Summary1. Anthropogenic noise is known to affect acoustic signal production in birds, frogs and mammals. These animals use different mechanisms to adjust their signals to elevated background noise levels (increase in signal amplitude, shift to higher frequencies, etc.). Previous studies have concentrated on behaviourally plastic changes in signal production as a result of elevated background noise levels. To our knowledge, long-term effects of anthropogenic noise on signal production have not yet been investigated. Moreover, strategies of invertebrate species to ensure acoustic signal transmission under anthropogenic noise have not been examined, so far. 2. We tested whether and how male Chorthippus biguttulus grasshoppers from noisy roadside habitats may adjust acoustic courtship signals to elevated background noise levels, compared with conspecifics from quiet control habitats. In this species, sexually selected male courtship signals serve to attract potential mating partners, which make the undisturbed transmission of signals in habitats with increased background noise levels crucial for male reproductive success. 3. Compared to males from control populations, males from roadside habitats produced songs with a significantly higher local frequency maximum under standardized, quiet recording conditions. This local frequency maximum (in the range of c. 6-9 kHz) overlaps with low-frequent road noise that has the potential to degrade or mask this part of the signals' frequency spectrum. 4. To our knowledge, this is the first evidence that insects from noisy habitats produce different acoustic signals than conspecifics from quiet habitats, possibly using a more permanent mechanism for signal adjustment than behavioural plasticity, which was found in different bird species adjusting to high background noise levels. Such an effect of anthropogenic noise has not been shown for any invertebrate species before, and our results suggest that similar strategies to avoid degradation or masking by noise (i.e. increase in carrier frequency) are used over a wide range of taxa, including both, vertebrates and invertebrates.
Summary1. Increasing levels of anthropogenic noise have the potential to mask signals of acoustically communicating species in their natural habitats. Animals in noise-polluted habitats typically adjust their signals away from high background noise levels to ensure successful signal transmission under challenging environmental conditions. Earlier we demonstrated that male Chorthippus biguttulus grasshoppers from roadside habitats produce courtship signals with elevated frequency components compared to conspecifics from non-roadside habitats.2. Here, we use a common garden approach to study the mechanisms underlying this response. We transferred grasshopper nymphs from seven roadside as well as five non-roadside habitats to the laboratory to rear half of them under noisy and the other half under quiet conditions in a full factorial two-by-two design. Courtship songs of adult males were later recorded under standardized quiet conditions. 3. Males exposed to road noise as nymphs produced signals with higher frequency components compared to males reared under quiet conditions, indicating developmental plasticity as a mechanism underlying the signal adjustment to anthropogenic noise in grasshoppers. 4. In addition, males originating from roadside habitats produced signals with higher frequency components and an increased syllable to pause ratio -a sexually selected signal trait -compared to males from non-roadside habitats. 5. Our results demonstrate for the first time that developmental plasticity may play an important role in song trait modifications in response to anthropogenic noise. Furthermore, they suggest that multiple roadside populations may have diverged in parallel, possibly in response to selection for minimizing signal masking by road noise.
Genome size is largely uncorrelated to organismal complexity and adaptive scenarios. Genetic drift as well as intragenomic conflict have been put forward to explain this observation. We here study the impact of genome size on sexual attractiveness in the bow-winged grasshopper Chorthippus biguttulus. Grasshoppers show particularly large variation in genome size due to the high prevalence of supernumerary chromosomes that are considered (mildly) selfish, as evidenced by non-Mendelian inheritance and fitness costs if present in high numbers. We ranked male grasshoppers by song characteristics that are known to affect female preferences in this species and scored genome sizes of attractive and unattractive individuals from the extremes of this distribution.We find that attractive singers have significantly smaller genomes, demonstrating that genome size is reflected in male courtship songs and that females prefer songs of males with small genomes. Such a genome size dependent mate preference effectively selects against selfish genetic elements that tend to increase genome size. The data therefore provide a novel example of how sexual selection can reinforce natural selection and can act as an agent in an intragenomic arms race. Furthermore, our findings indicate an underappreciated route of how choosy females could gain indirect benefits.
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