Silent katydid females are at higher risk of bat predation than acoustically signalling katydid males

Abstract

Males that produce conspicuous mate attraction signals are often at high risk of predation from eavesdropping predators. Females of such species typically search for signalling males and their higher motility may also place them at risk. The relative predation risk faced by males and females in the context of mate-finding using long-distance signals has rarely been investigated. In this study, we show, using a combination of diet analysis and behavioural experiments, that katydid females, who do not produce acoustic signals, are at higher risk of predation from a major bat predator, Megaderma spasma, than calling males. Female katydids were represented in much higher numbers than males in the culled remains beneath roosts of M. spasma. Playback experiments using katydid calls revealed that male calls were approached in only about one-third of the trials overall, whereas tethered, flying katydids were always approached and attacked. Our results question the idea that necessary costs of mate-finding, including risk of predation, are higher in signalling males than in searching females.

A novel acoustic-vibratory multimodal duet

“…In the neotropics, bats have been shown to locate male katydids by eavesdropping on their calls, and this has been hypothesized to drive low acoustic calling rates and the preferential use of an alternative channel of vibrational communication by neotropical pseudophylline males (Belwood and Morris, 1987;Morris et al, 1994;Heller, 1995 However, neither low call rates nor vibrational signaling has been reported in paleotropical pseudophylline males. In fact, playbacks of acoustic calls of male O. uninotatus were found to attract the predatory bat Megaderma spasma in only 30% of trials, compared with a 100% approach rate towards flying, non-stridulating females (Raghuram et al, 2015). This, combined with the fact that significantly more female bushcricket forewings were found in M. spasma's diet than male forewings, suggests higher predator pressure upon moving O. uninotatus females than calling males.…”

“…In this ecological context, female tremulation and male vibrotaxis may serve to shift the risk of mate localization away from females. This unique case of acoustic-vibratory duetting between O. uninotatus males and females may therefore be an evolutionary response to the higher predation risk faced by flying bushcricket females over calling males in paleotropical assemblages (Raghuram et al, 2015).…”

Insect Hearing

“…Examination of several species of the katydid genus Poecilimon , which show a diversity of both pair formation strategies and nuptial gift size, provides correlative support to this view (McCartney et al 2012 ). Because stationary males have to call for females to locate them, however, this hypothesis assumes that calling is less risky than searching, an assumption that is currently supported only by a couple of studies (Heller 1992 ;Raghuram et al 2015 ) and needs to be examined further in comparative studies on different katydid species.…”

“…The searching sex will have to move to fi nd the signaling sex and incurs an energetic cost as well as an increased risk of predation. Duetting may evolve when female search costs become too high: There is evidence that female katydids may incur predation risks as high as, or even signifi cantly higher than, calling males (Belwood and Morris 1987 ;Raghuram et al 2015 Fig. 3.1 Factors infl uencing the evolution of pair-forming strategies.…”

“…Male katydids calling from cages placed in the fi eld in Panamanian rain forests attracted bat predators, whereas similar cages baited with noncalling females did not (Belwood and Morris 1987 ). Playback of male katydid calls elicits approach by bat predators in fl ight cages/rooms in the neotropics (Belwood and Morris 1987 ), the palearctic , and the paleotropics (Raghuram et al 2015 ). There is no doubt that calling entails predation risk, but the question is whether this constitutes a suffi ciently strong selection pressure to infl uence call structure, calling, and searching behavior.…”