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This last strategy, modification of acoustic characteristics, holds ample opportunity for adaptation but also backs up against constraints specific to invertebrates. The sound pro- duction and receiving mechanisms and related exoskeleton features of many invertebrates (e.g., insects, spiders), which are manifestations of the sort represented by ripples in a hard- ened cuticle, are molded and set in place at the time of an organism’s final molt, the one in which they reach maturity (Figure 5). Herein lies a problem: on reaching maturity, many invertebrates have little room to plastically adjust the sounds they produce through structural adjustments. Additionally, many invertebrates are small and face size constraints in shift- ing the frequencies of their calls (Bennet-Clark, 1998).
Figure 4. Signal characteristics and their hypothesized robustness to masking noise. Left column: signal information can be conveyed across multiple acoustic characteristics. Right column: certain variations of each characteristic are predicted to be better at conveying information in the presence of masking noise than others (right to left, respectively). Information can be conveyed using temporal, amplitudinal (waveforms in white boxes), and spectral (spectrograms in gray boxes) properties. Modified from Raboin and Elias, 2019.
Figure 5. Substrate-borne “buzz” courtship signal and corresponding stridulating structure of the male jumping spider, Habronattus dossenus (order Araneae, family Salticidae). Front legs come down (1→2) as the abdomen oscillates at 65 Hz (1-2). This signal has a fundamental frequency at 65 Hz, with several harmonic frequencies (130, 195, and 260 Hz). A: body position, with numbers (1 and 2) illustrating movements of the forelegs and abdomen. B: position of one of the forelegs (in millimeters above the substrate). C: oscillogram of the substrate- borne signal. D: frequency characteristics of the substrate-borne signal. B-D are shown in the same time scale, with numbers (1 and 2) corresponding to the body movements illustrated in A. E and F: scanning electron micrographs (SEM) of the stridulating structure on the exoskeleton of a male H. dossenus. E: SEM of the posterior end of the head (cephalothorax). F, ridged file. F: SEM of the anterior end of the abdomen. S, locations of the scrapers. Adapted from Elias et al., 2003.
most consequential involve whether or not inverte- brates will habituate to anthropogenic sound or adapt via plasticity or evolution. For now, we can only guess at the answers. First, invertebrates can avoid anthropo- genic sound by migrating away from areas with human activity. Although this may be an especially useful strategy for those species with wings, migration may be out of the question for others, such as soil-dwelling species. Second, they can avoid a masking sound by shifting the times of day at which they communicate away from peak human activity. However, this strat- egy comes with the potential costs of communicating at nonoptimal times of day. Finally, they can adjust the acoustic characteristics of calls that contain infor- mation, such as amplitude, frequency and bandwidth, temporal structure, dimensionality, and modality, to avoid overlap with anthropogenic sound or increase their signal-to-noise ratio (Figure 4).
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