As outlined in Sect. 5.1.2, basal primates differ more in activity patterns and social cohesiveness within their social networks than monkeys and apes; nevertheless, they may establish individualized and permanent social bonds. Consequently, these primates face fundamental challenges to survive and reproduce, such as avoiding predators, limiting the potential for costly conflicts between competitors within and, if in sympatry, between species, coordinating movements in time and space while searching for common sleeping and/or feeding sites and reunions of mothers and infants, mates, pairs or groups, in particular when members are dispersed during foraging. In response to these challenges, a rich diversity of vocalizations evolved for acoustic signaling in basal primates (Table 5.1 and Sect. 5.2.2).

To the human ear, the vocalizations of basal primates sound fairly mysterious and even atypical for primates. The series of short, broadband, and high-frequency click, tsic, or zek noises (Fig. 5.2) used as infant isolation calls by bushbabies and lorises sound like high-frequency human tongue clicks, whereas the series of short, broadband, and low-frequency clicks given during grooming in lemurs (Scheumann et al. 2007b) sound like a purring cat. A series of low-frequency, broadband, and noisy grunts and growls (e.g., Zimmermann 1995a), which sound like hissing cats or growling/snarling dogs and acoustically mimic large and dangerous carnivores, occur across all nocturnal strepsirrhines and are associated with threatening situations. The cries, whines, or barks of the bushbabies (Otolemur, Galago), which are used during advertisement or in alarm situations, resemble human infant crying, whining, or a dog barking. The mews of ring-tailed lemurs or ruffed lemurs (Varecia variegata), which govern reunions, resemble the moaning meowing of cats, whereas the melodious low-frequency songs and duets in indris and sportive lemurs (Lepilemur), which are given in a territorial defense context, resemble howling in coyotes or wolves. The high-frequency/ultrasonic, multi-syllabic, and rapid frequency-modulated trills and chirps of mouse lemurs (Microcebus) and the songs of tarsiers (Tarsius, Carlito), which are produced in a reunion or mating context, sound like chirps in mating songs of crickets and grasshoppers. These sounds are barely perceptible or imperceptible to the human ear, but they resemble singing in birds when transferred to the human hearing range. The tonal, high-frequency, and almost constant-frequency whistles characteristic of cohesion or disturbance/alarm contexts in lorises (Perodicticus, Nycticebus, Loris), woolly lemurs (Avahi), tarsiers (Tarsius), and small-bodied lemurs (Cheirogaleus, Microcebus) sound quite similar to whistling sounds in humans. Note, however, that sounding like these other sound sources does not mean that the sound production mechanisms are comparable (also see Fitch 2010).

As in monkeys and apes, the high diversity of vocalizations in basal primates provides the potential for recognition even across distance and, consequently, for the gathering of dispersed members of social units at a particular site and a distinct time. A recent study in group-living lemurs pointed to the fact that vocal exchanges indicate the strength in social bonds between group members and allow grooming at a distance through vocal signaling networks (Kulahci et al. 2015). Furthermore, specific differences in acoustic signaling and recognition systems may represent an efficient premating isolation mechanism, contributing to species cohesiveness when living in sympatry (Zimmermann 2016).

Research studies on vocal communication in humans, monkeys, and apes have revealed that indexical and prosodic cues in voice are perceived and recognized (Altenmüller et al. 2013). Indexical cues are specific signatures that are linked to individuality, sex, kin, population, subspecies, or species. When conveyed vocally, these cues provide the substrate for recognizing the respective category without any additional visual cue. In contrast, prosodic or paralinguistic features constitute affective prosody (i.e., patterns of stress and intonation in acoustic expressions). Humans make use of these paralinguistic features in affective prosody not only to express what and how they feel but also to evoke or interpret these feelings in others, as well as to think about their own and others’ feelings as they make decisions. Although humans are unique in the goal-directed usage of prosodic cues in voice for decision making in groups and social manipulation, the acoustic communication of emotions and emotional contagiousness evoked by voice also are an important and potentially universal feature of monkey and ape societies (e.g., for governing group movements and reunions) (Altenmüller et al. 2013). Furthermore, monkeys and apes often vocally express emotions linked to specific individualized contexts, potentially leading to context-specific calls. When these calls are then responded to in a context-specific way without any additional visual cue, they are often termed functionally referential (see Wheeler and Fischer 2015; Snowdon, Chap. 6; Zuberbühler, Chap. 7). To what extent do basal primates share these building blocks in acoustic communication with monkeys and apes?

Current research on basal primates has revealed that these primates also exhibit a rich diversity of vocalizations in which indexical cues are conveyed, as is known for monkeys and apes. Vocal indexical cues are widespread across both groups of basal primates (the strepsirrhines and the tarsiers) and thus are present irrespective of social system and phylogeny. Individual-specific acoustic signatures are described from calls of all major phylogenetic groups of the strepsirrhines, such as the nocturnal, solitary-ranging mouse lemurs (Leliveld et al. 2011) and bushbabies (Kessler et al. 2014), the nocturnal, dispersed pair-living sportive lemurs (Rasoloharijaona et al. 2006), the cohesive pair-living woolly lemurs (Ramanankirahina et al. 2015), the cathemeral group-living crowned lemurs (Eulemur coronatus, Gamba and Giacoma 2007) and red-bellied lemurs (Eulemur rubiventer) (Gamba et al. 2011; Gamba et al. 2012b), the diurnal pair-living indris (Giacoma et al. 2010; Torti et al. 2013), and the group-living silky sifakas (Propithecus candidus) (Patel and Owren 2012) and ring-tailed lemurs (Macedonia 1986; Oda 2002). Individual-specific variation of long-distance calls was also revealed in Philippine tarsiers (Carlito, previously Tarsius, syrichta) (Řeháková-Petrů et al. 2012).

Sex-specific variation in acoustic dimensions also occurs in both groups of the basal primates, such as in sex-specific syllables of duetting songs in the nocturnal Milne Edwards’ sportive lemur (Lepilemur edwardsi) (Rasoloharijaona et al. 2006), the cathemeral crowned lemur (Gamba and Giacoma 2007), the diurnal indri (Giacoma et al. 2010; Torti et al. 2013), and nocturnal tarsiers (Niemitz et al. 1991; Nietsch 1999; Merker et al. 2010). The first evidence for vocal kin signatures and vocal kin recognition in basal primates was recently reported in gray mouse lemurs (Microcebus murinus) (Kessler et al. 2012, 2014).

Group, population, subspecies, and species-specific signatures in vocalizations are present in both groups of the basal primates. They are documented for strepsirrhines, such as the nocturnal, solitary-ranging lesser (e.g., Zimmermann 1990; Ambrose 2003) and larger bushbabies (Clark 1988; Masters 1991); the nocturnal, solitary-ranging lemurs (Hafen et al. 1998; Braune et al. 2008); the nocturnal, dispersed pair-living lemurs (Méndez-Cárdenas et al. 2008); the cathemeral group-living lemurs (Gamba et al. 2012a); the diurnal pair-living lemurs (Gamba et al. 2011) and group-living lemurs (Macedonia and Taylor 1985; Gamba et al. 2012b); and tarsiers (Nietsch 1999; Merker et al. 2009; Burton and Nietsch 2010).

In sum, vocal indexical cues are present in monkeys, apes, and humans (see Snowdon, Chap. 6; Zuberbühler, Chap. 7) but also in basal primates. Since these vocal cues are described from different phylogenetic clades (Zimmermann et al. 2013), such as tree shrews, bats, rodents, pigs, elephants, carnivores, dolphins and whales, vocal indexical cues likely represent universal features across all mammals, providing an important basis for sensory exploration, such as for the acoustic recognition of the respective categories. To what extent these acoustic cues for species, subspecies, population, group, sex, or kin recognition are used in the respective social system of the different phylogenetic groups of basal primates is an interesting but rather unexplored area in bioacoustic research.

Prosodic cues (referring to emotional/motivation states or potentially to particular categories in the external world, such as type of predator) are not only present in monkeys and apes but are also described for strepsirrhines, whereas nothing is known so far for tarsiers. As put forth by Morton (1977) and refined by August and Anderson (1987), motivation-structural rules should govern vocal production and favor the emergence of cross-taxa universals in vocal acoustics based on physiological constraints. Cross-taxa universals, conveying varying degrees of short-term stress or response urgency, were shown in the temporal pattern of different strepsirrhine vocalizations in which higher affect intensity, arousal, or response urgency was expressed by higher calling rates (Zimmermann et al. 2013). Likewise, a first cross-taxa comparison of the most commonly used calls in the vocal repertoire of lesser mouse lemurs of the genus Microcebus (Zimmermann 2016) pointed to cross-taxa universals in the acoustic contour of calls used either in the agonistic, pain, or alarm/disturbance context, whereas major species-specific divergences occurred in the acoustic contour of vocalizations used in the affiliation context (mating, group reunion, mother-infant reunion). These findings provide some support for the refined Morton motivation-structural rules and suggest that natural selection (predation, physiological constraints linked to short-term stress) limits cross-taxa vocal flexibility and favors universals in acoustic structure, whereas sexual and kin selection drives cross-taxa flexibility and divergence specifically.