How grasshoppers respond to road noise: developmental plasticity and population differentiation in acoustic signalling

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. 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. 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. 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. 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.