaptive for courting males to take advantage of the females’ sensory systems being tuned for prey cues by producing prey-like vibrations, as has been demonstrated in the water mite Neumania papillator.