It had been shown that Barbary macaque (Macaca sylvanus) mothers are able to individually recognise their offspring by its vocal signals, but it remained unclear which acoustical cues may account for such an ability. In order to address this issue from a basic perspective, about 1800 calls of infants and yearlings (N = 10) were investigated. The investigation applied a method of signal analysis which allowed to determine a large number of parameters (N = 84) for each vocal signal. The application of discriminant and cluster analyses provided the following results: (1) Animals differed in almost all call parameters. However, individuals were best identified by specific parameters which formed an individually characteristic parameter set. (2) Those parameters that facilitated the assignment of vocal patterns to a given individual usually were different among individuals. (3) Infants and yearlings achieved the same maximum value of correct assignment. However, infants achieved a reasonable assignment at a much smaller number of call parameters. (4) Cluster analysis of vocalisations revealed that Barbary macaques uttered individual versions of common call types. (5) When the discriminant analysis was rerun on the call clusters, the correct assignment could be improved from 81 % to 94% for infants and from 80.5% to 96% for yearlings. Our findings suggest that Barbary macaque mothers can recognise their offspring by more than one signal cue, and such a strategy may improve the recognition system's robustness against possible distortions caused by the environment. The pronounced differences in vocal patterns of young Barbary macaques may help mothers or other group members to readily learn and recognise the individually specific signal features. The methodological procedures described in this paper provide a powerful tool for an assessment of signal parameters also in other areas of vocal interactions.
I. The antiphonal duets of Cichladusa guttata (Turdidae, Aves) are often accompanied by spectacular wing movements. We have analysed the display, timing, coordination and function of this behaviour using sound films, audio and video recordings of free living and captured birds (Figs 1, 2). 2. The vocal duet contribution of the members of a pair appear well coordinated in timing, normally with no overlap of notes. Specific types of duet notes of the male or the female can release wing beats in the male (response time: 380 ± 80 msec.), but not vice versa. Additionally, note utterance can trigger the wing beat start (reaction time 30 ± 15 msec.) in the same bird, which is normally the male (Figs 4, 5, 6, 7, 8). 3. Wing beat performance is also affected by two further variables: "duet activity" and "wing beat periodicity". The duet activity which changes relatively slowly can be estimated from parameters of the behaviour (vocal density, vocal power, etc.). Auditory stimulation with conspecifics songs raises duet activity and thereby facilitates wing movements. This does not affect the "component of wing beat periodicity", which has a cycle length of 540 msec. (Fig. 3; Tab. i). 4. Normally, both wings are moved synchroneously. Experiments with restricted space on one side of a songster showed that wing beats can be blocked unilaterally. While a space restricted wing remained unmoved the other wing operated in full coordination to the duet program. 5. Due to the different affectations wing "escort" is optional. It occurs regularly, but, with high flexibility. When the variables promoting wing beats result in high values, high wing beats occur, when the values are low, low or no wing beats are observed (Fig. 9). 6. (Wing beats are supposed to support the general function described for the vocal duet contributions.) Specific functions discussed for the non-vocal contributions are: focussing and directing of attention (spectacular display via an additional signal channel; avoidance of habituation (optional and flexible occurrence) ; preventing distance signalling from disturbances in the acoustic channel (noise produced by birds singing simultaneously) ; distance measurement by identification of phase shifting between vocal and non-vocal behavioural rhythms.
Male blackbirds (Turdus merula) were confronted with modified and unmodified playback copies of their own songs presented within the birds normal territories (Fig. 1). The vocal responses of the birds were recorded and correlated to the playback songs in respect to pattern specificity (song matching) and time specificity (distinct latencies). The following results were obtained: 1. The initial element of a song played a key role. If it was erased or masked by a pre-element the birds did not match the playback copies. In contrast erasure of the second element or the rest of the song following the second element did not affect song matching (Figs 3, 4). 2. The second element had an additional effect. The matching preference increased with a shortening of the pause between the first two elements, it decreased when the second element was substituted by an alien element (Figs 2, 3, 4). 3. A series of initial elements (identical or different element types) led to an inhibition of song matching. In contrast, copies including a doubling of the first two or three elements had a supernormal effect (Figs 4, 5). 4. Three types of song responses could be discriminated: in type I responses (latency: L = 0.1 s) the birds did not match the stimulus; in type II responses (L = 0.25-0.8 s) song matching was preferred; type III responses (L > 1 s) showed no time specificity towards onset of playback songs, but still a preference of song matching (Figs 6, 7). 5. In general, late matching responses (type III) were less preferred than rapid ones (type II) but the effects of different song modifications remained the same. 6. In type II responses, the temporal triggering of the reply and the recognition of the heard song class were not completely linked. In contrast to the neglectable influence of the third elements they were well able to trigger a reply (Fig. 9).
In bird species in which territories are defended by both sexes, 'showing up together' is a widespread behaviour of mated individuals. To test whether the spatial proximity of simultaneously appearing mates would have a territorial signal function we presented robin chats (Cossypha heuglini H.) to mounts of conspecifics placed within their territories, Experimental variables were (1) distance between mounts, and (2) number of mounts. Results: Compared to single mount presentations, residents did not show significant differences in agonistic approaches towards the 'intruders', when mounts were positioned with a distance of 3-6 m to each other (spaced presentations). In contrast, when the distance between mounts was 0.3-0.6 m, (close presentations, simulating the close perching of mates) residents approached them significantly less often and less close (Tables 1 and 2). Such approach and attack inhibitions were not stronger when three or four mounts were offered instead of two. Residental pairs spent more time in non-vocal display behaviours than did single residents (Table 3). During playback of conspecific song after having been presented to a mount test, the residents' responses appeared to be influenced by whether they had received a spaced or a close mount test before. Our results have demonstrated the significance of visual pair recognition through clues from spatial proximity of birds. As with the vocal duet displays, the close perching of pair members may function in signalling cooperation during territorial confrontation with conspecifics.
Sounds of human laughter compose quite effectual stimuli that usually facilitate positive responses. We have studied the mechanisms of such effects and investigated how changes in particular acoustical signal parameters affect the evaluation of laughter. Effects were assessed by evaluating self-report data of human subjects who had been exposed to playbacks of experimentally modified laughter material and, for control, also to samples of natural laughter. The modified laughter phrases were generated by first analysing samples of natural laughter, and then using these data to synthesise new laughter material. Analyses of subjects' responses revealed that not only samples that resembled the rhythm of natural laughter (repetition interval of about 0.2 s) were evaluated positively. Instead we found that series with a wide range of repetition intervals were perceived as laughter. The mode of parameter changes within the model series had an additional clear effect on the rating of a given playback sample. Thus, an intra-serial variation of rhythm or pitch received ratings that were closer to ratings of natural laughter (control) than did a stereotyped patterning of stimuli. Especially stimuli with decreases in pitch were well suited to elicit positive reactions. In conclusion, our results showed that features of parameter variations can make human laughter particularly effectual.