High-pitch sounds small for domestic dogs: abstract crossmodal correspondences between auditory pitch and visual size.

Humans possess intuitive associations linking certain non-redundant features of stimuli-e.g. high-pitched sounds with small object size (or similarly, low-pitched sounds with large object size). This phenomenon, known as crossmodal correspondence, has been identified in humans across multiple different senses. There is some evidence that non-human animals also form crossmodal correspondences, but the known examples are mostly limited to the associations between the pitch of vocalizations and the size of callers. To investigate whether domestic dogs, like humans, show abstract pitch-size association, we first trained dogs to approach and touch an object after hearing a sound emanating from it. Subsequently, we repeated the task but presented dogs with two objects differing in size, only one of which was playing a sound. The sound was either high or low pitched, thereby creating trials that were either congruent (high pitch from small object; low pitch from large objects) or incongruent (the reverse). We found that dogs reacted faster on congruent versus incongruent trials. Moreover, their accuracy was at chance on incongruent trials, but significantly above chance for congruent trials. Our results suggest that non-human animals show abstract pitch sound correspondences, indicating these correspondences may not be uniquely human but rather a sensory processing feature shared by other species.

Audio-visual crossmodal correspondences in domestic dogs (Canis familiaris).

Crossmodal correspondences are intuitively held relationships between non-redundant features of a stimulus, such as auditory pitch and visual illumination. While a number of correspondences have been identified in humans to date (e.g. high pitch is intuitively felt to be luminant, angular and elevated in space), their evolutionary and developmental origins remain unclear. Here, we investigated the existence of audio-visual crossmodal correspondences in domestic dogs, and specifically, the known human correspondence in which high auditory pitch is associated with elevated spatial position. In an audio-visual attention task, we found that dogs engaged more with audio-visual stimuli that were congruent with human intuitions (high auditory pitch paired with a spatially elevated visual stimulus) compared to incongruent (low pitch paired with elevated visual stimulus). This result suggests that crossmodal correspondences are not a uniquely human or primate phenomenon and they cannot easily be dismissed as merely lexical conventions (i.e. matching 'high' pitch with 'high' elevation).

Vocal tract modelling in fallow deer: are male groans nasalized?

Males of several species of deer have a descended and mobile larynx, resulting in an unusually long vocal tract, which can be further extended by lowering the larynx during call production. Formant frequencies are lowered as the vocal tract is extended, as predicted when approximating the vocal tract as a uniform quarter wavelength resonator. However, formant frequencies in polygynous deer follow uneven distribution patterns, indicating that the vocal tract configuration may in fact be rather complex. We CT-scanned the head and neck region of two adult male fallow deer specimens with artificially extended vocal tracts and measured the cross-sectional areas of the supra-laryngeal vocal tract along the oral and nasal tracts. The CT data were then used to predict the resonances produced by three possible configurations, including the oral vocal tract only, the nasal vocal tract only, or combining the two. We found that the area functions from the combined oral and nasal vocal tracts produced resonances more closely matching the formant pattern and scaling observed in fallow deer groans than those predicted by the area functions of the oral vocal tract only or of the nasal vocal tract only. This indicates that the nasal and oral vocal tracts are both simultaneously involved in the production of a non-human mammal vocalization, and suggests that the potential for nasalization in putative oral loud calls should be carefully considered.

Evidence of biphonation and source-filter interactions in the bugles of male North American wapiti (Cervus canadensis).

With an average male body mass of 320 kg, the wapiti, ITALIC! Cervus canadensis, is the largest extant species of Old World deer (Cervinae). Despite this large body size, male wapiti produce whistle-like sexual calls called bugles characterised by an extremely high fundamental frequency. Investigations of the biometry and physiology of the male wapiti's relatively large larynx have so far failed to account for the production of such a high fundamental frequency. Our examination of spectrograms of male bugles suggested that the complex harmonic structure is best explained by a dual-source model (biphonation), with one source oscillating at a mean of 145 Hz (F0) and the other oscillating independently at an average of 1426 Hz (G0). A combination of anatomical investigations and acoustical modelling indicated that the F0 of male bugles is consistent with the vocal fold dimensions reported in this species, whereas the secondary, much higher source at G0 is more consistent with an aerodynamic whistle produced as air flows rapidly through a narrow supraglottic constriction. We also report a possible interaction between the higher frequency G0 and vocal tract resonances, as G0 transiently locks onto individual formants as the vocal tract is extended. We speculate that male wapiti have evolved such a dual-source phonation to advertise body size at close range (with a relatively low-frequency F0 providing a dense spectrum to highlight size-related information contained in formants) while simultaneously advertising their presence over greater distances using the very high-amplitude G0 whistle component.

The descended larynx is not uniquely human.

Morphological modifications of vocal anatomy are widespread among vertebrates, and the investigation of the physiological mechanisms and adaptive functions of such variants is an important focus of research into the evolution of communication. The "descended larynx" of adult humans has traditionally been considered unique to our species, representing an adaptation for articulate speech, and debate concerning the position of the larynx in extinct hominids assumes that a lowered larynx is diagnostic of speech and language. Here, we use bioacoustic analyses of vocalizing animals, together with anatomical analyses of functional morphology, to document descended larynges in red and fallow deer. The resting position of the larynx in males of these species is similar to that in humans, and, during roaring, red-deer stags lower the larynx even further, to the sternum. These findings indicate that laryngeal descent is not uniquely human and has evolved at least twice in independent lineages. We suggest that laryngeal descent serves to elongate the vocal tract, allowing callers to exaggerate their perceived body size by decreasing vocal-tract resonant frequencies. Vocal-tract elongation is common in birds and is probably present in additional mammals. Size exaggeration provides a non-linguistic alternative hypothesis for the descent of the larynx in human evolution.

Spectral acoustic structure of barking in roe deer (Capreolus capreolus). Sex-, age- and individual-related variations.

In roe deer, barking is a loud call commonly given by males and females during inter- or intraspecific interactions. The analysis of a set of 19 spectral variables computed on 560 calls revealed significant variation between sexes, individuals, and probably age classes. Discriminant analysis predicted the sex of an individual with a 93.5% probability from a small portion of the bark frequency range. Among six males, a linear combination of six variables predicted the identity of the barking individual with a 70% probability. These sexual and individual differences provide the potential for social recognition from vocalizations. These results are consistent with the hypothesis that barking in roe deer may allow remote signalling of presence, location and identity, and play an important role in the territorial system of this species.

Contexts and possible functions of barking in roe deer.

We studied the barking behaviour of free-ranging roe deer, Capreolus capreolus, in response to disturbance provoked by a human observer and in response to the playback of recorded barks. Three alternative functions of this behaviour were hypothesized: barking is an alarm call, a pursuit-deterrent call or a territorial call. Our observational data showed that, in the presence of a source of disturbance, solitary individuals barked more frequently than deer in groups, suggesting that barking does not serve to warn conspecifics of potential danger, but rather to inform any potential predator that it has been identified. The frequencies of both barking and counterbarking (barking of a second deer in response to the barks of an initiator) were inversely correlated with ambient luminosity, probably because the assessment of danger is more difficult when visibility is low. Males barked more frequently than females when disturbed. Moreover, when we played back a series of barks from within a buck's territory, this provoked counterbarking or aggressive behaviours rather than flight. Older bucks responded more frequently to playbacks than younger bucks. We suggest that while barking may initially have evolved as a signal to deter predator pursuit, it could play an important, secondary role in the territorial system of this species. Copyright 1999 The Association for the Study of Animal Behaviour.

Artificial neural networks as a classification method in the behavioural sciences.

The classification and recognition of individual characteristics and behaviours constitute a preliminary step and is an important objective in the behavioural sciences. Current statistical methods do not always give satisfactory results. To improve performance in this area, we present a methodology based on one of the principles of artificial neural networks: the backpropagation gradient. After summarizing the theoretical construction of the model, we describe how to parameterize a neural network using the example of the individual recognition of vocalizations of four fallow deer (Dama dama). With 100% recognition and 90% prediction success, the results are very promising.