INDIVIDUALITY IN THE VOICE OF FUR SEAL FEMALES: AN ANALYSIS STUDY OF THE PUP ATTRACTION CALL IN ARCTOCEPHALUS TROPICALIS

Like most otariids species, the Subantarctic fur seal breeds on land in large, dense colonies. Pups are confronted by the long and repetitive absences of their mother throughout lactation. At each mother's return, pups have to find her among several hundreds of congeners. This recognition process mainly relies on acoustic signals. We performed an acoustic analysis on 125 calls from 20 females recorded during the 1999–2000 breeding season on Amsterdam Island (Indian Ocean). Ten variables were measured in both temporal and frequency domains. To find the acoustic parameters supporting individual signature, we assessed the differences between individuals using Kruskall-Wallis univariate analysis of variance. For each variable, we also calculated the potential of individuality coding (PIC) as the ratio between the between-individual coefficient of variation and the mean value of the within-individual coefficients of variation. We found that the frequency spectrum, the characteristics of the frequency modulation of the initial and middle part of the call and the call duration exhibit an important individual stereotypy (PIC values ranging between 1.5 and 3), whereas features relative to amplitude and the frequency modulation of the final part of the call are weakly individualized (PIC values between 1 and 1.2).     

Amplitude modulation is a major marker of individual signature in lamb bleats

The source-filter theory of vocal production supports the idea that acoustic signatures are preferentially coded by the fundamental frequency (source-induced variability) and the distribution of energy among the frequency spectrum (filter-induced variability). By investigating the acoustic parameters supporting individuality in lamb bleats, a vocalization which mediates recognition by ewes, here we show that amplitude modulation – an acoustic feature largely independent of the shape of the acoustic tract – can also be an important cue defining an individual vocal signature. Female sheep (Ovis aries) show an acoustic preference for their own lamb. Although playback experiments have shown that this preference is established soon after birth and relies on a unique vocal signature contained in the bleats of the lamb, the physical parameters that encode this individual identity remained poorly identified. We recorded 152 bleats from 13 fifteen-day-old lambs and analyzed their acoustic structure with four complementary statistical methods (ANOVA, potential for individual identity coding PIC, entropy calculation 2^Hs, discriminant function analysis DFA). Although there were slight differences in the acoustic parameters identified by the four methods, it remains that the individual signature relies on both the temporal and frequency domains. The coding of the identity is thus multi-parametric and integrates modulation of amplitude and energy parameters. Specifically, the contribution of the amplitude modulation is important, together with the fundamental frequency F ₀ and the distribution of energy in the frequency spectrum.     

Sex recognition in brown skuas: do acoustic signals matter?

Bird vocalisations are often essential for sex recognition, especially in species that show little morphological sex dimorphism. Brown skuas (Catharacta antarctica lonnbergi), which exhibit uniform plumage across both sexes, emit three main calls: the long call, the alarm call and the contact call. We tested the potential for sex recognition in brown skua calls of 42 genetically sexed individuals by analysing 8–12 acoustic parameters in the temporal and frequency domains of each call type. For every call type, we failed to find sex differences in any of the acoustic parameters measured. Stepwise discriminant function analysis (DFA) revealed that sexes cannot be unambiguously classified, with increasing uncertainty of correct classification from contact calls to long calls to alarm calls. Consequently, acoustic signalling is probably not the key mechanism for sex recognition in brown skuas.     

Individuality of contact calls in the Greater Flamingo Phoenicopterus ruber and the problem of background noise in a colony

Acoustic communication has great importance in social relationships and especially in individual recognition in the Greater Flamingo Phoenicopterus ruber. However, it is constrained by the noisy environment of the colony. I performed an analysis of both the temporal and frequency patterns of contact calls and showed that the use of both is essential for discrimination between individuals. I made a search for the acoustic parameters which are used by the flamingos. It appears that the call duration, the slow amplitude modulation, the spectrum bandwidth and the repartition of energy among harmonics exhibit individuality. Recognition between the birds must be based on a multiparametric analysis, taking into account both spectral and temporal features of the calls. Indeed, no single parameter was sufficient to discriminate between individual birds used in my research. In the noisy environment of the colony, the transmission of individuality in a message is improved by this multiparametric coding, which can be considered as an adaptation to these extreme acoustic conditions. Moreover, the study revealed numerous acoustic convergences with another colonial bird species, the Emperor Penguin Aptenodytes forsteri , and both species seemed to use the same method to encode individuality in their vocalizations.     

Call-based species recognition in black-capped chickadees

Species recognition is essential for efficient communication between conspecifics. For this to occur, species information must be unambiguously encoded in the repertoire of each species’ vocalizations. Until now, the study of species recognition in songbirds has been focused mainly on male songs and male territorial behaviour. Species recognition of other learned vocalizations, such as calls, have not been explored, and could prove useful as calls are used in a wider range of contexts. Here, we present an experimental field study investigating the coding of species information in a learned vocalization, the ‘chick-a-dee’ call of the black-capped chickadee (Poecile atricapillus). By modifying natural calls in both temporal and spectral domains and by observing the vocal responses of black-capped chickadees following the playback of these modified calls, we demonstrate that species recognition in chickadees relies on several acoustic features including syntax, frequency modulation, amplitude modulation, and to a lesser extent, call rhythmicity and frequency range.     

Significance of temporal and spectral acoustic cues for sexual recognition in Xenopus laevis

As in many anurans, males of the totally aquatic species, Xenopus laevis, advertise their sexual receptivity using vocalizations. Unusually for anurans, X. laevis females also advertise producing a fertility call that results in courtship duets between partners. Although all X. laevis calls consist of repetitive click trains, male and female calls exhibit sex-specific acoustic features that might convey sexual identity. We tested the significance of the carrier frequency and the temporal pattern of calls using underwater playback experiments in which modified calls were used to evoke vocal responses in males. Since males respond differently to male and female calls, the modification of a key component of sexual identity in calls should change the male's response. We found that a female-like slow call rhythm triggers more vocal activity than a male-like fast rhythm. A call containing both a female-like temporal pattern and a female-like carrier frequency elicits higher levels of courtship display than either feature alone. In contrast, a male-like temporal pattern is sufficient to trigger typical male-male encounter vocalizations regardless of spectral cues. Thus, our evidence supports a role for temporal acoustic cues in sexual identity recognition and for spectral acoustic cues in conveying female attractiveness in X. laevis.     

A comparison of individual distinctiveness in three vocalizations of the dwarf mongoose (Helogale parvula)

Individual specificity can be found in the vocalizations of many avian and mammalian species. However, it is often difficult to determine whether these vocal cues to identity rise from “unselected” individual differences in vocal morphology or whether they have been accentuated by selection for the purposes of advertising caller identity. By comparing the level of acoustic individuality of different vocalizations within the repertoire of a single species, it is possible to ascertain whether selection for individual recognition has modified the vocal cues to identity in particular contexts. We used discriminant function analyses to determine the level of accuracy with which calls could be classified to the correct individual caller, for three dwarf mongoose (Helogale parvula) vocalizations: contact, snake, and isolation calls. These calls were similar in acoustic structure but divergent in context and function. We found that all three call types showed individual specificity but levels varied with call type (increasing from snake to contact to isolation call). The individual distinctiveness of each call type appeared to be directly related to the degree of benefit that signalers were likely to accrue from advertising their identity within that call context. We conclude that dwarf mongoose signalers have undergone selection to facilitate vocal individual recognition, particularly in relation to the species’ isolation call.     

Individuality in the Voice of the Emperor Penguin Aptenodytes forsteri: Adaptation to a Noisy Environment

The extreme coloniality in emperor penguins (Aptenodytes forsteri) demands that (1) individual recognition by the mutual display call must be supremely adapted to the total absence of nest sites — landmarks that would facilitate partner reunion, and (2) birds must transmit a precise message through an obstructed and noisy environment. A computerized sound analysis and subsequent data were submitted to univariate and multivariate statistics to determine individual-specific cues of the call. Temporal patterning of syllables, variables of the power spectrum (frequency and timbre features) and variables of a beat generated by two acoustic sources were analyzed (58 calls from 7 animals). Number and duration of syllables, fundamental frequencies and period of the beat were highly individual, with inter- significantly greater then intra-individual variation, enabling correct classification of the 58 calls. The respective relevance of temporal and spectral cues for individual recognition is discussed. In the acoustically hostile and noisy environment, beat was peculiarly emphasized, as it allowed great precision in the production of an amplitude modulation that was unlikely to be degraded by the environment.     

Biphonation May Function to Enhance Individual Recognition in the Dhole, Cuon alpinus

Biphonation (two independent fundamental frequencies in a call spectrum) represents one of the most widespread nonlinear phenomena in mammalian vocalizations. Recently, the structure of biphonations was described in detail; however, their functions are poorly understood. For the dhole (Cuon alpinus), biphonic calls represent a prominent feature of vocal activity. In this species, the biphonic call is composed of two frequency components – the high‐frequency squeak and the low‐frequency yap, which also occur alone as separate calls. In this study, we test the hypothesis that the complication of call structure, resulting from the joining of these calls into the biphonic yap–squeak may enhance the potential for individual recognition in the dhole. We randomly selected for analysis 30 high‐frequency squeaks, 30 low‐frequency yaps and 30 biphonic yap–squeaks per animal from five subadult captive dholes (450 calls in total). Discriminant analysis, based on 10 squeak parameter values, showed 80.7% correct assignment to a predicted individual. For 10 yap parameters, the correct assignment was only 44.7%. However, the analysis based on 10 parameters of the biphonic yap–squeak, selected as best contributing to discrimination, showed 96.7% correct assignment to a predicted individual. The results provide strong support for the hypothesis tested showing that the joining of two independent calls into a common vocalization may function to enhance individual recognition in the dhole.     

Individual Distinctiveness in Call Types of Wild Western Female Gorillas

Individually distinct vocalizations play an important role in animal communication, allowing call recipients to respond differentially based on caller identity. However, which of the many calls in a species'' repertoire should have more acoustic variability and be more recognizable is less apparent. One proposed hypothesis is that calls used over long distances should be more distinct because visual cues are not available to identify the caller. An alternative hypothesis proposes that close calls should be more recognizable because of their importance in social interactions. To examine which hypothesis garners more support, the acoustic variation and individual distinctiveness of eight call types of six wild western gorilla (Gorilla gorilla) females were investigated. Acoustic recordings of gorilla calls were collected at the Mondika Research Center (Republic of Congo). Acoustic variability was high in all gorilla calls. Similar high inter-individual variation and potential for identity coding (PIC) was found for all call types. Discriminant function analyses confirmed that all call types were individually distinct (although for call types with lowest sample size - hum, grumble and scream - this result cannot be generalized), suggesting that neither the distance at which communication occurs nor the call social function alone can explain the evolution of identity signaling in western gorilla communication.     

Recovering phylogenetic signal from frog mating calls

Goicoechea, N., De La Riva, I. & Padial, J. M. (2010). Recovering phylogenetic signal from frog mating calls. —Zoologica Scripta, 39, 411–154. Few studies have tried to analyse the phylogenetic information contained in frog mating calls. While some of those studies suggest that sexual selection deletes any phylogenetic signal, others indicate that frog calls do retain phylogenetic informative characters. Discordant results can be the outcome of disparate rates of character evolution and evolutionary plasticity of call characters in different groups of frogs, but also the result of applying different coding methods. No study to date has compared the relative performance of different coding methods in detecting phylogenetic signal in calls, hampering thus potential consilience between previous results. In this study, we analyse the strength of phylogenetic signal in 10 mating call characters of 11 related species of frogs belonging to three genera of Andean and Amazonian frogs (Anura: Terrarana: Strabomantidae). We use six quantitative characters (number of notes per call, note length, call length, number of pulses per note, fundamental frequency and dominant frequency) and four qualitative ones (presence/absence of: pseudopulses, frequency modulation in notes, amplitude modulation in notes and amplitude modulation in pulses). We code quantitative characters using four different coding and scaling methods: (i) gap‐coding, (ii) fixed‐scale, (iii) step‐matrix gap‐weighting with between‐characters scaling, and (iv) step‐matrix gap‐weighting with between‐states scaling. All four coding methods indicate that frog calls contain phylogenetic information. These results suggest that divergent selection on frog mating calls may not always be strong enough to eliminate phylogenetic signal. However, coding methods strongly affect the amount of recoverable information. Step‐matrix gap‐weighting with between‐characters scaling and gap‐coding are suggested as the best methods available for coding quantitative characters of frog calls. Also, our results indicate that the arbitrariness in selecting character states and the method for scaling transitions weights, rather than the number of character states, is what potentially biases phylogenetic analyses with quantitative characters.     

Long-distance communication of acoustic cues to social identity in African elephants

Research on long-distance vocal communication in mammals has tended to focus on the maximum distances over which a vocal signal might be physically detectable. For example, because elephants and some whales communicate using infrasonic calls, and low frequencies are particularly resilient to attenuation, it has often been assumed that these species can communicate over very long distances. However, a wide range of acoustic characteristics typically carry information on individual identity in mammalian calls, and frequency components crucial for social recognition could be distorted or lost as distance from the source increases. We used long-distance playback experiments to show that female African elephants, Loxodonta africana, can recognize a contact call as belonging to a family or bond group member over distances of 2.5 km, but that recognition is more usually achieved over distances of 1-1.5 km. We analysed female contact calls to distinguish source- and filter-related vocal characteristics that have the potential to code individual identity, and rerecorded contact calls 0.5-3.0 km from the loudspeaker to determine how different frequencies persist with distance. Our analyses suggest that the most important frequency components for long-distance communication of social identity may be well above the infrasonic range. When frequency components around 115 Hz become immersed in background noise, once propagation distances exceed 1 km, abilities for long-distance social recognition become limited. Our results indicate that the possession of an unusually long vocal filter, which appears to incorporate the trunk, may be a more important attribute for long-distance signalling in female African elephants than the ability to produce infrasound. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

The Complex Acoustic Design of an Advertisement Call in Male Mouse Lemurs (Microcebus murinus,Prosimii, Primates) and Sources of its Variation

During the breeding season sexually active male mouse lemurs produce a trill call used for advertising. The trill call displays a variable, high pitched, complex frequency and amplitude modulated acoustical structure and is given with a high sound intensity. The different sources of its variation in structure and usage have to be determined to understand its complex acoustical design and function. We investigated the vocal morphology of the trill call of males with known age and weight as well as the behavioral context, in order to evaluate sources of its variation. Cluster and discriminant analyses revealed that the structure of trill calls showed a high degree of individual stereotypy in contrast to a high interindividual variability. A set of three distinct variables (fundamental frequency of call onset, bandwidth of 3rd frequency modulation and end frequency of 7th modulation) was sufficient to identify individual senders with almost 90% probability. The structure of the trill call of adult males was independent of age, weight and context and was stable over at least one breeding season. Its individually distinct call structure and use in relation to social status provide the potential for individual recognition and for assessing the “quality” of a male.     

Variation in pika (Ochotona collaris,O. princeps) vocalizations within and between populations

Understanding geographic call variation can resolve evolutionary and behavioural questions, yet the factors influencing divergent acoustic signals remain poorly understood in mammals. We explored call variation between collared pikas in Yukon and Alaska and American pikas in Alberta, and between individuals within a population of collared pikas. Classification trees were used to determine the extent of call divergence between populations and the elements of calls driving these differences. Pika populations had significant differences in call structure, and individual pikas were classified to their correct populations with up to 94% accuracy. To investigate possible mechanisms responsible for interspecific variability, we tested the acoustic adaptation hypothesis by using a playback experiment to explore whether American and collared pika calls transmit with less degradation across their own species' habitat than the habitat of their congener. We found no support for the acoustic adaptation hypothesis. Geographic call variation in these two species of pikas likely reflects genetic divergence, and may be a result of separate evolutionary histories. We calculated the potential for individual coding for both time and frequency measurements of calls. High frequency harmonics showed greater between‐ than within‐individual variation, and may act as sources of information regarding individual identity.     

Mother-lamb acoustic recognition in sheep: a frequency coding

Ewes of the domestic sheep ( Ovis aries ) display selective maternal investment by restricting care to their own offspring and rejecting alien young. This trait relies on individual recognition processes between ewes and lambs. Whereas identification at the udder is only olfactory, distance recognition is performed through visual and acoustic cues. We studied the effectiveness and modalities of mutual acoustic recognition between ewes and lambs by spectrographic analysis of their vocal signatures and by playbacks of modified calls in the field. Our results show that ewes and their lambs can recognize each other based solely on their calls. The coding of identity within the vocal signatures, previously unknown in sheep, is similar in lamb and ewe: it uses the mean frequency and the spectral energy distribution of the call, namely the timbre of the call. These results point out a simple signature system in sheep that uses only the frequency domain. This engenders a signal with low information content, as opposed to some highly social birds and mammal species that may integrate information both in the temporal and spectral domains. The simplicity of this system is linked to the roles played by vision and olfaction that corroborate the information brought by the vocal signature.     

Individual recognition of contact calls by pygmy marmosets

Individual recognition by voice has been frequently assumed to be an important phenomenon in the social species. The present study on pygmy marmosets (Cebuella pygmaea) demonstrated that there are distinctive acoustic features in each of two contact calls by which individuals could be distinguished from each other. Individual pygmy marmosets responded differently depending on which type of contact call was given and on the identity of the vocalizing animal. Playbacks of individual calls through hidden speakers produced some evidence for differential individual responses, but only when they originated from familiar locations. These results indicate that pygmy marmosets recognize each other on the basis of their contact calls.     

Advertising individual identity by mother and adolescent contact calls in Siberian wapiti Cervus elaphus sibiricus

Individualistic contact calls facilitate mother‐offspring reunion after separation. However, in many mammals, both the acoustic structure and individuality of contact calls differ between mother and young. In contrast, in Siberian wapiti Cervus elaphus sibiricus, contact calls are similar in the acoustics between mother and young, whereas effects of this similarity on vocal individuality were not investigated. In this study, we analyzed acoustic differences between closed‐mouth (nasal) and open‐mouth (oral) contact calls and examined individuality of the most usual oral calls of 19 Siberian wapiti (9 hinds and 10 5–6‐month adolescents) emitted in response to mother‐offspring separation. In the oral calls, the values of frequency and power variables were higher than in the nasal calls. Calls of hinds and adolescents did not differ by the maximum fundamental frequency and duration, whereas the peak frequency was higher in the young. Discriminant function analysis (DFA) based on 11 acoustic variables of oral calls accurately classified to individual 92.5% of hind calls and 96.9% of adolescent calls (chi‐square test for differences between hinds and adolescents, p = 0.19). Variables mainly contributing to vocal identity (duration, start, and maximum fundamental frequency) were the same in calls of mothers and adolescents. We conclude that similarities in the acoustics calls of mothers and adolescents mean that they do not differ in their potential for encoding individual identity, suggesting a mutual process of mother‐offspring vocal recognition in Siberian wapiti.     

A hitherto unknown hybridization between Chilean and South Polar skua

 Three skua specimens with a colouration pattern of the Chilean skua were observed at Potter Peninsula, King George Island, Antarctica. Two of these were breeding birds. However, they showed a pattern of mitochondrial DNA typical for South Polar skua. These birds thus represent hybrids originating from male Chilean skua. The third bird could not be caught. It appeared to be indistinguishable from Chilean skua. Received: 9 October 1995/Accepted: 17 March 1996     

Selective phonotaxis in <Emphasis Type="Italic">Neoconocephalus nebrascensis</Emphasis> (Orthoptera: Tettigoniidae): call recognition at two temporal scales

The calls of many Orthopteran species are comprised of a simple trill of pulses, the temporal pattern of which is important for call recognition. Male Neoconocephalus nebrascensis produce pulses with a temporal structure typical for the genus. However, they modify this pattern by grouping their pulses into verses, thereby creating a higher order temporal structure. The importance of the pulse pattern and verse structure for call recognition in N. nebrascensis was determined using a walking compensator. Females required the conspecific pulse pattern for call recognition, responding only when the intervals between pulses were short or absent. Females also required the verse structure for call recognition, and recognized the verse structure only when the amplitude modulation depth between verses and pauses exceeded 18 dB. We discuss that the verse recognition mechanism is a derived trait adapted for pre-mating isolation. We hypothesize that the unusually large amplitude modulation required for verse recognition forces males to synchronize their calls in order to preserve an attractive pattern. Call synchrony appears to be the outcome of cooperation, rather than competition, in this species.     

The separate and combined effects of harmonic structure, phase, and FM on female preferences in the barking treefrog (Hyla gratiosa)

In natural advertisement calls of the barking treefrog, Hyla gratiosa, a small amount of incoherent frequency modulation (FM) is present. Incoherency in the FM of a call creates inharmonicity and phase changes between its frequency components. In this study, the combined and separate effects of the harmonic structure, phase spectrum, and FM of an advertisement call on female choice were tested. The harmonic structure of a call can have a direct effect on female preference; females showed a significant preference for static-inharmonic calls over static-harmonic calls. Neither differences in phase or FM alone conferred a preference in two choice tests. However, when FM is present in both calls it does influence female preference for harmonic structure -namely harmonic calls become preferable to inharmonic calls. This reversal of female preference for inharmonicity in a call by the presence of FM suggests that call parameters may interact, and thereby effect mate choice.Abbreviations AP amphibian papilla - BP basilar papilla - FM frequency modulation - PM phase modulation - HS harmonic structure - GB Gaussian Band