David A. Helweg

GEOGRAPHIC VARIATION IN SOUTH PACIFIC HUMPBACK WHALE SONGS

Every winter, (male) humpback whales (Megaptera novaeangliae) produce long complex songs. Song content is dynamic and singers incorporate changes as they occur, thus song is shared through cultural transmission. We compared songs recorded in winter migratory termini in Tonga, New Caledonia, Eastern Australia, and on migration paths off Eastern Australia and New Zealand, in the winter of 1994. Seven themes were shared by all regions, with an additional two themes shared by all but Tonga. Differences in regional variants were most pronounced between Tongan and Eastern Australian song. New Caledonian and Kaikouran song were more similar to songs from Eastern Australia rather than Tonga. These regional differences were stable across the season. The results suggest some migratory exchange among widely separate wintering regions of Area V, consistent with tag recovery data, but the time and location at which song sharing occurs remains speculative.

Classification of dolphin echolocation clicks by energy and frequency distributions

Dolphins demonstrate an adaptive control over echolocation click production, but little is known of the manner or degree with which control is exercised. Echolocation clicks (N approximately 30,000) were collected from an Atlantic bottlenose dolphin (Tursiops truncatus) performing object discrimination tasks in order to investigate differential click production. Seven categories of clicks were identified using the spectral conformation and relative position of -3 and -10 dB peaks. A counterpropagation network utilizing 16 inputs, 50 hidden units, and 8 output units was trained to classify clicks using the same spectral variables. The network classified novel clicks with 92% success. Additional echolocation clicks (N > 24,000) from two other dolphins were submitted to the network for classification. Classified echolocation clicks were analyzed for animal specific differences, changes in predominant click type within click trains, and task-related specificity. Differences in animal and task performance may influence click type and click train length.

Cultural change in the songs of humpback whales (Megaptera novaeangliae) from Tonga

Some humpback whales migrate annually from Antarctic feeding grounds to the seas around the Tongan Islands to give birth and mate. The Tongan humpbacks are considered part of Southern Hemisphere Group V that splits during migration, some swimming to Eastern Australia and others to various Polynesian Islands. During this time long complex songs are produced. The song is thought to be a male breeding display and may serve either as an intra-sexual or an inter-sexual signal or both. It is in a constant state of change that occurs every season. Since these changes are directional they cannot be described by drift, and singers incorporate changes as they occur, thus song must be shared through cultural transmission. This investigation describes the cultural changes that occurred in 158 songs recorded from Tongan humpbacks through the 1990s. The rate of change differed within years, some themes were retained for as much as five years and others were lost after only two years. The farther apart the years the less similar are the songs, as in the humpback songs of the Northern Hemisphere. The largest number of changes seems to have occurred in the early 1990s where all themes seemed to have been lost and new ones originated. What initiates these changes remains speculative, but we assess some hypotheses in relation to humpback whale behaviour and cultural transmission in avian song.

Acoustic identification of female Steller sea lions (Eumetopias jubatus)

Steller sea lion (Eumetopias jubatus) mothers and pups establish and maintain contact with individually distinctive vocalizations. Our objective was to develop a robust neural network to classify females based on their mother-pup contact calls. We catalogued 573 contact calls from 25 females in 1998 and 1323 calls from 46 females in 1999. From this database, a subset of 26 females with sufficient samples of calls was selected for further study. Each female was identified visually by marking patterns, which provided the verification for acoustic identification. Average logarithmic spectra were extracted for each call, and standardized training and generalization datasets created for the neural network classifier. A family of backpropagation networks was generated to assess relative contribution of spectral input bandwidth, frequency resolution, and network architectural variables to classification accuracy. The network with best overall generalization accuracy (71%) used an input representation of 0-3 kHz of bandwidth at 10.77 Hz/bin frequency resolution, and a 2:1 hidden:output layer neural ratio. The network was analyzed to reveal which portions of the call spectra were most influential for identification of each female. Acoustical identification of distinctive female acoustic signatures has several potentially important conservation applications for this endangered species, such as rapid survey of females present on a rookery.

Against the humpback whale sonar hypothesis

A rebuttal is presented to the article of L.N. Frazer and E. Mercado (ibid., vol. 25, pp. 160-182, 2000), who presented a sonar model for humpback whale song, is presented. This rebuttal considers the noise-limited form of the sonar equation, current understanding of humpback whale behavior and the characteristics of humpback whale songs, along with arguments from an evolutionary perspective. Arguments from all of these different aspects do not support the model of Frazer and Mercado.

Recognition of aspect-dependent three-dimensional objects by an echolocating Atlantic bottlenose dolphin

We examined the ability of a bottlenose dolphin (Tursiops truncatus) to recognize aspect-dependent objects using echolocation. An aspect-dependent object such as a cube produces acoustically different echoes at different angles relative to the echolocation signal. The dolphin recognized the objects even though the objects were free to rotate and sway. A linear discriminant analysis and nearest centroid classifier could classify the objects using average amplitude, center frequency, and bandwidth of object echoes. The results show that dolphins can use varying acoustic properties to recognize constant objects and suggest that aspect-independent representations may be formed by combining information gleaned from multiple echoes.

Acoustic basis for recognition of aspect-dependent three-dimensional targets by an echolocating bottlenose dolphin.

The relationships between acoustic features of target echoes and the cognitive representations of the target formed by an echolocating dolphin will influence the ease with which the dolphin can recognize a target. A blindfolded Atlantic bottlenose dolphin (Tursiops truncatus) learned to match aspect-dependent three-dimensional targets (such as a cube) at haphazard orientations, although with some difficulty. This task may have been difficult because aspect-dependent targets produce different echoes at different orientations, which required the dolphin to have some capability for object constancy across changes in echo characteristics. Significant target-related differences in echo amplitude, rms bandwidth, and distributions of interhighlight intervals were observed among echoes collected when the dolphin was performing the task. Targets could be classified using a combination of energy flux density and rms bandwidth by a linear discriminant analysis and a nearest centroid classifier. Neither statistical model could classify targets without amplitude information, but the highest accuracy required spectral information as well. This suggests that the dolphin recognized the targets using a multidimensional representation containing amplitude and spectral information and that dolphins can form stable representations of targets regardless of orientation based on varying sensory properties.

Multiecho processing by an echolocating dolphin

Bottlenose dolphins (Tursiops truncatus) use short, wideband pulses for echolocation. Individual waveforms have high-range resolution capability but are relatively insensitive to range rate. Signal-to-noise ratio (SNR) is not greatly improved by pulse compression because each waveform has small time-bandwidth product. The dolphin, however, often uses many pulses to interrogate a target, and could use multipulse processing to combine the resulting echoes. Multipulse processing could mitigate the small SNR improvement from pulse compression, and could greatly improve range-rate estimation, moving target indication, range tracking, and acoustic imaging. All these hypothetical capabilities depend upon the animals ability to combine multiple echoes for detection and/or estimation. An experiment to test multiecho processing in a dolphin measured detection of a stationary target when the number N of available target echoes was increased, using synthetic echoes. The SNR required for detection decreased as the number of available echoes increased, as expected for multiecho processing. A receiver that sums binary-quantized data samples from multiple echoes closely models the N dependence of the SNR required by the dolphin. Such a receiver has distribution-tolerant (nonparametric) properties that make it robust in environments with nonstationary and/or non-Gaussian noise, such as the pulses created by snapping shrimp.

Using a biomimetric neural net to model dolphin echolocation

A biomimetic neural network was used to model the ability of a bottle nosed dolphin to recognize aspect-dependent geometric objects. Each echo train was recorded and an Integrator Gateway Network (IGN) was trained to discriminate among the objects using spectra of the object echoes. The IGN classifies objects using an average-like sum of the spectra from successive echoes. However, combining echoes may reduce classification accuracy if the spectra vary from echo to echo. The dolphin and the IGN learned to recognize the geometric objects, even though orientation was free to vary. The process of recognition using cumulated echoes was robust with respect to nonstationary raw input. The results were interpreted as evidence for the formation of aspect-independent representations of the objects.<<ETX>>

Mapping acoustic sensitivity about the dolphin’s head: A look at the peripheral hearing system

The hypothesis that echolocating dolphins best receive acoustic signals over the pan bones of the lower jaw is widely accepted. Studies in echolocation and hearing have assumed that those areas serve as the dolphin’s peripheral hearing system. The research that established that model, however, does not exclude other potential sound reception sites and suggests that additional areas of the head may be acoustically sensitive and perhaps frequency dependent. Using jawphones, relative hearing thresholds for representative frequencies (10, 30, 60, and 90 kHz) were behaviorally measured at over 40 sites on a dolphin’s head. Iso‐sensitivity curves were constructed and projected onto the image of a dolphin’s head based on these measurements. The results suggest sensitivity to high frequency along the lower jaw with greater sensitivity forward of the pan bone area, sensitivity to low frequency around the external auditory meatus, and an acoustic asymmetry with greater sensitivity favoring the right side of the hea...