Information theory analysis of patterns of modulation in the advertisement call of the male bullfrog, Rana catesbeiana

Variations in advertisement call modulations do not influence vocal interactions in bullfrog choruses

Chorusing male bullfrogs naturally vary the number of modulations within their advertisement call notes. A field playback experiment investigated whether these variations affect males' evoked vocal responses. Vocal responses were quantified manually and automatically by quantifying acoustic energy. The numbers of calls, number of notes, latency of response, and detected-note acoustic energy did not vary significantly across playback stimuli for focal males or the entire chorus, suggesting that variations in modulation number do not carry relevant information to males. Future work can determine whether modulation cues may function in sexual selection and affect female response.

Estimating chorusing activity by quantifying total acoustic energy

Passive acoustics provides a powerful method for localizing vocalizing animals and estimating species abundance. A passive acoustics method previously used to census dense populations of flying bats is applied here to estimate chorusing activity of male bullfrogs vocalizing against anthropogenic noise. There are significant links between manual counts of the numbers of advertisement call notes and automatically detected notes and two measures of acoustic energy. These data provide a foundation for the use of acoustic energy measures to census vocal activity in different habitats.

Swarm of Sound-to-Light Conversion Devices to Monitor Acoustic Communication Among Small Nocturnal Animals

[abstFig src='/00290001/24.jpg' width='300' text='Sound-to-light conversion devices, Fireflies, in Oki Island and their lighting pattern of frog calling' ] While many robots have been developed to monitor environments, most studies are dedicated to navigation and locomotion and use off-the-shelf sensors. We focus on a novel acoustic device and its processing software, which is designed for a swarm of environmental monitoring robots equipped with the device. This paper demonstrates that a swarm of monitoring devices is useful for biological field studies, i.e., understanding the spatio-temporal structure of acoustic communication among animals in their natural habitat. The following processes are required in monitoring acoustic communication to analyze the natural behavior in the field: (1) working in their habitat, (2) automatically detecting multiple and simultaneous calls, (3) minimizing the effect on the animals and their habitat, and (4) working with various distributions of animals. We present a sound-imaging system using sound-to-light conversion devices called “Fireflies” and their data analysis method that satisfies the requirements. We can easily collect data by placing a swarm (dozens) of Fireflies and record their light intensities using an off-the-shelf video camera. Because each Firefly converts sound in its vicinity into light, we can easily obtain when, how long, and where animals call using temporal analysis of the Firefly light intensities. The device is evaluated in terms of three aspects: volume to light-intensitycharacteristics, battery life through indoor experiments, and water resistance via field experiments. We also present the visualization of a chorus of Japanese tree frogs (<span class=”bold”>Hyla japonica</span>) recorded in their habitat, that is, paddy fields.

Syntactic structure and geographical dialects in the songs of male rock hyraxes

Few mammalian species produce vocalizations that are as richly structured as bird songs, and this greatly restricts the capacity for information transfer. Syntactically complex mammalian vocalizations have been previously studied only in primates, cetaceans and bats. We provide evidence of complex syntactic vocalizations in a small social mammal: the rock hyrax (Procavia capensis: Hyracoidea). We adopted three algorithms, commonly used in genetic sequence analysis and information theory, to examine the order of syllables in hyrax calls. Syntactic dialects exist, and the syntax of hyrax calls is significantly different between different regions in Israel. Call syntax difference is positively correlated to geographical distance over short distances. No correlation is found over long distances, which may reflect limited dispersal movement. These findings indicate that rich syntactic structure is more common in the vocalizations of mammalian taxa than previously thought and suggest the possibility of vocal production learning in the hyrax.

Sound imaging of nocturnal animal calls in their natural habitat

We present a novel method for imaging acoustic communication between nocturnal animals. Investigating the spatio-temporal calling behavior of nocturnal animals, e.g., frogs and crickets, has been difficult because of the need to distinguish many animals' calls in noisy environments without being able to see them. Our method visualizes the spatial and temporal dynamics using dozens of sound-to-light conversion devices (called "Firefly") and an off-the-shelf video camera. The Firefly, which consists of a microphone and a light emitting diode, emits light when it captures nearby sound. Deploying dozens of Fireflies in a target area, we record calls of multiple individuals through the video camera. We conduct two experiments, one indoors and the other in the field, using Japanese tree frogs (Hyla japonica). The indoor experiment demonstrates that our method correctly visualizes Japanese tree frogs' calling behavior. It has confirmed the known behavior; two frogs call synchronously or in anti-phase synchronization. The field experiment (in a rice paddy where Japanese tree frogs live) also visualizes the same calling behavior to confirm anti-phase synchronization in the field. Experimental results confirm that our method can visualize the calling behavior of nocturnal animals in their natural habitat.

Bats and frogs and animals in between: evidence for a common central timing mechanism to extract periodicity pitch

Widely divergent vertebrates share a common central temporal mechanism for representing periodicities of acoustic waveform events. In the auditory nerve, periodicities corresponding to frequencies or rates from about 10 Hz to over 1,000 Hz are extracted from pure tones, from low-frequency complex sounds (e.g., 1st harmonic in bullfrog calls), from mid-frequency sounds with low-frequency modulations (e.g., amplitude modulation rates in cat vocalizations), and from time intervals between high-frequency transients (e.g., pulse-echo delay in bat sonar). Time locking of neuronal responses to periodicities from about 50 ms down to 4 ms or less (about 20-300 Hz) is preserved in the auditory midbrain, where responses are dispersed across many neurons with different onset latencies from 4-5 to 20-50 ms. Midbrain latency distributions are wide enough to encompass two or more repetitions of successive acoustic events, so that responses to multiple, successive periods are ongoing simultaneously in different midbrain neurons. These latencies have a previously unnoticed periodic temporal pattern that determines the specific times for the dispersed on-responses.

Spatial location influences vocal interactions in bullfrog choruses

A multiple sensor array was employed to identify the spatial locations of all vocalizing male bullfrogs (Rana catesbeiana) in five natural choruses. Patterns of vocal activity collected with this array were compared with computer simulations of chorus activity. Bullfrogs were not randomly spaced within choruses, but tended to cluster into closely spaced groups of two to five vocalizing males. There were nonrandom, differing patterns of vocal interactions within clusters of closely spaced males and between different clusters. Bullfrogs located within the same cluster tended to overlap or alternate call notes with two or more other males in that cluster. These near-simultaneous calling bouts produced advertisement calls with more pronounced amplitude modulation than occurred in nonoverlapping notes or calls. Bullfrogs located in different clusters more often alternated entire calls or overlapped only small segments of their calls. They also tended to respond sequentially to calls of their farther neighbors compared to their nearer neighbors. Results of computational analyses showed that the observed patterns of vocal interactions were significantly different than expected based on random activity. The use of a multiple sensor array provides a richer view of the dynamics of choruses than available based on single microphone techniques.

Analyzing acoustic interactions in natural bullfrog (Rana catesbeiana) choruses

Analysis of acoustic interactions between animals in active choruses is complex because of the large numbers of individuals present, their high calling rates, and the considerable numbers of vocalizations that either overlap or show close temporal alternation. The authors describe a methodology for recording chorus activity in bullfrogs (Rana catesbeiana) using multiple, closely spaced acoustic sensors that provide simultaneous estimates of sound direction and sound characteristics. This method provides estimates of location of individual callers, even under conditions of call overlap. This is a useful technique for understanding the complexity of the acoustic scene faced by animals vocalizing in groups.