Separating overlapping echolocation: An updated method for estimating the number of echolocating animals in high background noise levels.

Much can be learned by investigating the click trains of odontocetes, including estimating the number of vocalizing animals and comparing the acoustic behavior of different individuals. Analyzing such information gathered from groups of echolocating animals in a natural environment is complicated by two main factors: overlapping echolocation produced by multiple animals at the same time, and varying levels of background noise. Starkhammar et al. [(2011a). Biol. Lett. 7(6), 836-839] described an algorithm that measures and compares the frequency spectra of individual clicks to identify groups of clicks produced by different individuals. This study presents an update to this click group separation algorithm that improves performance by comparing multiple click characteristics. There is a focus on reducing error when high background noise levels cause false click detection and recordings are of a limited frequency bandwidth, making the method applicable to a wide range of existing datasets. This method was successfully tested on recordings of free-swimming foraging dolphins with both low and high natural background noise levels. The algorithm can be adjusted via user-set parameters for application to recordings with varying sampling parameters and to species of varying click characteristics, allowing for estimates of the number of echolocating animals in free-swimming groups.

Detailed analysis of two detected overlaying transient components within the echolocation beam of a bottlenose dolphin (Tursiops truncatus).

Dolphin echolocation clicks measured far off-axis contain two time-separated components. Whether these components overlap and appear as a single signal on axis has received little attention. Here, the scaled reassigned spectrogram analysis was used to examine if bottlenose dolphin (Tursiops truncatus) clicks measured near- or on-axis of the echolocation beam contained overlapping components. Across click trains, the number of overlapping components spatially varied within the echolocation beam. Two overlapping components were found to predominantly occur in the upper portion of the beam, whereas the lower portion of the beam predominantly contained a single component. When components overlapped, the trailing component generally had a higher center frequency and arrived less than 5 µs after the leading component. The spatial relationship of components was consistent with previous findings of two vertically distinct beam lobes with separated frequency content. The two components in the upper portion of the beam possibly result from a single transient click propagating through a geometrically dispersive media; specifically, the slower sound speed of the dolphin melon's core slightly delays the more directional, high frequency energy of the click, whereas the less directional, lower frequency energy propagates through more peripheral but higher sound speed portions of the melon.

Objective detection and time-frequency localization of components within transient signals.

An automatic component detection method for overlapping transient pulses in multi-component signals is presented and evaluated. The recently proposed scaled reassignment technique is shown to have the best achievable resolution for closely located Gaussian shaped transient pulses, even in heavy disruptive noise. As a result, the method automatically detects and counts the number of transients, giving the center times and center frequencies of all components with considerable accuracy. The presented method shows great potential for applications in several acoustic research fields, where coinciding Gaussian shaped transients are analyzed. The performance is tested on measured data from a laboratory pulse-echo setup and from a dolphin echolocation signal measured simultaneously at two different locations in the echolocation beam. Since the method requires little user input, it should be easily employed in a variety of research projects.

Bottlenose dolphin communication during a role-specialized group foraging task.

A division of labor with role specialization is defined as individuals specializing in a subtask during repetitions of a group task. While this behavior is ubiquitous among humans, there are only four candidates found among non-eusocial mammals: lions, mice, chimpanzees, and bottlenose dolphins. Bottlenose dolphins in the Cedar Keys, Florida, engage in role specialized "driver-barrier feeding", where a "driver" dolphin herds mullet towards "barrier" dolphins. Thus trapped, the mullet leap out of the water where the dolphins catch them in air. To investigate whether dolphins use acoustic cues or signals to coordinate this behavior, vocalizations were recorded before and during driver-barrier feeding. Results of fine-scale audio and video analysis during 81 events by 7 different driver individuals suggest that barrier animals coordinate movements during these events by cueing on the driver's echolocation. Analysis of dolphin whistle occurrence before driving events versus another foraging technique, which does not involve role specialization, revealed significantly higher whistle production immediately prior to driver-barrier events. Possible whistle functions include signaling motivation, recruiting individuals to participate, and/or behavioral coordination. While the use of cues and signals is common in humans completing role-specialized tasks, this is the first study to investigate the use of vocalizations in the coordination of a role-specialized behavior in a non-human mammal.

Frequency-dependent variation in the two-dimensional beam pattern of an echolocating dolphin.

Recent recordings of dolphin echolocation using a dense array of hydrophones suggest that the echolocation beam is dynamic and can at times consist of a single dominant peak, while at other times it consists of forward projected primary and secondary peaks with similar energy, partially overlapping in space and frequency bandwidth. The spatial separation of the peaks provides an area in front of the dolphin, where the spectral magnitude slopes drop off quickly for certain frequency bands. This region is potentially used to optimize prey localization by directing the maximum pressure slope of the echolocation beam at the target, rather than the maximum pressure peak. The dolphin was able to steer the beam horizontally to a greater extent than previously described. The complex and dynamic sound field generated by the echolocating dolphin may be due to the use of two sets of phonic lips as sound sources, or an unknown complexity in the sound propagation paths or acoustic properties of the forehead tissues of the dolphin.

Separating overlapping click trains originating from multiple individuals in echolocation recordings.

Recordings of the acoustic activity of free-swimming groups of echolocating dolphins increase the likelihood of collecting overlapping click trains, originating from multiple individuals, in the same set of data. In order to evaluate the click properties of each individual based on such recordings it is necessary to identify which clicks originate from which animal. This paper suggests a computationally efficient strategy to separate overlapping click trains originating from multiple free-swimming bottlenose dolphins, enabling echolocation analysis at an individual level on several animals. This technique is based on sequential matching of the frequency spectra of successive clicks. The clicks are grouped together as individual click trains if the correlation coefficients between clicks are higher than a pre-set threshold level. The robustness of the algorithm is tested by adding artificially generated white Gaussian noise and comparing the results with other comparable commonly used methods based on inter-click intervals, centroid frequencies, and amplitude levels. The described method is applicable to a variety of experimental and observational contexts, e.g., those regarding echolocation development of calves, the hypothesized acoustic "etiquette" among dolphins when investigating the same object, and the possible occurrence of eavesdropping in large dolphin pods.

Scaled reassigned spectrograms applied to linear transducer signals.

This study evaluates the applicability of scaled reassigned spectrograms (ReSTS) on ultrasound radio frequency data obtained with a clinical linear array ultrasound transducer. The ReSTS's ability to resolve axially closely spaced objects in a phantom is compared to the classical cross-correlation method with respect to the ability to resolve closely spaced objects as individual reflectors using ultrasound pulses with different lengths. The results show that the axial resolution achieved with the ReSTS was superior to the cross-correlation method when the reflected pulses from two objects overlap. A novel B-mode imaging method, facilitating higher image resolution for distinct reflectors, is proposed.

47-channel burst-mode recording hydrophone system enabling measurements of the dynamic echolocation behavior of free-swimming dolphins.

Detailed echolocation behavior studies on free-swimming dolphins require a measurement system that incorporates multiple hydrophones (often >16). However, the high data flow rate of previous systems has limited their usefulness since only minute long recordings have been manageable. To address this problem, this report describes a 47-channel burst-mode recording hydrophone system that enables highly resolved full beamwidth measurements on multiple free-swimming dolphins during prolonged recording periods. The system facilitates a wide range of biosonar studies since it eliminates the need to restrict the movement of animals in order to study the fine details of their sonar beams.

An echolocation visualization and interface system for dolphin research.

The present study describes the development and testing of a tool for dolphin research. This tool was able to visualize the dolphin echolocation signals as well as function as an acoustically operated "touch screen." The system consisted of a matrix of hydrophones attached to a semitransparent screen, which was lowered in front of an underwater acrylic panel in a dolphin pool. When a dolphin aimed its sonar beam at the screen, the hydrophones measured the received sound pressure levels. These hydrophone signals were then transferred to a computer where they were translated into a video image that corresponds to the dynamic sound pressure variations in the sonar beam and the location of the beam axis. There was a continuous projection of the image back onto the hydrophone matrix screen, giving the dolphin an immediate visual feedback to its sonar output. The system offers a whole new experimental methodology in dolphin research and since it is software-based, many different kinds of scientific questions can be addressed. The results were promising and motivate further development of the system and studies of sonar and cognitive abilities of dolphins.