Original investigationIndividual vs. non-individual acoustic signalling in African woodland dormice (Graphiurus murinus)
Introduction
Acoustic signalling represents an important communication channel among animals (Wiley and Richards, 1978): conveying information about individual identity by acoustic communication is a fundamental feature for species that need to maintain social contact among group members, particularly those inhabiting low-visibility habitats (Bouchet et al., 2012). Encoding individual identity within acoustic signals allows animals to communicate such information from a distance, without the need of establishing visual contact between emitter and recipient, which would be difficult in structurally complex habitats such as forests (Maynard Smith and Harper, 2003). Individuality has been often recorded as ‘signatures’ found in social calls, widely used especially by birds and mammals to communicate with conspecifics (Matrosova et al., 2011, Kremers et al., 2012, Janik and Sayigh, 2013, Cornec et al., 2014). Among mammals, individual signatures are most common in species with complex and stable social structures; comparative studies on rodents, for example, indicate that vocal complexity relates to that of the species’ social structure (Freeberg et al., 2012, Pollard and Blumstein, 2012, Bouchet et al., 2013).
The ‘distance communication hypothesis’ (Bouchet et al., 2012) predicts that, within the vocal repertoire of a species, signals used for short- or long-distance communication may be subject to different selective pressures. In close contact, animals provide non-vocal (visual of olfactory) individual cues, whereas long-distance communication, taking place in absence of direct contact, is more likely to encode individual identity, status or quality. This hypothesis has received support mainly from studies on primates (Macedonia, 1986, Mitani et al., 1996, Bouchet et al., 2012) as well as on the social giant otter (Mumm et al., 2014), but whether it also holds valid for small mammals is yet to be proven.
Dormice (Gliridae) are highly vocal rodents and many species produce a variety of acoustic signals, audible to humans as well as ultrasonic (Boratynski et al., 1999, Hutterer and Peter, 2001, Madikiza, 2010, Ancillotto et al., 2014). The secretive, nocturnal habits of most dormice species (Webb and Skinner, 1994) make their social systems and communication signals difficult to unveil: observations on the African woodland dormouse Graphiurus murinus, the species dealt with in the present study, and the hazel dormouse Muscardinus avellanarius (Juškaitis, 2008, Madikiza et al., 2010) suggest that their social systems involve non-random association among individuals, with fission-fusion dynamics across small groups (Madikiza et al., 2011).
In this work we test the distance communication hypothesis on the vocal repertoire of the social rodent G. murinus predicting that long-range vocalizations will show a higher degree of individual variability than vocalizations broadcast by individuals close to each other. We assume that such variation provides the structural basis for conveying individual information.
The vocal behaviour of G. murinus is poorly known, thus we first analyse the acoustic repertoire of the species and associate vocalization types with non-vocal behaviours; then, we test our hypothesis selecting two signal types that best contrast short vs. long-distance communication.
Section snippets
Model species, experimental individuals and housing conditions
G. murinus is an arboreal small-sized glirid (weight: 24–34 g; head-body length: 78–113 mm; tail length: 58–94 mm) common throughout central and southern Africa, where it inhabits woodlands, scrublands and suburban gardens (Madikiza, 2010). This species is also widespread as a pet in Europe and North America, where it is commercially known as ‘African pygmy dormouse’ (Suckow et al., 2012).
We considered 15 adult subjects originating from two groups kept at the “Charles Darwin” Department of “La
Vocal repertoire and non-vocal behaviour
We recorded 363 vocalizations classified into 8 different types (labelled a–h hereafter, see Table 2, Fig. 1). Call parameters differed significantly across such types (all p < 0.05, df = 7). The entire vocal repertoire ranged between 5.0–56.6 kHz: most vocalizations consisted of frequency-modulated syllables spanning between audible and ultrasonic frequencies (Table 2). All but two (d and e) vocalization types were made of multiple notes (up to 36). Adults of both sexes emitted vocalizations a–e,
G. murinus calls and the “distance communication hypothesis”
We show that G. murinus possesses a rich vocal repertoire, largely spanning into the ultrasonic range and comprising complex vocalizations, a fundamental prerequisite to encode information (Wiley and Richards, 1978). Given its propensity to vocalize, G. murinus is an excellent candidate as model species for investigating the evolution and adaptive significance of vocal complexity in mammals.
By comparing aggressive and contact calls we found that only the latter, serving long-range
Acknowledgement
We are indebted to two anonymous reviewers for their valuable comments which greatly helped us to improve this article.
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