Plant-borne vibrations modulate calling behaviour in a tropical amphibian

Communication via Biotremors in the Veiled Chameleon (Chamaeleo calyptratus): Part I- Biotremor Production and Response to Substrate-Borne Vibrations

Biotremors are vibrations, usually surface waves along the boundary of a medium, produced by an organism. While substrate-borne vibrations are utilized by different reptile species, true conspecific communication via biotremors has not yet been demonstrated in lizards. Recent research revealed that the veiled chameleon (Chamaeleo calyptratus) produces biotremors. The prerequisites for any communication system are the ability of an organism to produce and detect a signal. We tested C. calyptratus behavioral responses to vibrations by placing them on a dowel attached to a shaker, emitting vibrations of 25, 50, 150, 300, and 600 Hz and compared their locomotory velocity before and after the stimulus. Adult chameleons exhibited a freeze response to 50 and 150 Hz, while juveniles exhibited a similar response to frequencies between 50 and 300 Hz. In a second experiment, chameleons were induced to produce biotremors via experimenter contact. These biotremors ranged in mean fundamental frequency from 106.4 to 170.3 Hz and in duration from 0.06 to 0.29 s. Overall, two classes of biotremors were identified, "hoots" and "mini-hoots," which differed significantly in mean relative signal intensity (-7.5 and -32.5 dB, respectively). Juvenile chameleons 2 months of age were able to produce biotremors, suggesting this behavior may serve a wide range of ecological functions throughout ontogeny. Overall, the data demonstrate that C. calyptratus can both produce and detect biotremors that could be used for intraspecific communication.

Effect of natural abiotic soil vibrations, rainfall and wind on anuran calling behavior: a test with captive-bred midwife toads (Alytes obstetricans)

Anurans are known to detect vibrations, but few studies explore relationships between vibrations and resultant behaviors. We studied the reaction of calling captive-bred male midwife toads (Alytes obstetricans) to the randomized playback of a vibrational crescendo stimulus train. We considered two sources of natural abiotic vibrational stimuli: rainfall and wind. Rainfall was expected to induce calling and wind was expected to inhibit it. Playback experiments with two synthetic tones (200 Hz and 300 Hz) tested the sensitivity to pure tones and could possibly reveal a hearing sensitivity trend between these frequencies. The toads did not increase call rate in response to rainfall vibrations and only one of the five wind stimulus levels caused a significant decrease in call rate. This limited response could be explained, because the tested toads came from a captive population, where emergence may not be mediated by rainfall vibrations. We found that A. obstetricans is highly sensitive to very low frequencies, which could explain the sensitivity observed to vibrational stimuli. Playback of a random crescendo stimulus train proves to be a valid approach for addressing behavioral questions. However, the use of a captive population may have been a limitation in the clarity of the results.

Beyond sound: Bimodal acoustic calls used in mate-choice and aggression by red-eyed treefrogs

Airborne sound signals function as key mediators of mate-choice, aggression, and other social interactions in a wide range of vertebrate and invertebrate animals. Calling animals produce more than sound, however. When displaying on or near a solid substrate, such as vegetation or soil, they also unavoidably excite substrate vibrations due to the physics of sound production and of acoustic propagation, and these vibrations can propagate to receivers. Despite their near ubiquity, these vibrational signal components have received very little research attention, and in vertebrates it is completely unknown whether they are relevant to mate-choice, an important driver of evolutionary divergence. Here we show that female red-eyed treefrogs are more than twice as likely to choose a male mating call when airborne sound is paired with its corresponding substrate vibrations. Furthermore, males of the same species are more aggressive towards and display a greater range of aggressive behaviors in response to bimodal (sound and vibration) versus unimodal (sound or vibration alone) calls. In aggressive contexts, at least, air- and substrate-borne signal components function non-redundantly. These results are a clear demonstration that vibrations produced by a calling animal can function together with airborne sound to markedly enhance the function of a signal. If this phenomenon proves widespread, this finding has the potential to substantially influence our understanding of the function and evolution of acoustic signals.

Hearing without a tympanic ear

The ability to sense and localize sound is so advantageous for survival that it is difficult to understand the almost 100 million year gap separating the appearance of early tetrapods and the emergence of an impedance-matching tympanic middle ear - which we normally regard as a prerequisite for sensitive hearing on land - in their descendants. Recent studies of hearing in extant atympanate vertebrates have provided significant insights into the ancestral state(s) and the early evolution of the terrestrial tetrapod auditory system. These reveal a mechanism for sound pressure detection and directional hearing in 'earless' atympanate vertebrates that may be generalizable to all tetrapods, including the earliest terrestrial species. Here, we review the structure and function of vertebrate tympanic middle ears and highlight the multiple acquisition and loss events that characterize the complex evolutionary history of this important sensory structure. We describe extratympanic pathways for sound transmission to the inner ear and synthesize findings from recent studies to propose a general mechanism for hearing in 'earless' atympanate vertebrates. Finally, we integrate these studies with research on tympanate species that may also rely on extratympanic mechanisms for acoustic reception of infrasound (<20 Hz) and with studies on human bone conduction mechanisms of hearing.

Hay meadow vibroscape and interactions within insect vibrational community

Our experiences shape our knowledge and understanding of the world around us. The natural vibrational environment (vibroscape) is hidden to human senses but is nevertheless perceived and exploited by the majority of animals. Here, we show that the vibroscape recorded on plants in a temperate hay meadow is a dynamic low-frequency world, rich in species-specific vibrational signals. The overall vibroscape composition changed throughout the season and also depended on the plant species, as well as on the spatial position of individual plants within the meadow. Within the studied community, vibrationally signaling species sharing this communication channel avoided interference primarily by partitioning vibrational space on a fine temporal scale. The vibroscape is a reliable source of information in the environment and expands our understanding of ecological and evolutionary processes.

A common computational principle for vibrotactile pitch perception in mouse and human

We live surrounded by vibrations generated by moving objects. These oscillatory stimuli propagate through solid substrates, are sensed by mechanoreceptors in our body and give rise to perceptual attributes such as vibrotactile pitch (i.e. the perception of how high or low a vibration’s frequency is). Here, we establish a mechanistic relationship between vibrotactile pitch perception and the physical properties of vibrations using behavioral tasks, in which vibratory stimuli were delivered to the human fingertip or the mouse forelimb. The resulting perceptual reports were analyzed with a model demonstrating that physically different combinations of vibration frequencies and amplitudes can produce equal pitch perception. We found that the perceptually indistinguishable but physically different stimuli follow a common computational principle in mouse and human. It dictates that vibrotactile pitch perception is shifted with increases in amplitude toward the frequency of highest vibrotactile sensitivity. These findings suggest the existence of a fundamental relationship between the seemingly unrelated concepts of spectral sensitivity and pitch perception.

The features of vibrations provide key information on the surrounding environment. Here the authors show that a common computational principle underlies vibrotactile pitch perception in both mice and humans.

The role of hyoid muscles in biotremor production in Chamaeleo calyptratus

The production of biotremors has been described in veiled chameleons (Chamaeleo calyptratus), but the mechanism by which they are produced is unknown. We gathered muscle activation data via electromyography (EMG), with simultaneous recordings of biotremors using an accelerometer, to test for the role of hyoid muscles in biotremor production. We recorded a mean biotremor frequency of 150.87 Hz for females and 136.01 Hz for males. The durations of activity and the latencies to onset and offset for the M. sternohyoideus profundus (SP), M. sternohyoideus superficialis (SS), Mm. mandibulohyoideus (MH) and M. levator scapulae (LS) were all significantly correlated with biotremor durations and biotremor onset and offset, respectively. Linear mixed-effect regression model comparisons of biotremor duration indicated that models containing either the MH and/or the SP and LS account for the most variation in biotremor duration. Twitch times for the SP (100 ms) and the SS (132 ms) at field active body temperature, however, were individually too slow to produce the biotremors at the observed frequency without alteration after production by other anatomical structures. These results implicate the SP, SS, MH and LS in the production of biotremors, but the exact mechanism of production requires further study.

Male serrate-legged treefrogs adjust competition strategies according to visual or chemical cues from females

There is increasing evidence that many anurans use multimodal cues to detect, discriminate and/or locate conspecifics and thus modify their behaviors. To date, however, most studies have focused on the roles of multimodal cues in female choice or male-male interactions. In the present study, we conducted an experiment to investigate whether male serrate-legged small treefrogs (Kurixalus odontotarsus) used visual or chemical cues to detect females and thus altered their competition strategies in different calling contexts. Three acoustic stimuli (advertisement calls, aggressive calls and compound calls) were broadcast in a randomized order after a spontaneous period to focal males in one of four treatment groups: combined visual and chemical cues of a female, only chemical cues, only visual cues and a control (with no females). We recorded the vocal responses of the focal males during each 3 min period. Our results demonstrate that males reduce the total number of calls in response to the presence of females, regardless of how they perceived the females. In response to advertisement calls and compound calls, males that perceived females through chemical cues produced relatively fewer advertisement calls but more aggressive calls. In addition, they produced relatively more aggressive calls during the playback of aggressive calls. Taken together, our study suggests that male Kodontotarsus adjust their competition strategies according to the visual or chemical cues of potential mates and highlights the important role of multisensory cues in male frogs' perception of females.

Anthropogenic substrate-borne vibrations impact anuran calling

Anthropogenic disturbance is a major cause of the biodiversity crisis. Nevertheless, the role of anthropogenic substrate vibrations in disrupting animal behavior is poorly understood. Amphibians comprise the terrestrial vertebrates most sensitive to vibrations, and since communication is crucial to their survival and reproduction, they are a suitable model for investigating this timely subject. Playback tests were used to assess the effects of substrate vibrations produced by two sources of anthropogenic activity– road traffic and wind turbines– on the calling activity of a naïve population of terrestrial toads. In their natural habitat, a buried tactile sound transducer was used to emit simulated traffic and wind turbine vibrations, and changes in the toads’ acoustic responses were analyzed by measuring parameters important for reproductive success: call rate, call duration and dominant frequency. Our results showed a significant call rate reduction by males of Alytes obstetricans in response to both seismic sources, whereas other parameters remained stable. Since females of several species prefer males with higher call rates, our results suggest that anthropogenically derived substrate-borne vibrations could reduce individual reproductive success. Our study demonstrates a clear negative effect of anthropogenic vibrations on anuran communication, and the urgent need for further investigation in this area.

Gular pouch diversity in the Chamaeleonidae

Numerous chameleon species possess an out-pocketing of the trachea known as the gular pouch. After surveying more than 250 specimens, representing nine genera and 44 species, we describe two different morphs of the gular pouch. Species of the genera Bradypodion and Chamaeleo, as well as Trioceros goetzei, all possess a single gular pouch (morph one) formed from ventral expansion of soft tissue where the larynx and trachea meet. Furcifer oustaleti and Furcifer verrucosus possess from one to four gular pouches (morph two) formed by the expansion of soft tissue between sequential hyaline cartilage rings of the trachea. In Trioceros melleri, examples of both morphs of the gular pouch were observed. Morphometric data are presented for 100 animals representing eight species previously known to possess a gular pouch and two additional species, Bradypodion thamnobates and Bradypodion transvaalense. In the species with the absolutely and relatively largest gular pouch, Chamaeleo calyptratus, a significant difference was found between sexes in its width and volume, but not its length. In C. calyptratus, we show that an inflated gular pouch is in contact with numerous hyoid muscles and the tongue. Coupled with the knowledge that C. calyptratus generates vibrations from the throat region, we posit that the tongue (M. accelerator linguae and M. hyoglossus) and supporting hyoid muscles (i.e., Mm. sternohyoideus profundus et superficialis and Mm. mandibulohyoideus) are involved in the production of vibrations to produce biotremors that are amplified by the inflated gular pouch and used in substrate-borne communication.

Feature-selective encoding of substrate vibrations in the forelimb somatosensory cortex

The spectral content of skin vibrations, produced by either displacing the finger across a surface texture¹ or passively sensing external movements through the solid substrate^2,3, provides fundamental information about our environment. Low-frequency flutter (below 50 Hz) applied locally to the primate fingertip evokes cyclically entrained spiking in neurons of the primary somatosensory cortex (S1), and thus spike rates in these neurons increase linearly with frequency^4,5. However, the same local vibrations at high frequencies (over 100 Hz) cannot be discriminated on the basis of differences in discharge rates of S1 neurons^4,6, because spiking is only partially entrained at these frequencies⁶. Here we investigated whether high-frequency substrate vibrations applied broadly to the mouse forelimb rely on a different cortical coding scheme. We found that forelimb S1 neurons encode vibration frequency similarly to sound pitch representation in the auditory cortex^7,8: their spike rates are selectively tuned to a preferred value of a low-level stimulus feature without any temporal entrainment. This feature, identified as the product of frequency and a power function of amplitude, was also found to be perceptually relevant as it predicted behaviour in a frequency discrimination task. Using histology, peripheral deafferentation and optogenetic receptor tagging, we show that these selective responses are inherited from deep Pacinian corpuscles located adjacent to bones, most densely around the ulna and radius and only sparsely along phalanges. This mechanoreceptor arrangement and the tuned cortical rate code suggest that the mouse forelimb constitutes a sensory channel best adapted for passive 'listening' to substrate vibrations, rather than for active texture exploration.