The complex auditory scene at leks: balancing antipredator behaviour and competitive signalling in an acoustic moth

International audienceLepidoptera life history theory pyralid moth sexual selection trade-off ultrasound signal Although sexual activity in many animal species is reduced when predation pressure intensifies, such reduction may be attenuated in accordance with age, demography or sexual competition. For example, males in lekking aggregations might forgo evasive behaviour and continue their signalling activity when exposed to predation for various reasons: the pressure to engage in signal competition with neighbours outweighs the risk of a predator attack, the per capita risk of attack is lower on larger leks, signals from neighbours within the lek mask predator cues, or limitations on general attention prevent a lekking male from simultaneously signalling and monitoring predators. We addressed the problem of balancing antipredator behaviour and signal competition in an acoustic pyralid moth, Achroia grisella, in which males gather in leks and broadcast an ultrasonic mating call. There is evidence that A. grisella can be menaced by substrate-gleaning bats and that singing males generally become silent upon perceiving bat echolocation signals or pulsed ultrasound bearing the characteristics of these signals. In this study, the incidence and duration of these silence responses were greatly reduced in lekking males compared with solitary individuals. Moreover, a moderate reduction in silence responses persisted when we broadcast, to individual males, song from a lek followed by bat echolocation stimuli. Thus, while signal masking may play a role in attenuating antipredator behaviour in lekking males, other factors, including signal competition and dilution of predation pressure, are probable influences as well. Studies of mating behaviour in both vertebrate and invertebrate species commonly note marked reductions or changes in sexual activity in the presence of predators (Burk 1982; Magnhagen 1991; Fuller & Berglund 1996). Because male signalling is often conspicuous to natural enemies as well as to females (Zuk & Kolluru 1998), males are expected to broadcast their advertisements at reduced intensity, intermittently or not at all when predators are perceived. Similarly, the orientation movements of both sexes during pair formation may reveal their presence to third parties (eavesdrop-pers) and females may therefore be less receptive to males and male signals (Farris et al. 1998; Dill et al. 1999; Jones et al. 2002), or discriminate less among local males (Hedrick & Dill 1993; Danchin & Cézilly 2005), if predation intensifies. Courtship and copulation pose additional risks (Sih et al. 1990; Koga et al. 1998; but see Gwynne 1989 for data indicating an absence of risk) and may also be shortened, or avoided and deferred until a safer moment. The attention to one's partner that normally occurs during courtship may be incompatible with the vigilance necessary to monitor predators, and the immobility during copulation may render escape from predators difficult or impossible. These expectations are general predictions, which may be modified in accordance with the principles of life history theory (Candolin 1998). For example, as animals age and the expected number of future mating opportunities diminishes, the decision equation that modulates the conflicting demands of current reproduction versus survival and the possibility of future reproduction may change such that males continue signalling and females remain receptive and orient towards signalling males under higher levels of perceived predation (Lafaille et al. 2010). Thus, individuals of either sex would not forgo terminal mating opportunities under some conditions (Clutton-Brock 1984). Social behaviour and the competition inherent within groups introduce the possibility of additional modifications to the partitioning of effort between current reproduction versus survival and potential future reproduction. From the male perspective, when individuals advertise in the vicinity of conspecifics, the phenomenon of signal competition arises in which each male may be under pressure at least to match the broadcasts of his neighbour

Priority of precedence: receiver psychology, female preference forleading calls and sexual selection in insect choruses

International audienceIn species in which males display collectively, females may evaluate display features that arise specificallyin groups in addition to basic features of signal energy. For example, in acoustic insects and anuransthat chorus, males often adjust their song timing relative to neighbours, and females may pay attentionto these adjustments in timing. Many laboratory studies show how males may delay the phase of theirsong rhythm with respect to a song stimulus such that they call just prior to the stimulus; similarly,females may choose a male whose song rhythm leads a neighbour by a brief interval. However, theimportance of male phase adjustments and female attention to call order in actual choruses hasremained poorly understood. We studied female choice in laboratory choruses in the bushcricketEphippiger diurnus, a species with preferences for leading calls but also for longer calls and fasterrhythms, features representing broadcast energy. Although chorusing males varied in all features, wefound that females largely focused on call order. This overriding preference for call order may be reflectedby the prevalence of male phase adjustment throughout the population, a feature that wouldhave been subject to strong sexual selection.We found that the broadcast of leading calls within a choruswas not repeatable among males but rather shifted among the chorus participants. This observation toomay reflect the strong selection imposed by female choice for call order, and it may have implications forthe role of receiver psychology in the evolution of mate choice. 2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved