Hearing for Defense

Auditory predator cues decrease herbivore survival and plant damage

The high fitness cost of predation selects prey capable of detecting risk cues and responding in ways that reduce their vulnerability. While the impacts of auditory predator cues have been extensively researched in vertebrate prey, much less is known about invertebrate species' responses and their potential to affect the wider food web. We exposed larvae of Spodoptera exigua, a slow-moving and vulnerable herbivore hunted by aerial predators, to recordings of wasp buzzing (risk cue), mosquito buzzing (no-risk cue), or a no-sound control in both laboratory and field settings. In the laboratory, wasp buzzing (but not mosquito buzzing) reduced survival relative to the control; there was, however, no effect on time to or weight at pupation in survivors. In the field, wasp buzzing reduced caterpillar herbivory and increased plant biomass relative to the control treatment. In contrast, mosquito buzzing reduced herbivory less than wasp buzzing and had no effect on plant biomass. The fact that wasp cues evoked strong responses in both experiments, while mosquito buzzing generally did not, indicates that caterpillars were responding to predation risk rather than sound per se. Such auditory cues may have an important but largely unappreciated impacts on terrestrial invertebrate herbivores and their host plants.

The mechanics of acoustic signal evolution in field crickets

Field crickets (Family Gryllidae, Subfamily Gryllinae) typically produce tonal calls with carrier frequencies in the range 3-8 kHz. In this study, we explored the use of a finite element model (FEM) of the stridulatory apparatus of a field cricket, Gryllus bimaculatus, based on experimental measurements of resonator geometry and mechanical properties, to predict the measured call carrier frequencies of eight other field cricket species, ranging between 3 and 7 kHz. The model allowed accurate predictions of carrier frequencies for all eight species to within a few hundred hertz from morphological measurements of their resonators. We then used the model to explore the plausible evolutionary design space for field cricket call carrier frequency along the axes of resonator size and thickness, and mapped the locations of the nine experimentally measured species in this design space. Although the nine species spanned the evolutionarily conserved spectrum of carrier frequency and body size in field crickets, they were clustered in a small region of the available design space. We then explored the reasons for this apparent evolutionary constraint on field cricket carrier frequencies at both the lower and higher limit. We found that body size and sound radiation efficiency were the main constraints at the lower limits, whereas the energetics of stridulation using the clockwork mechanism may pose a constraint at higher frequencies.

Acoustic particle motion detection in the snapping shrimp (Alpheus richardsoni)

Many crustaceans produce sounds that might be used in communication. However, little is known about sound detection in crustaceans, hindering our understanding of crustacean acoustic communication. Sound detection has been determined only for a few species, and for many species, it is unclear how sound is perceived: as particle motion or sound pressure. Snapping shrimp are amongst the loudest and most pervasive marine sound sources. They produce snaps during interactions with conspecifics, and they also interact with soniferous heterospecifics. If they can hear, then sound could facilitate key behavioral interactions. We measured the auditory sensitivity of the snapping shrimp, Alpheus richardsoni, using auditory evoked potentials in response to a shaker table that generated only particle motion and an underwater speaker that generated both particle motion and sound pressure. Auditory detection was most sensitive between 80 and 100 Hz, and auditory evoked potentials were detected up to 1500 Hz. Snapping shrimp responded to both the shaker table and the underwater speaker, demonstrating that they detect acoustic particle motion. Crushing the statocyst reduced or eliminated hearing sensitivity. We conclude that snapping shrimp detect acoustic particle motion using the statocyst, they might detect conspecifics and heterospecifics, and hearing could facilitate key behavioral interactions.

Phylogenomic analysis sheds light on the evolutionary pathways towards acoustic communication in Orthoptera

Acoustic communication is enabled by the evolution of specialised hearing and sound producing organs. In this study, we performed a large-scale macroevolutionary study to understand how both hearing and sound production evolved and affected diversification in the insect order Orthoptera, which includes many familiar singing insects, such as crickets, katydids, and grasshoppers. Using phylogenomic data, we firmly establish phylogenetic relationships among the major lineages and divergence time estimates within Orthoptera, as well as the lineage-specific and dynamic patterns of evolution for hearing and sound producing organs. In the suborder Ensifera, we infer that forewing-based stridulation and tibial tympanal ears co-evolved, but in the suborder Caelifera, abdominal tympanal ears first evolved in a non-sexual context, and later co-opted for sexual signalling when sound producing organs evolved. However, we find little evidence that the evolution of hearing and sound producing organs increased diversification rates in those lineages with known acoustic communication.

What Does an Insect Hear? Reassessing the Role of Hearing in Predator Avoidance with Insights from Vertebrate Prey

Insects have a diversity of hearing organs known to function in a variety of contexts, including reproduction, locating food, and defense. While the role of hearing in predator avoidance has been extensively researched over the past several decades, this research has focused on the detection of one type of predator-echolocating bats. Here we reassess the role of hearing in antipredator defense by considering how insects use their ears to detect and avoid the wide range of predators that consume them. To identify the types of sounds that could be relevant to insect prey, we first review the topic of hearing-mediated predator avoidance in vertebrates. Sounds used by vertebrate prey to assess predation risk include incidental sound cues (e.g., flight sounds, rustling vegetation, and splashing) produced by an approaching predator or another escaping prey, as well as communication signals produced by a predator (e.g., echolocation calls, songs) or nonpredator (e.g., alarm calls). We then review what is known, and what is not known, about such sounds made by the main predators and parasitoids of insects (i.e., birds, bats, terrestrial vertebrates, and invertebrates) and how insects respond to them. Three key insights emerged from our review. First, there is a lack of information on how both vertebrate and insect prey use passive sound cues produced by predators to avoid being captured. Second, while there are numerous examples of vertebrate prey eavesdropping on the calls and songs of predators and nonpredators to assess risk, there are currently no such examples for eared insect prey. Third, the hearing sensitivity of many insects, including those with ears considered to be dedicated to detecting bats or mates, overlaps with both sound cues and signals generated by nonbat predators. Sounds of particular relevance to insect prey include the flight sounds and calls of insectivorous birds, the flight sounds of insect predators and parasitoids, and rustling vegetation sounds of birds and terrestrial predators. We conclude that research on the role of insect hearing in predator avoidance has been disproportionally focused on bat-detection, and that acoustically-mediated responses to other predators may have been overlooked because the responses of prey may be subtle (e.g., ceasing activity, increasing vigilance). We recommend that researchers expand their testing of hearing-mediated risk assessment in insects by considering the wide range of sounds generated by predators, and the varied responses exhibited by prey to these sounds.

Hearing with exceptionally thin tympana: Ear morphology and tympanal membrane vibrations in eneopterine crickets

The receiver sensory system plays a crucial role in the evolution of new communication signals in insects. Among acoustic communicating crickets, the tribe Lebinthini (Eneopterinae) has evolved a unique communication system in that males produce exceptionally high-frequency calls and females respond with vibratory signals to guide males towards them. In this study, we describe nine species of Eneopterinae in which the sound receiving structures have undergone considerable morphological changes. We revealed that the anterior tympanal membrane (ATM) of the ear was extremely thin, as little as 0.35 µm thick, and to the best of our knowledge, this is the thinnest tympanal membrane found in crickets thus far. Measurements of tympanum vibrations obtained from Lebinthus bitaeniatus demonstrated a strong sensitivity towards higher frequencies. The finding also coincides with the neuronal tuning of ascending neurons and the behavioural response of the Lebinthini. The morphologically specialized ATM and its mechanical sensitivity for high frequencies, therefore, may have driven the sensory exploitation of an anti-predator behaviour that led to the evolution of a new communication system known for this group of crickets. The hypothetical phylogenetic origin of the investigated tympanal ears is discussed.

Evolutionary diversification of the auditory organ sensilla in Neoconocephalus katydids (Orthoptera: Tettigoniidae) correlates with acoustic signal diversification over phylogenetic relatedness and life history

Neoconocephalus Tettigoniidae are a model for the evolution of acoustic signals as male calls have diversified in temporal structure during the radiation of the genus. The call divergence and phylogeny in Neoconocephalus are established, but in tettigoniids in general, accompanying evolutionary changes in hearing organs are not studied. We investigated anatomical changes of the tympanal hearing organs during the evolutionary radiation and divergence of intraspecific acoustic signals. We compared the neuroanatomy of auditory sensilla (crista acustica) from nine Neoconocephalus species for the number of auditory sensilla and the crista acustica length. These parameters were correlated with differences in temporal call features, body size, life histories and different phylogenetic positions. By this, adaptive responses to shifting frequencies of male calls and changes in their temporal patterns can be evaluated against phylogenetic constraints and allometry. All species showed well-developed auditory sensilla, on average 32-35 between species. Crista acustica length and sensillum numbers correlated with body size, but not with phylogenetic position or life history. Statistically significant correlations existed also with specific call patterns: a higher number of auditory sensilla occurred in species with continuous calls or slow pulse rates, and a longer crista acustica occurred in species with double pulses or slow pulse rates. The auditory sensilla show significant differences between species despite their recent radiation, and morphological and ecological similarities. This indicates the responses to natural and sexual selection, including divergence of temporal and spectral signal properties. Phylogenetic constraints are unlikely to limit these changes of the auditory systems.