Nocturnal “humming” vocalizations: adding a piece to the puzzle of giraffe vocal communication
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- Baotic, A., Sicks, F. & Stoeger, A.S. BMC Res Notes (2015) 8: 425. doi:10.1186/s13104-015-1394-3
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Recent research reveals that giraffes (Giraffa camelopardalis sp.) exhibit a socially structured, fission–fusion system. In other species possessing this kind of society, information exchange is important and vocal communication is usually well developed. But is this true for giraffes? Giraffes are known to produce sounds, but there is no evidence that they use vocalizations for communication. Reports on giraffe vocalizations are mainly anecdotal and the missing acoustic descriptions make it difficult to establish a call nomenclature. Despite inconclusive evidence to date, it is widely assumed that giraffes produce infrasonic vocalizations similar to elephants. In order to initiate a more detailed investigation of the vocal communication in giraffes, we collected data of captive individuals during day and night. We particularly focussed on detecting tonal, infrasonic or sustained vocalizations.
We collected over 947 h of audio material in three European zoos and quantified the spectral and temporal components of acoustic signals to obtain an accurate set of acoustic parameters. Besides the known burst, snorts and grunts, we detected harmonic, sustained and frequency-modulated “humming” vocalizations during night recordings. None of the recorded vocalizations were within the infrasonic range.
These results show that giraffes do produce vocalizations, which, based on their acoustic structure, might have the potential to function as communicative signals to convey information about the physical and motivational attributes of the caller. The data further reveal that the assumption of infrasonic communication in giraffes needs to be considered with caution and requires further investigations in future studies.
A lion “roars”, a dog “barks”, an elephant “trumpets”, but what does a giraffe sound like? Indeed, to date, our knowledge about giraffe (Giraffa camelopardalis sp.) vocal communication is limited due to a lack of systematic scientific investigations, while other aspects of giraffe behaviour have now received more research attention [1–4].
In contrast to the prevalent opinion that giraffe herds are loose amalgamations of non-bonded individuals, recent behavioural research on long-term data indicates that they possess a structured, fission–fusion social system in which herd composition is apparently based upon social associations that often reflect kinship [5–7]. Social affiliation and attachment among individuals have also been observed in captive giraffes [8–10].
Species with a fission–fusion society (such as the African elephant Loxodonta africana, African buffalo Syncerus caffer, spotted hyaena Crocuta crocuta and the chimpanzee Pan troglodytes) often exhibit a sophisticated vocal communication system to facilitate social dynamics [11–16]. Important vocalization types include long-distance contact calls that convey individual identity [11, 17–19] as well as vocalizations to confirm and strengthen social bonding when reunited after temporary separation.
Although giraffes do have a well-developed larynx and laryngeal nerves [20–22], it was long suggested that due to the long neck, giraffes might have problems to produce an air-flow of sufficient velocity to induce self-sustained vocal fold vibrations . Notwithstanding, giraffes are, in principle, capable of producing sounds . On YouTube there is a video of a newborn calf at a zoo emitting loud bellows while being restrained by keepers to examine its health state . Giraffes do not seem to use vocalizations regularly, but they have further been (anecdotally) described to, “bleat”, “brrr”, “burst”, “cough”, “growl”, “grunt”, “low” “moan”, “moo”, “sneeze”, “snore” or “snort” [23, 25–28]. The snort seems to be the most commonly heard vocalization and has been documented in varying contexts such as being alarmed, annoyed, or when approaching each other [10, 29]. Snorts and bursts are broad-band signals with no harmonic structure (and thus no measurable fundamental frequency); they seem to be produced by a sudden burst of air out of the nostrils . During 700 h of vocal recordings (by day) of giraffes in three zoological institutions, Hurgitsch  recorded 72 vocalizations, mostly snorts.
Apart from this, no acoustic descriptions have been provided for the different types of vocalizations listed above. This makes it impossible to assess whether these are different call types or whether the authors used different terms for similar sounds.
It has been suggested (again rather anecdotally) that giraffes do communicate using infrasonic vocalizations (the signals are verbally described to be similar—in structure and function—to the low-frequency, infrasonic “rumbles” of elephants) [27, 30]. It was further speculated that the extensive frontal sinus of giraffes  acts as a resonance chamber for infrasound production. Moreover, particular neck movements (e.g. the neck stretch) are suggested to be associated with the production of infrasonic vocalizations.
Despite these reports of giraffe sounds, there is no clear evidence that giraffes indeed use acoustic signals to communicate with each other . Acoustic communication describes the interchange of information between at least two individuals, where an acoustic signal (typically a vocalization) is being directly transmitted from a sender and perceived by a receiver, that alters the behaviour of the communicating animals [33, 34]. Although grunts and snorts are produced in agonistic interactions [28, for personal observation for a grunting adult female giraffe see Additional file 1], it is unclear what role the acoustic signals play compared to the visual, tactile and olfactory cues.
In general, Artiodactyla are highly vocal. Acoustic behaviour and vocalizations of several species have been intensively studied, showing that acoustic cues have a functional relevance in reproductive contexts. Examples include the saiga Saiga t. tatarica , the red deer Cervus elaphus  and the North American bison Bison bison . Acoustic signals are also important for mother–infant recognition, such as in the goat Capra hircus , cattle Bos taurus , sheep Ovis aries , eland antelope Taurotragus oryx, red deer Cervus elaphus, reindeer Rangifer tarandus, mule deer Odocoileus hemionus, white-tailed deer Odocoileus virginianus and the pronghorn Antilocapra americana . Hurgitsch , however, who recording during a birth at Vienna Zoo and later on recorded the mother–calf unit regularly (during daytime), did not document vocal communication between the giraffe mother and her offspring, which is highly uncommon for mammals.
We aim at further investigating vocal communication in giraffes in more detail and collected data of captive individuals during day and night. We particularly focussed on detecting tonal, infrasonic or sustained vocalizations.
By presenting these calls, which have not been acoustically described elsewhere, we want to encourage acoustic research in giraffes. However, we strongly suggest developing an automatic system that helps analysing great amount of acoustic data and related behavioural contexts in order to instigate more research on giraffe vocal behaviour.
Data were collected over several months in three European zoos: Berlin Tierpark in Germany, Copenhagen Zoo in Denmark and Vienna Zoo, Austria. We conducted nocturnal indoor and diurnal outdoor recordings. During both recording conditions no other animal species were housed together with the giraffes. The animals had access to the outdoor enclosure throughout the zoo opening hours depending on season and weather condition at all 3 institutions. Giraffe indoor facilities cover the following area size: 600 m2 at Berlin Tierpark, 120 m2 at Copenhagen Zoo and 130 m2 at Vienna Zoo. At Berlin Tierpark each individual was kept in its individual stall overnight. Giraffes at Copenhagen Zoo are generally kept together during the night, but during data collection one pregnant female was separated from the rest of her herd (indoor stall can be divided into 2–4 separate compartments if required). During data collection at the giraffe barn at Vienna Zoo, the giraffe bull was separated from its group during the night.
Number of giraffes including age (year-month), year of data collection and total recording time for each institution
Rec time (hh:mm:ss)
Giraffa c. rothschildi
21 Years 5 months
1 Year 1 month
24 Years 5 months
1 Year 8 months
12 Years 2 months
9 Years 4 months
8 Years 2 months
4 Years 7 months
4 Years 2 months
22 Years 6 months
1 Year 3 months
13 Years 3 months
10 Years 5 months
9 Years 3 months
5 Years 8 months
5 Years 3 months
2 Years 9 months
14 Years 9 months
11 Years 11 months
10 Years 8 months
7 Years 2 months
6 Years 8 months
Giraffa c. reticulata
7 Years 11 months
7 Years 7 months
1 Year 9 months
7 Years 8 months
10 Years 8 months
Giraffa c. rothschildi
20 Years 9 months
8 Years 6 months
Oct − Dec, 2007
14 Years 3 months
8 Years 6 months
We visually inspected 908 h and 50 min of nocturnal, and 38 h and 22 min of diurnal recordings for tonal, infrasonic or sustained signals and annotated 65 calls using sound spectrograms (fast Fourier transform method; Gaussian window shape; window lengths: 0.02 s; time steps: 1000; frequency steps: 250; dynamic range: 35–40 dB) generated in PRAAT 5.4.01 DSP package . We did not inspect and listen to short broad noise-bands (and thus putative bursts or snorts) because these are very similar to most other ambient cracking and bumping sounds). Annotated calls were extracted to separate WAV sound files and analysed in a custom-written semi-automatic tool in Matlab [following 42]. This enabled tracing the contour of the fundamental frequency (F0) in the spectrogram. To compute a Fourier spectrogram for the frequency range of 0–800 Hz we used a frame size of 30 ms and a step size of 3 ms.
List of 11 acoustic parameters and their definitions
F0 start (Hz)
Fundamental frequency at the beginning of the vocalization
F0 mid (Hz)
Fundamental frequency at the middle of the vocalization
F0 end (Hz)
Fundamental frequency at the end of the vocalization
F0 min (Hz)
Lowest fundamental frequency
F0 max (Hz)
Highest fundamental frequency
F0 range (Hz)
Difference between minimum and maximum fundamental frequency
F0 mean (Hz)
Arithmetic average frequency across a call
F0 median (Hz)
Central point from data points in ascending order of the F0 contour
Time between onset to the end of call
Statistical analyses were performed using SPSS software version 22 . We conducted descriptive statistics to obtain general information about the recorded vocalizations by examining the arithmetic averages and standard deviations for all acoustic parameters.
Arithmetic mean, standard deviation, minimum and maximum for acoustic parameters extracted from giraffe humming vocalizations
N hums = 34
N hums = 22
N hums = 9
F0 start (Hz)
82.54 ± 29.32
109.8 ± 27.54
100.57 ± 40.81
F0 mid (Hz)
76.48 ± 18.81
110.02 ± 27.04
93.35 ± 29.05
F0 end (Hz)
73.9 ± 19.41
110.7 ± 34.21
87.23 ± 35.98
F0 min (Hz)
63.98 ± 18.23
83.88 ± 20.64
77.23 ± 27.4
F0 max (Hz)
94.69 ± 23.44
143.67 ± 29.61
116.68 ± 39.05
F0 range (Hz)
30.71 ± 14.59
59.79 ± 29.83
39.45 ± 21.57
F0 mean (Hz)
79.25 ± 18.52
110.26 ± 22.07
95.6 ± 31.64
F0 median (Hz)
78.98 ± 18.67
108.77 ± 22.56
94.18 ± 30.26
1.42 ± 1.04
0.95 ± 0.36
1.52 ± 0.87
At present, a systematic assessment of giraffe vocal behaviour is missing. Up to now the only scientifically documented giraffe vocalizations are atonal snorts or bursts through the nostrils [10, 32]. One reason for this is that, compared to other social-living mammals, giraffes seem very taciturn.
In this study we analysed hundreds of hours of acoustic recordings from captive giraffes in three institutions and documented a sound that has never been structurally described in the scientific literature before—the “hum”. Although we could not identify the calling individuals, the giraffes definitely produced the recorded sounds because we documented similar vocalizations in three different institutions without any additional co-housing species.
The “hum” is a low-frequency vocalization with a rich harmonic structure and of varying duration. Since it was not possible to determine the calling individual, we are currently unable to prove that this sound is indeed used for communication or to give information about the behavioural context and prospective information content. Although we cannot provide behavioural data, we would like to note that at all 3 zoos all giraffes where kept under similar housing conditions during night times. At Copenhagen Zoo the pregnant cow was separated from her herd, while at Vienna Zoo the giraffe bull was kept separate from the rest. Berlin Tierpark kept each giraffe in an individual stall, however calves where kept together with their mothers. At Copenhagen Zoo hums occurred approximately within 2 h before sunrise, while at the other two zoos, hums occurred mainly in the middle of the night. These patterns might provide suggestive hints that in giraffe communication the “hum” might function as a contact call, for example, to re-establish contact with herd mates.
Nonetheless, the rich harmonic structure and the frequency modulation indicate that this type of vocalization has the potential to convey relevant information to receivers.
Interestingly, these vocalizations have so far been recorded only at night. Even giraffe keepers and zoo managers stated that they have never heard these vocalizations before. Anatomical investigations indicate that giraffes have excellent vision with potentially long-range visual acuity, which would provide a means of communication between widely separated conspecifics . Recent social behaviour research has shown that giraffes spend a significant portion of their vigilance towards social partners , suggesting that perception and utilization of visual communication cues are highly developed in the giraffe communication system. Giraffes might use vocalizations more often once vision is limited (e.g. at night time). Future studies should test in a well established experimental setting whether giraffes are more vocal when visual communication cues are absent.
We found no evidence for giraffe infrasonic communication in our data set even though it is widely assumed that giraffes communicate in this manner. The lack of systematic assessment, detailed spectrographic descriptions and presentations or sound examples of giraffe infrasonic signals have not prevented researchers from suggesting adaptive explanations (e.g. keeping vocal contact) or from accepting as fact the idea that giraffes produce infrasound (via Helmholtz resonance, not vocal fold production) to communicate [23, 30–32]. We concede that giraffes in captivity, housed within the same enclosure, might not need to use infrasonic signals to communicate (such signals may be used mainly for long-distance communication when vision is eliminated). Still, neither Bashaw 2003 , nor Hurgitsch 2011 , nor we could find evidence for giraffe infrasonic communicative signals. Accordingly, such communication in giraffes should remain in a mere hypothesis status.
To re-emphasize, our measurements and the calculations mentioned above give no information about identity, vocal tract length and age class of the caller. In general, vocalizations can be used for transferring various information about, for example, individuality, age, gender, arousal, dominance hierarchies or reproductive states . In this study, however, due to absent behavioural data during acoustic recordings, we are unable to make any statement about the context-specific use, or the potential active or passive communicative role of humming.
Another more inherent issue in regards to call nomenclature is worth mentioning. The use of specific terms for giraffe vocalizations in earlier reports [23, 25] were based on the calls’ phonetics and authors’ subjective sound perception, respectively, and not due to comparative and quantitative methods to objectively describe distinct types of vocalizations. Classification of animal sounds requires comparative analyses among individuals; there is, however, no general agreement on how to best categorize calls . The humming vocalizations presented in this study might be the same type of vocalization as one of those reported earlier, but missing acoustic recordings from these reports hinder objectively comparing the data.
The present findings emphasize that vocalizations should be taken into account when studying giraffe social and communicative behaviour. At the same time, we draw attention to the fact that detecting giraffe vocalizations is intricate and time consuming. Clearly, it would be even more difficult to record vocalizations of free-ranging giraffes. This makes zoos or sanctuaries optimal sites for initial exploration. A next step for future studies should be to develop an automatic acoustic monitoring/detecting system linked with concurrent video recordings. This would enable detecting, annotating and recording vocalizations along with the corresponding behaviour and help identify the calling individual.
Furthermore, another possibility to examine vocalizations and potential infrasound production (though behaviourally invasive) in giraffes could be by separating a giraffe spatially from its herd during the night (when visual stimuli are absent and individuals can barely see or locate each other). In addition, playback experiments would help reveal whether giraffes show behavioural reactions in response to conspecific vocalizations.
These approaches could yield novel insights into giraffe vocal behaviour.
This non-experimental research meets all applicable international, national and/or institutional guidelines for the care and use of animals. The nature of the study was purely observational: No invasive methodologies were applied at any point of the study. Berlin Tierpark, Copenhagen Zoo and Vienna Zoo approved data collection for the study. All procedures were in accordance with European Union law.
AB conducted recordings, analysed the data and contributed to the writing of the paper. FS supported data collection and commented on the manuscript. AS designed the study, interpreted the data and led the writing up of the paper. All authors read and approved the final manuscript.
The authors thank each participating zoo for its cooperation in facilitating the data collection at their institution. We are also very grateful to all giraffe keepers who contributed to obtaining acoustic recordings by installing and retrieving the recording units between closing and opening hours, and for listening to the recordings. We further acknowledge Vienna Zoo for providing acoustic data material from 2007 as well as Jutta Kirchner for providing a giraffe photograph for the cover page taken at Vienna Zoo. The authors would also like to thank the editor and two anonymous reviewers for their valuable comments and suggestions to improve the manuscript. A. Stoeger and A. Baotic were supported by the FWF Austrian Science Fund (No. P26448).
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
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