Bouts of rutting calls
This study analysed the rutting calls of male impala, emitted in compact bouts comprising three types of roars and two types of snorts. The bouts were complex displays, containing roars, putatively produced by the vocal folds, and snorts, produced by explosive expirations through the nose, as documented by our video footage. In other ruminants, bouts of rutting calls can also include different call types: roars, growls and grunts in goitred gazelle [36] or harsh and common long and short roars in red deer [4, 39, 62, 64, 65]. However, among the studied species of ruminants, only male impala bouts include both roars and snorts.
Male impala frequently use their bouts of rutting calls to attract potential female mating partners and to defend them against rival males. Therefore, this vocal display is a dominant part of male impala rutting behaviour and might be costly regarding the energy budget [14,15,16, 18]. Bouts of rutting calls of male impala follow each other with large intervals (of at least a few minutes), indicating that their production could be exhausting for a caller. In addition, roar emission by males involves other potentially exhaustive activities, as retraction of the larynx by maximal contraction of the strap muscles and pronounced inhalatory and exhalatory phases supported by strong intermittent contractions of the abdominal muscles [18]. Apparently, this also applies to the advertising display in male koalas, which also produce their sequences of pant-calls at large intervals and accompanied by pronounced flank movements [66].
Moreover, male impala often emit their bouts of rutting calls while running [18] and, thus, making their production particularly challenging in terms of energy expenditure. Male impala bouts contained many rutting calls (13.5 on average, up to a maximum of 38) and were of long duration (on average 20.6 s). For comparison, bouts of rutting calls in another bovid, the goitred gazelle, also produced while running, contain only 2.67 calls per bout on average at an average bout duration of 1.28 s [36]. In cervids (red deer stags), commonly producing their bouts of rutting roars from a standing posture, the bouts are also shorter and contain less calls on average than in male impala: 2.6 roars at an average bout duration of 1.63 s in Corsican red deer Cervus elaphus corsicanus [64], 2.11 roars at an average bout duration of 3.41 s in Iberian red deer C. e. hispanicus [39] and 3.18 roars at an average bout duration of 3.73 s in Pannonian red deer C. e. hippelaphus [62].
Roars
The estimated length of the maximally extended vocal tract of male impala did not differ between roar types. Apparently, the additional short inhalations in the interrupted roars and pant-roars did not affect the degree of larynx retraction and vice versa. Similarly, the fundamental and formant frequencies of the roars were not affected by the additional inhalations in the interrupted and pant-roars (except a weak influence on the third formant F3). Male impala retract the larynx down to a mid-neck position [18], i.e. less far than the other ruminant species known to retract the larynx down towards the sternum [34,35,36, 39, 42]. The fundamental frequency of male impala rutting roars was low (50.5 Hz) and comparable to the low fundamental frequency in the rutting groans of male fallow deer (from 21 to 39 Hz, 28.2 Hz on average) [24, 45] and in the rutting roars of male goitred gazelle (from 17.2 to 27.8 Hz, 22.0 Hz on average) [36]. In fallow deer and goitred gazelle, there is a strong sexual dimorphism of the larynx and vocal folds [41]. In contrast, there is no obvious sexual dimorphism of larynx size in impala and the male larynx is not enlarged. Probably, the low fundamental frequency of impala roars results from the enlargement and unique structure of the massive male vocal folds [18].
Pant-calls
In male impala, the exhalatory phase displayed clear pulses, evidently resulting from the regular vibration of the sound source (most probably the vocal folds), whereas during the inhalatory phase, the sound was noisy (Fig. 2a). This difference between the exhalatory and inhalatory phases is reminiscent of the koala male advertising calls [66] and the striped possum Dactylopsila trivirgata mating calls [48], produced at both phases of respiration, in spite of the distinctive mechanism of sound production in the koala [46, 67]. In contrast, clear pulsation is evident at both exhalatory and inhalatory phases of the purring vocalizations of felids [68, 69], probably because of the involvement of active muscle contractions for producing the exhalatory and inhalatory phases of these calls [70].
In male impala, there is a gradual shortening of both exhalations and inhalations from continuous roars via interrupted roars towards pant-roars and further to the pant parts of the pant-roars. From continuous via interrupted to pant-roars, exhalations and inhalations alternated more and more rapidly, and the time spent inhaling progressively increased. In the pant-parts of the pant-roars, within particular exhalation-inhalation cycles, the inhalations were shorter than the exhalations. In contrast, in the pant-calls of a marsupial species, the striped possum, the exhalations were always shorter than the subsequent inhalations [48].
In male impala, both the pant-roars and the interrupted roars were two times longer than the purely exhalatory continuous roars. Pant-roaring as well as short additional inhalations within the interrupted roars, might therefore promote the production of longer roars. The longer duration of the roars and the corresponding longer duration of the bouts of rutting calls may be important for advertising male quality. Both the extra inhalations and the longer calls might be challenging for the male respiratory system and the entire male physiology and energy budget. In koala, pant-calling in advertising/pair-bonding contexts may be similarly challenging to both sexes [46, 48].
Pant-calls appear to be an adaptation for producing impressive vocal displays as they have an increased audibility [49, 50, 71]. However, pant-calls may function differently across taxa. Whereas in impala only males produce pant-roars and only in a rutting context, both sexes of striped possums and koalas produce pant-calls during pair formation, probably not only for mate attraction but also as an agonistic or territorial display [47, 48]. In male and female koalas, pant-calls announce individual identity, age and sex [47, 66]. In male and female southern white rhinoceros Ceratotherium simum, pant-calls announce individual identity and age in different social contexts [51, 72]. In female baboons, pant-calls represent copulation calls [52,53,54,55], announcing individual identity [53] and proximity to ovulation [54]. In male chimpanzee, pant hoots announce male quality [73, 74], testosterone levels [75], individual identity [76] and social context [77]. In hairy armadillo Chaetophractus vellerosus, pant-calls function as distress vocalizations [78]. Loud calls at both respiratory phases were also reported in donkeys Equus asinus [79], but the function of these calls has not yet been clarified. Ultimately, pant-calls occur in many species but they are rarely emitted in all taxa. The contexts in which pant-calls occur are diverse, ranging from aggressive to peaceful, and their functional meaning is poorly understood.
Origin of pant-calling from thermoregulatory behaviour
From an evolutionary perspective, respiratory panting evolved as an adaptation for thermoregulation to avoid overheating by evaporative cooling [80,81,82]. In the course of further evolution, panting, by an expansion of its function, could have acquired an additional or, by a change of function [83], even a sole acoustic role, e.g. for enhancing the impressiveness of vocalizations, as in male impala (this study), rhinos [51, 72], koalas [46], striped possums [48] and lions Panthera leo [84]. Indirect evidence for this comes from the fact that pant-calling occurs only in animals living in hot climates, as koalas [85], striped possums [86], rhinos [51, 72], chimpanzee [73], lions [87] and impala [16].
Impala experience thermal stress between 35 °C and 50 °C of ambient temperature and their breathing rate at such temperatures can exceed 200 cycles per minute [88]. Therefore, panting might be adaptive for evaporative cooling in male impala. As a consequence, pant-roaring in rutting male impala could have evolved as a mechanism to avoid overheating during their exaustive rutting vocal display. The ability of males to use pant-roars for effective evaporative cooling may even be an indirect trait of male quality. Possibly, those males which are capable of producing long rutting vocal displays without overheating because of their efficient thermoregulation are more attractive for females.
Another possible adaptation against overheating in male impala is tongue protrusion during the rutting vocal display. This behaviour might also involve a thermoregulatory function besides its potential role as a visual display [18]. Thermoregulatory and advertising roles of tongue protrusion during the rutting vocal display have already been discussed for Iberian red deer Cervus elaphus hispanicus [39]. Often, pure thermoregulatory panting is accompanied by tongue protrusion both in carnivores (e.g. domestic dogs Canis familiaris) and in ruminants (e.g. reindeer Rangifer tarandus) [80,81,82, 89, 90] indicating the thermoregulatory role of tongue protrusion. However, the rutting period of goitred gazelle occurs at relatively low temperatures, and a wide opening of the mouth with tongue protrusion has also been documented for the vocal display of male goitred gazelle [36, 91]. Presumably, the advertising role of tongue protrusion prevails over its thermoregulatory role in male goitred gazelle.
Nevertheless, tongue protrusion in male impala during pant-roar emission might be a behavioural relict of an ancestral purely thermoregulatory panting [81, 82]. Perhaps, tongue protrusion has been retained to prevent or restrict the formation of an adequate oropharyngeal seal and, thereby, to facilitate inhalation and exhalation through the mouth, as well as through the nose, during pant-roar emission. The observed roar-synchronous wide opening of the nostrils would support this assumption. During deep open mouth panting in dogs and reindeer, the airflow is bidirectional in both nose and mouth but only a small fraction of the air is routed through the nose because of its higher resistance [92]. Consequently, the bulk of the air passes in and out through the oropharynx [80, 81]. From a bioacoustical perspective, this would suggest an involvement of the nasal vocal tract in pant-roar production although to a much lesser degree than of the oral vocal tract.
An involvement of the nasal vocal tract in production of the rutting groans has been suggested for male fallow deer [93]. In male impala, the values of the oral vocal tract length calculated on the basis of formants and the values obtained by anatomical and video analyses are very close (they practically coincide) [18]. This contradicts the involvement of the nasal vocal tract in impala roaring. In red deer, some rutting calls start nasally and then shift to purely oral calling. This is clearly visible in spectrograms, as the nasal part is always much fainter than the oral part [26]. However, the spectrograms of male impala roars do not reveal any nasal parts. This suggests that nasalization during roaring in male impala is negligibly small or lacking. Accordingly, the roar-synchronous opening of the nostrils might be used for increasing the inhalation efficiency by allowing oronasal inhalations.
Rutting and alarm snorts
We found that rutting snorts of both types (usual snorts and roar-snorts) did not differ from each other in any acoustic variable, whereas the alarm snorts were shorter and had a lower upper quartile, i.e. their sound energy was stronger concentrated in the middle frequency range. However, there was substantial overlap regarding duration and upper quartile between samples of rutting and alarm snorts. Hence, recipient animals can hardly discriminate between rutting and alarm snorts by judging from the hearing of a single snort. This is especially important in natural habitats, where the call duration can be distorted by echo, and distribution of sound energy depends on the distance to a caller [94,95,96]. The prominent difference between alarm snorts of the series and rutting snorts of the bouts arises from the specific sequence combination pattern of snorts. Alarm snorts are separated from one another by large intervals, whereas the intervals between rutting snorts within bouts are short. Sometimes, rutting snorts alternate with roars, whereas alarm snorts do not. Therefore, it is not the acoustic structure of individual snorts but the temporal sequence pattern and the association with another call type or not that defines snorts as either rutting or alarm snorts. These results are consistent with findings indicating the importance of call combinations in the communicative systems of mammals, including impala, and birds [97,98,99].
Playback experiments showed that male impala [99], like female topi [57], respond to any isolated snorts as to alarm calls. In contrast, the combination of snorts and roars provoked an aggressive reaction in male impala [99], whereas in female impala it provoked only an increase in their movements but did not alarm them [100]. Further research is necessary to reveal the precise functions of different call types of male impala and their use in relation to season and climate conditions.
Impala therefore, like topi antelope, display snorts in both an alarm and a rutting context [57]. The rutting snorts of male impala may function to attract the attention of receptive females and delay their departure from a male’s harem or territory. However, in topi, the snorts are the only calls described in the rutting context, whereas male impala produce bouts of roars and snorts during their rutting behaviour.
In many ruminants, snorts are used as alarm calls, for review see [56]. However, among bovids, only in topi [57] and in impala (this study) the alarm snorts are included in the courtship display. In cervids, there are observations of manipulative use of alarm barks by red deer stags for promoting defensive bunching by the hinds and thus to increase the cohesion of a stag’s harem [5].
Aside from ruminants, males of two rodent species (Pallas’s squirrel Callosciurus erythraeus and Belding’s ground squirrel Urocitellus beldingi) also produce alarm calls in relation to sexual behaviour in precopulatory and postcopulatory contexts [101, 102]. The hypothetical function of these alarm calls is manipulation of females via a sensory exploitation mechanism, allowing the calling males postcopulatory mate guarding to avoid matings with other males and subsequent sperm competition [101, 102]. Aside from mammals, the superb lyrebird, Menura novaehollandiae uses alarm calls in a sexual context during mating [103].
The rutting activity of impala depends on climatic conditions. A clearly restricted rutting season with high levels of roaring activity exists only in subtropical zones with seasonal changes of temperature, day length and rainfall, as e.g. in Namibia [18]. In habitats with a tropical climate, as e.g. in Kenya, impala breed continuously and dominant impala males permanently herd females and defend them against rival males, producing roaring activity at a moderate level [104].