Costs of Calling in Tettigoniid Orthoptera: A Case Study of Requena verticalis (Tettigoniidae: Listroscelidinae)

The tettigoniid Requena verticalis Walker controls the apportioning of energy between calling and spermatophylax production during courtship and copulation. Males delay the onset of calling following mating and during this period nutrients are mobilized to the production of a large nutrient-rich spermatophylax as well as to the production of sound. Experiments, using males reared on high and low quality diets (Simmons et al. 1993) examined the distribution of metabolic resources during this post-copula refractory period. An experimental set of males was allowed to call in a period immediately following copulation, while a control group of animals was prevented from calling by waxing the wings together. Males maintain spermatophylax size, regardless of the quality of the diet, but delayed the onset of calling; those on a high quality diet were able to invest energy into further reproduction. Contrary to prediction, males prevented from calling did not produce a larger spermatophylax, which suggests that males control the apportioning of energy reserves during this critical period. It is argued that where energy reserves are required for the production of a spermatophylax, less energy may be available for calling and so calling strategies should evolve that allow for the more efficient use of metabolic energy for sound production. Calling efficiency is a measure of how males convert metabolic energy to sound power. Efficiency was estimated by measuring the metabolic costs of calling and the power of sound radiation from the calling insect. Metabolic costs of calling in R. verticalis are compared with those of conehead tettigoniids such as Neoconocephalus robustus where calling appears to require more energy per unit of sound. The efficiency of conversion of metabolic energy to sound in R. verticalis is 6.4% while that for N. robustus is 2% (Bailey et al. 1993). Another cost of calling is exposure by the male to acoustically orienting predators. Such predators may include tachinid flies, insectivorous bats and predaceous females. I discuss experiments where a species of vespertilionid bat is shown to orient to the call of R. verticalis (Hosken et al. in press). Males may also be subject to cannibalism by orienting females. I describe experiments where females are released within 2m of calling males and where males show a high degree of female rejection. In the confines of a small container, where each sex cannot retreat from the other, a male smaller than the female is far more likely to be eaten by the female than males paired with females of comparable size. In this context, I discuss how male coyness (a reluctance to mate) may be due, in part, to sexual role reversal and thereby under sexual selection, or due to female cannibalism, and thus selected through survival.