Sexually Dimorphic Intrasexual Duetting in an Otherwise Monomorphic Green Lacewing (Neuroptera, Chrysopidae, Chrysoperla plorabunda): Sexual Selection or Sex Recognition?

Songs produced during heterosexual duets in a green lacewing, C. plorabunda, are sexually monomorphic. However, individuals of either sex will also engage in intrasexual duets, which can exhibit sexual dimorphism. We confined males and females together in various combinations in a small arena to study the phenotypes, behavioral interactions, and functional roles of duetting in this species. The goal was to test whether sexual selection or sex recognition provided the better explanation of song sexual dimorphism. We determined that the monomorphic form of the intrasexual duet was long and stable, and could take place either between males or between females. Such “standard” intrasexual duetting songs were acoustically indistinguishable from heterosexual songs. However, males could also engage other males in special “fast duets” that sped up and terminated abruptly. Equivalent fast duets were not part of the female repertory. Fast duetting songs between males differed significantly from other types of male or female duetting songs in every measurable characteristic, but their role in the mating system was ambiguous. Contrary to one prediction of the sexual selection hypothesis, fast duetting between males occurred less often in situations where it might be the most useful to males in securing mates, i.e., during male-male-female interactions (trios). In addition, fast songs that started, ended, both started and ended, or neither started nor ended duets were acoustically indistinguishable, making it unlikely that females were choosing males based on such variation. However, songs that “both started and ended” fast duets were associated with a significant mating advantage, indicating a possible role for fast duetting in male-male sexual competition. Because the alternative hypothesis of sex recognition was also supported by some of our results, we conclude that aggressive qualities of male-male fast duets probably mediate intrasexual selection, while their increasing tempo serves as an adaptive response to promote rapid sex recognition by truncating unproductive and potentially dangerous intrasexual duetting.     

Escalation of aggressive vocal signals: a sequential playback study

Rival conspecifics often produce stereotyped sequences of signals as agonistic interactions escalate. Successive signals in sequence are thought to convey increasingly pronounced levels of aggressive motivation. Here, we propose and test a model of aggressive escalation in black-throated blue warblers, presenting subjects with two sequential and increasingly elevated levels of threat. From a speaker outside the territorial boundary, we initiated an interaction (low-threat level), and from a second speaker inside the territory, accompanied by a taxidermic mount, we subsequently simulated a territorial intrusion (escalated threat level). Our two main predictions were that signalling behaviours in response to low-threat boundary playback would predict signalling responses to the escalated within-territory threat, and that these latter signalling behaviours would in turn reliably predict attack. We find clear support for both predictions: (i) specific song types (type II songs) produced early in the simulated interaction, in response to boundary playback, predicted later use of low-amplitude ‘soft’ song, in response to within-territory playback; and (ii) soft song, in turn, predicted attack of the mount. Unexpectedly, use of the early-stage signal (type II song) itself did not predict attack, despite its apparent role in aggressive escalation. This raises the intriguing question of whether type II song can actually be considered a reliable aggressive signal. Overall, our results provide new empirical insights into how songbirds may use progressive vocal signalling to convey increasing levels of threat.     

Soft songs in male ortolan buntings are used in an aggressive context but are not an aggressive signal

Many bird species produce low‐amplitude acoustic signals that have been poorly studied in comparison with loud, broadcast songs used for mate attraction and repelling rivals. In some birds, these soft signals were found to be emitted in an antagonistic context and were the most reliable predictor of a subsequent physical attack. The function of this signal is poorly understood, and several hypotheses have been proposed to explain the mechanisms and possible functions of such low‐amplitude signals. The subject of this study is the ortolan bunting, a small passerine species that produces soft songs during territorial defence. In this study, we aim to study whether the soft songs of the ortolan bunting are a signal of increased aggressiveness by testing if they meet the context, prediction and response criteria of aggressive signals. We simulated stranger male intrusion into a focal male territory with three different playback experiments. We found no significant differences in the male responses to the taxidermic model regardless of whether they were or were not producing soft songs in response. The males responded more strongly to loud songs than to soft songs during the simulated intrusions, and the males did not treat soft songs as a predictor of conflict escalation. Although soft songs clearly appeared during territorial encounters and were not present during spontaneous singing before the intrusions, our results did not support the hypothesis that soft songs indicate aggressive character. We suggest that soft songs in the ortolan bunting are intentionally used by birds to modify their intentions or target‐specific individuals within a close range.     

Sexual identity and sexual attractiveness of a gynandromorph of the lawn ground cricket,Polionemobius mikado (Orthoptera: Trigonidiidae)

A gynandromorph adult of the lawn ground cricket Polionemobius mikado (Shiraki, 1913) (Orthoptera: Trigonidiidae) was collected from a natural population. It had complete male forewings and a female ovipositor at the end of abdomen. A normal male that encountered the gynandromorph performed a courtship song and tried to transmit a spermatophore, whereas a normal female was indifferent to the gynandromorph. The gynandromorph showed aggressive behavior toward the normal male but not against the normal female. The gynandromorph raised its forewings immediately after the antenna touched the body of the normal male and female, but no sound was produced. Overall, the gynandromorph behaved as a male, but was courted by conspecific males. Thus, the sexual identity of the gynandromorph was masculine, but sexual attractiveness was feminine.     

Context-dependent effects of castration and testosterone treatment on song in male European starlings

Most seasonally breeding songbirds display dramatic seasonal fluctuations in plasma testosterone (T) levels and mate attraction behaviors, including song. However, males of some songbird species, such as the European starling (Sturnus vulgaris), continue to sing at high levels after the breeding season, when T levels are basal. In male starlings song during the breeding season functions mainly to attract mates, whereas song during the nonbreeding season appears unrelated to reproduction. This suggests that song produced in a context unrelated to female courtship, unlike song directed toward females, is not regulated by plasma T. In captive males housed in large outdoor aviaries we explored the relationship between plasma T and song produced during the breeding season within and outside a courtship context. This was achieved by determining the effects of castration and subsequent T treatment on song and mate attraction behaviors in both the presence and the absence of a female. Compared to intact males, castrated males did not show reduced song activity in the absence of a female for at least 6 months after the operation, strongly suggesting that the expression of noncourtship song is not regulated by plasma T. Likewise, we found that experimentally elevating T levels in castrated males did not affect noncourtship song rates. However, control castrated males receiving empty implants tended to show reduced noncourtship song rates after implantation. This may have been due to a suppressive effect caused by the presence of the T-implanted castrated males in the same aviary. In contrast, courtship singing was clearly controlled by plasma T: it was abolished by castration and restored by subsequent T replacement when males were housed both individually and in a group situation. High plasma levels of T also appeared necessary for the activation of three other behavioral traits critical for mate attraction, namely, nesthole occupancy, spending time (singing) in a nesthole, and carrying green nesting material into a nesthole.