Behav Ecol Sociobiol (2017) 71: 164 DOI 10.1007/s00265-017-2381-6 ORIGINAL ARTICLE Plasticity of signaling and mate choice in a trilling species of the Mecopoda complex (Orthoptera: Tettigoniidae) I. Krobath1 & H. Römer1 & M. Hartbauer1 Received: 13 June 2017 /Revised: 28 August 2017 /Accepted: 9 September 2017 /Published online: 24 October 2017 # The Author(s) 2017. This article is an open access publication Abstract isolation, compete with another male, or sing in the presence Males of a trilling species in the Mecopoda complex produce of a female. These results indicate that the AM-motif plays a conspicuous calling songs that consist of two motifs: an dominant role especially in close-range courtship and that amplitude-modulated motif with alternating loud and soft seg- males are challenged in finding a balance between attracting ments (AM-motif) and a continuous, high-intensity trill. The females and saving energy during repeated acoustic function of these song motifs for female attraction and com- interactions. petition between males was investigated. We tested the hy- pothesis that males modify their signaling behavior depending Significance statement on the social environment (presence/absence of females or Males of acoustic insects often produce conspicuous calling rival males) when they compete for mates. Therefore, we an- songs that have a dual function in male-male competition and alyzed acoustic signaling of males in three different situations: mate attraction. High signal amplitudes and signal rates are (1) solo singing, (2) acoustic interaction with another male, associated with high energetic costs for signal production. We and (3) singing in the presence of a female. In addition, the would therefore predict that males adjust their signaling be- preference of females for these song motifs and further song havior according to their perception of the social context. Here parameters was studied in two-choice experiments. As expect- we studied signal production and mate choice in a katydid, ed, females showed a preference for conspicuous and loud where males switch between loud and soft song segments in a song elements, but nevertheless, males increased the propor- dynamic way. Additionally, we examined the attractiveness of tion of the AM-motif in the presence of a female. In acoustic different song elements in female choice tests. Our results interactions, males reduced bout duration significantly com- show how males of this katydid deal with the conflict of re- pared to both other situations. However, song bouts in this maining attractive for females and competing with a costly situation still overlapped more than expected by chance, signal with rivals. which indicates intentionally simultaneous singing. A multi- variate statistical analysis revealed that the proportion of the AM-motif and the duration of loud segments within the AM- Keywords Katydid . Female choice . Signal plasticity . motif allow a reliable prediction of whether males sing in Inter-male competition . Receiver preference . Complex calling song Communicated by S. Sakaluk Abbreviations * M. Hartbauer [email protected] RMS Root mean square AM-motif Amplitude-modulated motif 1 Institute of Zoology, University of Graz, Universitätsplatz 2, DC Duty cycle A-8010 Graz, Austria SPL Sound pressure level 164 Page 2 of 14 Behav Ecol Sociobiol (2017) 71: 164 Introduction immediately affects the males’ calling song production, driv- ing increased signal complexity (Akre and Ryan 2011). Males of many acoustic insect species produce conspicuous After an arrival of or encounter with a female, how- calling songs to attract females from a distance and, at the ever, males often begin emitting a different, less con- same time, compete with rival males. These calling songs spicuous song. The courtship song of crickets is struc- often exhibit high amplitudes and signal rates, which are song turally distinct from the calling song and varies in its parameters associated with high costs. Costs for the sender sound pressure level (SPL), carrier frequency, and tem- arise as a result of energy investment during sound production poral pattern (Otte and Alexander 1983; Balakrishnan (Prestwich 1994) and/or the risk of unintentionally attracting and Pollack 1996;Brown1999). Male bladder grass- eavesdropping predators or parasitoids (Zuk and Kolluru hoppers decrease the SPL of their calling songs from 1998). Greenfield (1994a) proposed that song parameters such about 100 dB to about 70 dB after receiving the soft as high amplitude, song duration, and other exaggerated signal reply of a female. This reduces competition with rival features are manifest expressions of competition between males that have alternative mating tactics (Alexander males, the development of which is the outcome of co- and van Staaden 1989). In both cases, switching from evolutionary processes between senders and receivers (see a long-distance to a short-distance signal may facilitate also Kokko et al. 2003 and Lailvaux and Irschick 2006). private communication between the sexes and therefore One important factor that probably influences characteris- indicates a distinctive function of both signal types tics of signal displays is the social context in which signal (i.e., preparing the female to be mounted during copu- production occurs. Except for duetting species, in which one lation). In other species with different types of songs, sex responds to the other sex, acoustic signaling for males is however, the function of such a switch is less clear (for rather ‘speculative’ (Zimmermann et al. 1989), since calling areviewonBsoft signaling^ see Reichard and males cannot predict the effectiveness of their signaling until Anderson 2015). the female arrives. Males often sing in choruses with other In the current study, we hypothesized that males of a males within earshot of each other. In this situation, they at- trilling katydid species of the Mecopoda complex adjust tempt to increase the attractiveness of their call relative to that their calling behavior to the current social situation, of their rivals by increasing their signaling effort (Dadour i.e., the presence of females or competing males. 1989; Latimer 1981;Morrisetal.2002). In the presence of These males produce complex calling songs that have rivals, for example, stickleback males invest more energy in alternating loud and soft song segments (AM-motif) as red coloration (Sin-Yeon and Velando 2014) and weakly elec- well as trills with a consistent, high amplitude (105 dB tric fish increase pulse amplitude and phase duration of their SPL at a distance of 15 cm, Fig. 1). The high ampli- electric signals (e.g., Gavassa et al. 2013). Further examples tude of these calls indicates their function in long- for an increased energy investment in signal production of distance mate attraction (Krobath 2013). Field record- males are the field cricket Teleogryllus commodus, which ex- ings made in the native habitat (tropical rainforests in hibits a higher calling effort which in turn reduces their life Malaysia) revealed that neighboring males often sing at span (Callander et al. 2013) and the calling songs and invest- the same time within earshot of each other. If masking ment in spermatophores of the bushcricket Ephippiger by intense trills is critical in male-male competition, we diurnus, which also depend on the acoustic environment and would expect that males in a social context with other male’s age (Rebar and Greenfield 2017). Increasing signaling males increase the proportion of trills. If, however, effort is costly for the sender due to the inefficiency of the available energy for calling is critical, males may save conversion of muscle power into acoustic power (Prestwich energy during song interactions and adjust the propor- 1994; Gerhardt and Huber 2002). Signals produced in a cho- tion of costly (loud) and less costly (soft) song seg- rus are often more complex and include different signal com- ments in the AM-motif of the song. To test these hy- ponents. For example in Hyla microcephala, females prefer potheses, we compared the calling songs produced by multi-note over single-note calls, which stimulates the males males singing in isolation with those of the same males to increase both the call rate and number of call notes in a when they were free to interact acoustically with a chorus situation (Schwartz 1986). Such competitive signaling competitor. Because the two song motifs could poten- is associated with higher energetic demands (Wells and Taigen tially be directed toward a different audience (i.e., ei- 1989). A similar preference for complex calls is exhibited by ther female or rival male(s)), we also studied the call- the tungara frog (Physalaemus pustulosus), where competing ing songs produced by males in the presence of females males add chucks to whine calls in order to exploit a sensory and measured the amount of time spent in singing the bias in the receivers, increasing the attractiveness of their calls AM-motif and trill. Similar to other species, sexual se- (Ryan and Rand 1981; Ryan and Keddy-Hector 1992, but see lection by female choice may act as a driving force for Ron 2008). In P. pustulosus, the presence of females the evolution of complex calling songs in this species. Behav Ecol Sociobiol (2017) 71: 164 Page 3 of 14 164 Fig. 1 Temporal pattern of a AM-motif Trill calling song of a trilling a Mecopoda species. a The song bout of a solo singing male usually consists of an amplitude- modulated motif and a trill. b Loud segments are made up of high amplitude syllables (left), whereas syllables of the soft seg- Amplitude ment have lower amplitudes (right). c Temporal pattern of re- 50 s peatable units (RU) of the loud segment (left) and the soft seg- b ment (right) Amplitude 0.3 s RU of loud RU of soft c segments segments Amplitude 28ms 14ms Therefore, we studied the attractiveness of the two song rainforest in Malaysia in 2010 and 2011.