Is song-type matching a conventional signal of aggressive intentions?

Song-type matching is a singing strategy found in some oscine songbirds with repertoires of song types and at least partial sharing of song types between males. Males reply to the song of a rival male by subsequently singing the same song type. For type matching to serve as an effective long-distance threat signal, it must be backed up by some probability of aggressive approach and impose some type of cost on senders that minimizes the temptation to bluff. Western subspecies of the song sparrow exhibit moderate levels of song-type sharing between adjacent males and sometimes type match in response to playback of song types they possess in their repertoires. Interactive playback experiments were used in order to examine the subsequent behaviour of type-matching birds and to quantify the responses of focal birds to type-matching versus non-matching stimuli. Birds that chose to type match the playback of a shared song type subsequently approached the speaker much more aggressively than birds that did not type match. Moreover, birds approached a type-matching stimulus much more aggressively than a non-matching stimulus. These results and consideration of alternatives suggest that type matching in song sparrows is a conventional signal in which honesty is maintained by a receiver retaliation cost against bluffers.     

Elevating perceived predation risk modifies the relationship between parental effort and song complexity in the song sparrow Melospiza melodia

Adult‐directed predation risk elevates costs of parental care, and may modify relationships between sexually selected ornaments and parental effort by accentuating the tradeoff between survival and parental investment. We assessed multiple hypotheses regarding the relationship between maternal effort, paternal effort, and the sexually selected trait of male song complexity in the song sparrow Melospiza melodia. Further, we explored whether experimentally elevating perceived adult‐directed predation risk near nests affected these relationships. We quantified two dimensions of song complexity: song repertoire size and residual syllable number (the relative number of syllables for a given song repertoire size). Under elevated perceived predation risk, but not in the absence of the predator stimuli, females mated to males with higher residual syllable number displayed higher nestling provisioning rates and performed a greater proportion of nestling provisioning trips. In other words, elevating perceived predation risk induced a pattern of maternal investment consistent with differential allocation. In contrast, under elevated perceived predation risk, only, females performed a lesser proportion of provisioning trips when mated to males with large song repertoire sizes. Further, consistent with the good parent hypothesis, males with large song repertoire sizes displayed lower latencies to return to the nest, independent of the predator stimuli. Results suggest that residual syllable number may reflect some aspect of male genetic quality, such that females are more willing to maintain maternal effort while facing heightened predation risk. On the other hand, females may gain paternal benefits when mated to males with large song repertoires. Our study supports the hypothesis that increased costs of parental care associated with predation risk may induce relationships between sexually selected traits and parental behavior, which may increase the strength of sexual selection. Additionally, results suggest that different aspects of song complexity may fulfill non‐equivalent signaling roles.     

Ejaculate size varies with socio-sexual situation in an insect

Abstract.

• 1 Males operate within a finite energy budget and cannot produce limitless supplies of sperm. On the other hand, when a female mates with a second male while still containing fertile sperm from a rival male, selection should favour the male that inseminates more sperm. Optimal strategy should thus be for males to exercise discretion in the allocation of sperm to individual females. Assuming the outcome of sperm competition to be based on either the raffle or kamikaze principles, the sperm competition hypothesis predicts a positive association between the probability that the sperm will find themselves in competition with sperm from rival males and the number of sperm inseminated.
• 2 The beetle, Tenebrio molitor L., behaves in accordance with this hypothesis. Males accompanied by a rival male before and during mating inseminate more sperm per ejaculate than unaccompanied males. Accompanied males are also faster to initiate mating and more likely to show post-copulatory guarding. Adjustment of number of sperm inseminated was shown by males subjected to both long-term (5 days) and short-term (5 min) exposure to potential intrasexual competitors. Individual males exposed to both levels also demonstrated the ability to adjust their ejaculate according to socio-sexual situation.
• 3 We conclude that male T.molitor adjust the number of sperm they inseminate according to some perception of the risk of sperm competition.

     

A Life History Trade-Off Between Flight Ability and Reproductive Behavior in Male Field Crickets (<Emphasis Type="Italic">Gryllus texensis</Emphasis>)

Male field crickets (Gryllus texensis) that differ in flight ability incur a life history trade-off between flight ability and reproduction, where flight ability comes with a male fitness cost. In courtship trials, flight-capable males produced courtship song, a necessary signal for mating success, with a significantly lower probability than flight-incapable males. The trade-off was evident in young males, and a similar trend occurred in older males. Males that lost the ability to fly through histolysis of flight muscles produced courtship song with a similar probability as males incapable of flight for their entire lives. Time of day did not affect the expression of the trade-off. Neither male morph nor time of day influenced female mating behavior.     

Misleading mollies: surface- but not cave-dwelling Poecilia mexicana males deceive competitors about mating preferences

Individuals providing misleading information to conspecifics may benefit from deception at the receiver's expense. A recent study (Plath et al., Curr Biol 18:1138–1141, 2008c) suggested that male Atlantic mollies (Poecilia mexicana) deceive rival males about their preferred mate. Here, we contrasted potentially deceptive behavior in surface-dwelling P. mexicana males to males of the cave form of that species (the cave molly). Unlike many other cavefishes, cave mollies have retained functional eyes and readily respond to visual stimuli. Males could interact freely with two females (large and small), and an audience male was visually presented during the second part of the tests. When observed during mate choice, males reduced their mating activity, but this reduction was significantly weaker in cave mollies. Overall, the expression of mating preferences (determined through frequencies of nipping and thrusting) declined in front of an audience; again, this effect was significantly weaker in the cave form. Reduced sexual activity and reduced expression of mating preferences can be interpreted as an attempt of the focal male to avoid unintended interception of information by the rival male. Surface but not cave molly males directed their first sexual interaction (when being observed by the rival male) towards the initially non-preferred female, suggesting that surface-dwelling males deceive rival males about their mating preferences. Deception by the focal males may be an adaptation to avoid sperm competition, since other males in their social environment may use public information and copy the focal male's mate choice. It seems that sending deceptive signals is evolutionarily regressed in the cave molly, since mate choice copying is unlikely to occur under naturally dark conditions, and also the potential to deceive rivals about mating preferences is probably very limited.     

Sexual differences in cricket calling song recognition

Summary Phonotactic behavior was studied in male crickets,Teleogryllus oceanicus. Tethered flying males were presented with electronically synthesized calling song models in a two-choice phonotaxis assay, and their song preferences were determined and compared with previous findings for females.Males are poorer at discriminating between songs than females; they do not display choice behavior as frequently as females, and the choices they do make are not as consistent as those of females (Figs. 3, 4). T. oceanicus calling song is composed of rhythmically different chirp and trill sections. The selectivity of males for these two components differs from that of females. Females prefer chirp to trill, but the opposite is true for males (Fig. 5B-F). Males are similar to females in that they prefer either a conspecific song model or its separate components to a heterospecific model (Fig. 5A, G, H).Behavioral and neural implications of these findings are discussed.     

Male singing behaviour and female presence in the territory in whitethroats Sylvia communis

Unmated male songbirds usually change their vocal behaviour when females enter their territories. Either the males court the females by changing the rate or pattern with which their normal long-ranging full songs are emitted, or they shift to special displays and long- or short-ranging vocalisations. In this study we quantified how female presence and behaviour affect the singing behaviour of male whitethroats. In the presence of a female the male frequently performed song flights, maybe to locate the female before it was courted, with sequences of diving-song displays. The courtship was interrupted by periods of perch songs. Female dscharp calls and short movements made the males initiate or resume courtship, whereas short horizontal jumps made the males intensify their courtship. Overall, the males changed their dual-function song activity in females' presence by emitting fewer perch songs and more flight songs. The quiet diving songs were only emitted during direct courtship of a female. The song types emitted immediately before, during, and after courtship are all highly variable, thus allowing for a quick assessment of the male's song repertoire. The courtship was also interrupted by periods of male woid calling, a call that is known to have a deterring effect on rival males. Bouts of woid calls were usually followed by song flights, again probably to locate the female that might have moved out of sight, or maybe to locate potential rival intruders. The latter was supported by an increased intrusion rate during female presence. Communicated by P. McGregor     

Matched Counter-singing Signals Escalation of Aggression in Black-capped Chickadees (Parus atricapillus)

Black-capped chickadees (Parus atricapillus) have the ability to shift their songs up and down over a wide range of absolute frequencies. Males can shift their songs over 465 ± 52.9 (SE) Hz. During the dawn chorus, males shift their songs by 80 Hz or more every 41 ± 8.8 (SE) songs, but it appears that males can sing at any frequency within their range. Frequency shifting may allow males to match counter-sing with rival males; that is, to switch song output to match that of a rival. During simultaneously recorded dawn choruses, however, there was no correlation over time in the frequency of neighbouring males' songs, nor was there a correlation over time in the size of shifts between their songs. Moreover, males did not match the frequencies of songs presented on a played-back tape at the edge of their territories during the dawn chorus. Matching was observed during some bouts of counter-singing between males. In these cases, matched counter-singing was highly associated with escalation of the conflict. We suggest that frequency matching in this species may be a graded signal that allows the singer to direct aggression towards a particular rival.     

Fighting talk: complex song elicits more aggressive responses in a vocally complex songbird

Song complexity in many songbirds is a trait subject to sexual selection. It is often associated with male territorial defence. Empirical studies testing differential male responses to rival song in vocally complex songbirds have, however, been scarce. We conducted playback experiments of the endemic New Zealand Tui Prosthemadera novaeseelandiae to test the aggressive response of territorial male Tui to rival songs with differing complexity levels. Overall, complex songs evoked significantly stronger responses from territorial males than did simple songs. Following playback of complex songs, focal males approached the playback more closely and rapidly, and responded with songs of higher complexity than they did to playback of simple songs. This suggests males could both distinguish between different levels of complexity within the Tui repertoire, and perceive a more complex song as a greater territorial threat. Our study is one of the first to demonstrate strong aggressive responses to increased levels of song complexity in a songbird species with highly complex vocalizations.     

Aggressive displays by male House Wrens are composed of multiple components that predict attack

Aggressive signals should predict whether the sender of the signal will attack the receiver, yet this criterion has been little studied. We conducted experiments with male House Wrens (Troglodytes aedon) in north‐central Illinois in 2009 to test the hypothesis that rates of song delivery and wing‐quivering (putative aggressive signals) signal aggressive intent. We simulated a conspecific territorial intrusion by combining playback of male song with a male taxidermic mount, predicting that these signals would be related to a male's likelihood of attacking a conspecific. All males (N = 37) sang in response to the intrusion. Males attacking the mount sang at significantly higher rates and performed significantly more wing quivers than males that did not attack. In addition, all males that attacked the mount performed wing quivers (9/9), whereas only 53.6% (15/28) of males that did not attack did so (P = 0.011). Our results are consistent with the hypothesis that both song and wing quivering indicate a signaler's aggressive intent and that these signals are likely components of a multi‐component, hierarchical display.     

Breeding Experience and not Age Modulates the Song Development of Pied Flycatchers (Ficedula hypoleuca)

In songbirds, the development of the species‐specific adult song involves a learning process that varies in extension. In species that incorporate new song elements throughout life (open‐ended learners), variation in male song composition could be the result of either age or breeding experience. Using data from 16 yr of fieldwork on pied flycatchers (Ficedula hypoleuca), we aimed to disclose the individual contribution of these two factors on the species song characteristics, as well as their relation with morphology and plumage color changes. Finally, we explored whether any of the song or physical features could predict the probability of males returning to the breeding site. We found that the song characteristics of the first‐time breeders did not differ between age classes, except for the total number of syllables per song, which was higher in the 1‐year‐old than in the 2‐year‐old males. However, we found that song variables associated with complexity (song and sample versatilities and repertoire size), increased significantly from the first to the second breeding season. Males showed delayed plumage maturation, with 1‐year‐old males being browner than the 2‐year‐old males independently of their breeding experience. Morphology, however, was not affected by age or breeding experience. The probability that males returned to the breeding site was not associated with song or physical features. Considering that some song learning occurs during the breeding period and that some males may skip the first breeding season, selective pressures may have been established for song complexity to be an honest indicator of breeding experience rather than age.     

Male Competition over Access to Females in a Spider with Last-Male Sperm Precedence

Agonistic behaviour between male cellar spiders (Pholcus phalangioides) was investigated to test whether (1) size difference determines which male achieves access to the female, (2) males are able to monopolize access to the female until egg laying and whether (3) female resource value increases before egg laying because of last‐male sperm precedence. We further investigated whether (4) there is variation in time and energy spent on courtship and copulation depending on the degree of sperm competition, i.e. with or without rival present. In three experimental settings we introduced two males of either different or similar sizes, or a single male to a female. The mating units were constantly video‐observed until the females produced their first egg sac. Experience, ownership and female resource value in terms of body size was controlled. Our results show that larger males achieve almost exclusive access to females. Size symmetrical settings resulted in increased fighting activity and duration but dominance did not influence mating success. If copulations were disturbed by the rival male, copulations were terminated earlier in symmetrical settings compared with asymmetrical settings. In 94.8% of trials only one copulation took place, suggesting that the copulating male successfully monopolized access to the female. Males confronted with a rival copulated longer but courted significantly shorter than lone males. Although the last male to copulate sires 88% of the offspring in P. phalangioides, neither fighting nor courtship activity increased before the female laid a batch of eggs. This suggests that males have no indication of the timing of oviposition.     

The Influence of Physical and Acoustic Experience on Sequential Mate Preference in the Cricket Gryllus bimaculatus. Is Song Important?

Female Gryllus bimaculatus alter their mate choice based on size depending on previous experience with males. Male crickets sing to attract mates and several studies have identified call parameters important in female choice. We tested the hypotheses that exposure to acoustic stimuli before and/or after mating, from males of different sizes, in isolation and together with physical exposure influences female choice (willingness to mate and spermatophore retention time [SRT]). Females exposed to ad lib. song of multiple males post-mating had a shorter SRT than females in acoustic isolation. Exposure to ad lib. song of multiple males prior to mating had no effect on SRT. Females did not alter SRT depending on exposure to acoustic stimuli from males of different sizes either post or ante mating. Females exposed to acoustic and physical stimuli (though gauze) from large males had a shorter SRT than those exposed to small males but only when the exposure was post-mating. We were unable to identify any correlation between call parameters and body size in G. bimaculatus. Females use male song to locate potential mates but physical exposure to males is needed to allow females to judge male size and this exposure only influences SRT if it takes place post-mating.     

Temporal pattern changes in the calling song of the katydidMygalopsis marki Bailey in response to conspecific song (Orthoptera: Tettigoniidae)

Males of Mygalopsis markiBailey (Tettigoniidae: Orthoptera) alter the temporal structure of their song in response to other competing males. The song of males calling in aggregations has a high variance in the number of syllables per chirp, with short intervals between each chirp. In contrast, the temporal pattern of the song of isolated males is more evenly spaced, with an increase in length of the interchirp intervals and low variance in the number of syllables per chirp. In order to simulate a calling male moving closer to a male in an aggregation, a playback technique was adopted whereby the recorded calling song of a male was presented to itself via a loudspeaker in increments of 2dB. The change in song pattern of the resident male involved a reduction in the number of syllables per chirp and an increase in the interchirp interval, with the number of chirps per second remaining constant. This reduction in the output of the song, instead of not calling as a result of an acoustic contest, may still allow males to continue calling for females.     

Wing-flick Signals in the Courtship of the African Cave Cricket,Phaeophilacris spectrum

The male of the African cave cricket Phaeophilacris spectrum (Saltatoria: Grylloidea: Phalangopsidae) possesses tegmina without stridulatory organs, and both females and males lack tympanal organs. Therefore acoustic communication in the usual sense, which is typical for most crickets, is absent in this species. However, adaptations of the wing articulation allow the males to flick their wings forward over their heads. During courtship these movements are performed in series of 4–5 wing-flicks at a rate of 8–12 Hz (called wing-flick series = WFS) which elicit low-frequency air movements. Such signals may have taken over the functional significance of an acoustic ‘courtship song’. A quantitative analysis of the cricket's behaviour showed that in terms of frequency and total duration, ‘wing-flick series’ and ‘rocking’ behaviour are the most prominent signals of the male's courtship display. Moreover, analysis of the spatial relationships between male and female demonstrates that the wing-flicking is directed towards the female. The response of a female to a wing-flicking, courting male seems mainly to be calmed down and to reach a passive, receptive state, necessary for subsequent copulation. Females confronted with a male's courtship display exhibited a significantly higher threshold to disturbing stimuli than uncourted ones.     

Dangerous love? Predation risk does not affect female mate choice in blue‐black grassquits

Predation risk may be an important factor affecting female mate choice. Hypothetically, females could choose extravagantly ornamented males that survive in high predation risk environments. However, this decision could be different if choosing a conspicuous male under high predation risk is costly for females or results in reduced offspring survival. In such contexts, females could become indifferent to male quality or prefer inconspicuous males. We tested this idea using captive blue‐black grassquits (Volatinia jacarina, Linnaeus, 1766), a species in which males perform conspicuous leap displays coupled with songs during the breeding season, which presumably subjects females and offspring to higher predation risk. Females were placed in an arena with speakers on opposite sides emitting male courtship songs. One speaker emitted songs at a high rate (proxy for a conspicuous male) while the other speaker broadcast songs at a low rate (proxy for a less conspicuous male). While the female evaluated the two male songs, a third speaker emitted vocalizations characterizing three levels of risk: adult predator, nest predator, and no‐risk control. Females showed no preference for either male stimuli across the predation risk treatments. This lack of preference relative to frequency of male vocal displays suggests that leap‐song frequency is not used by females during mate choice. We suggest that in addition to its role in courtship, male grassquit displays also signal status to other males when competing for territories. Thus, we propose that predation risk does not directly influence blue‐black grassquit intersexual selection and that females in this species may exercise indirect mate choice, choosing social mates based on male ability to establish and defend a territory, and relying secondarily upon other aspects of male display attributes, such as its visual components.     

Independence of Sexual and Anti-Predator Perceptual Functions in an Acoustic Moth: Implications for the Receiver Bias Mechanism in Signal Evolution

The receiver bias model for the evolution of mating signals proposes that selection favors male displays that happen to stimulate a general, ancestral perception in females such that receptivity and successful courtship increase. If these male signals do arise, however, the female perception will operate in two contexts, the original, typically non‐sexual, one and courtship. We may then ask whether these two functions represent the same or distinct traits, which may be under separate neural and genetic control. We studied this question in Achroia grisella, a pyralid moth species in which males attract females, with an ultrasonic mating song. Hearing in pyralid moths is widespread and originated in an anti‐predator context – the perception and avoidance of echolocating bats – and it is inferred that the male song found in A. grisella, and in several other pyralid species, arose subsequently via a receiver bias mechanism: Females perceiving male‐produced ultrasound and responding with anti‐predator behavior normally exhibited in the presence of bat echolocations may have inadvertently increased the likelihood of successful courtship. We measured hearing responses in both sexual and anti‐predator contexts in inbred lines developed from an A. grisella population. Significant inter‐line variance was observed for sensitivity thresholds for female response to male song and for both female and male responses to synthetic bat echolocation signals. Female responses to male song and to synthetic echolocations differed markedly in sensitivity, and the median sexual and anti‐predator responses in the various lines were not correlated. However, a higher level of similarity occurs between the female and male anti‐predator responses. Thus, genetic variance for sexual and anti‐predator responses appears to exist, and the two responses may represent independent traits. These results imply that when male signals originate via a receiver bias mechanism, female perception does not necessarily remain fixed in its ancestral state but has the potential for continued modification.     

Phonotactic Behavior of Male Field Crickets (Gryllus texensis) in Response to Acoustic Calls From Conspecific Males

We studied male phonotactic behaviors elicited by acoustic cues that simulate conspecific male songs in the field cricket, Gryllus texensis. Males exhibited significant positive phonotaxis in response to the simulated song stimuli, but showed no such response to atypical song stimuli. We found no significant relationship between males’ own calling behavior and their phonotactic responses to the stimuli. Analyses indicated that larger males exhibited greater phonotactic responses, which may indicate a greater tendency to engage in aggressive interactions if size is an indicator of fighting ability. Male phonotactic responses were significantly weaker than those exhibited by females, and adult males did not exhibit stronger responses with increasing age as has been documented for females. Observed sex differences in the strengths of phonotactic responses may reflect differences in the fitness-payoffs of responding. That is, females are under strong selection pressure to respond to male songs and subsequently mate. In contrast, males responding to acoustic signals from other males need not precisely locate the signaler but would likely move to areas where females are likely to be found. Alternatively, males might benefit from avoiding areas with calling males and establishing their own calling stations away from competing males.     

Short records of marsh warbler ( Acrocephalus palustris) song provide indices that correlate with nesting success

The migrant Marsh Warbler (Acrocephalus palustris) has a complex song repertoire, but such complexity makes quantitative comparison of songs between individual males both time consuming and challenging. We investigate a streamlined method of song analysis that uses 2-min records of song to provide simpler relative indices of repertoire size, including the use of capture-recapture and species-richness models. For each male, three attributes of song were determined: the song complexity, the total repertoire elements and the estimated repertoire size based on the Burnham and Overton jackknife method. Males with higher song indices tend to have greater nesting success, suggesting that even short records of song can indicate male quality to prospective female mates. Why should male Marsh Warblers have long and sustained songs when only 2-min records correlate with nesting success? Assuming that song advertises the quality of the male, and that the quality of such advertisement is sustained throughout the males song period, we argue that the female may need to have only brief exposure initially to that song to assess its quality and hence the quality of the male. More continuous periods of song may reinforce that initial choice by the female, and allow males to remain conspicuous to transient females moving around the territories of potential male mates.     

Functional Aspects of Singing in Male and Female Uraeginthus bengalus (Estrildidae)

Both sexes of the African estrildid species Uraeginthus bengalus sing regularly. Song structure and the context of singing were studied in males and females under aviary conditions. It was found that:

• 1 Males and females have a similar song differing merely in strophe length, medial length being 1.38 ± 0.47 s in females and 2.58 ± 0.63 s in males.
• 2 Males sing during nesting, laying, incubation and broodcare, but also long before, with a remarkable activity peak at the beginning of incubation, whereas females hardly sing at all during the egg-laying and incubating phases, and their singing peak is long before laying starts.
• 3 In 52 % of all cases the male responds to his female's song during the pre-incubation period. His reaction includes singing, courtship display and elements introducing copulation.