Male singing strategies
We observed patterns of singing by night and season. Additionally, we observed variable singing effort across individuals, and more interestingly, varying strategies of singing in relation to space use. Multiple ecological and social factors can influence singing effort. The variation in singing effort across the six month dry season supports seasonality of this behavior, aligning with previous observations (Vaughan 1976, McWilliam 1987). For songbirds and singing mammals (e.g. (Smith et al. 1997, Brenowitz 2004, Coudrat et al. 2015, Smotherman et al. 2016a), singing effort is seasonal and regulated by environmental cues such as temperature and daylight, and subsequent physiological changes such as testosterone levels (Nelson et al. 1990). Additional variation in singing output can relate to male fitness. Male sac-winged bats (Saccopteryx bilineata ) with lower frequency buzzes in their territory songs have higher fitness (Behr et al. 2006). For the lekking lesser short-tailed bat, Mystacina tuberculata , smaller males have greater song output and higher fitness (Toth and Parsons 2018). We observed thatC. cor males with smaller testes sang more, potentially as a tradeoff for energetic output.
Beyond singing effort, we observed two main singing and space use strategies: individuals spending a large proportion of singing at particular trees or spending small amounts of time singing at more trees. The latter strategy is a reflection of more movement around the territory and resulted in larger core areas of use. These strategies could be influenced by social factors including the location and proximity of neighbors, and ecological factors including the amount of cover, and the type and height of trees on the territory. Exposed perches increased the energetic cost of singing due to higher thermoregulatory costs in willow warblers (Ward and Slater 2005). Tree type and habitat can influence the transmission ability of songs through the habitat (Blumenrath and Dabelsteen 2004), and has been shown to affect the decisions of animals while choosing perches. Chaffinches, for example, prefer to sing in the upper canopy of pines for better transmission of songs (Krams 2001). Male black-crested gibbons (Nomascus concolor ) choose trees near key food and sleeping sites, but also select the highest trees on ridges or slopes for singing to increase vocal transmission (Fan et al. 2009). Kloss gibbons (Hylobates klossii ) also choose emergent trees of the rain forest on their home ranges (Whitten 1982). Perch height can also have an effect on social dynamics of rival territory holders. Nightingales change their singing output in response to the perceived perch height of neighbors (Sprau et al. 2012). Lastly, predation is a cost for loud, conspicuous signals that may influence behavior (Mo*ller et al. 2005), such as greater perch switching (Marler 1956). Krams (2001) found that chaffinch males move to lower canopy perches in response to sparrowhawk models (Krams 2001). The lower frequencies of C. cor song syllable (between 8 and 10 Khz) (Smarsh and Smotherman 2015a) are within the audiogram of barn owls, a bat predator that may influence behavior (Baxter et al. 2006, Lima and O’Keefe 2013). Personality can create variability in response to predation risk, in which bolder individuals are less influenced by a predator. More explorative and risk-taking male collared flycatchers (Ficedula albicollis ) sing at lower perches in the presence of a human observer (Garamszegum et al. 2008). These personality traits can be consistent in individuals, regardless of body condition (Dammhahn and Almeling 2012). The shy-bold continuum of behavioral variability could thus be an important factor in singing and movement strategies (Wilson et al. 1994).
CONCLUSIONS
Our data provide a clear, quantitative link between the nighttime spatial patterns and communication behaviors of male Cardioderma cor. For a “whispering” bat using quiet echolocation, singing is likely an efficient mechanism for advertising and defending a small foraging territory rather than continually flying about or eavesdropping on the echolocation of passerby. Heart-nosed bat singing is tightly linked to perches on foraging areas, with variation in strategy of tree use and subsequent core area size. Singing location is an excellent proxy for territory presence, but the variation in behavior and space use during the course of the dry season and throughout the night demonstrates the importance of different levels of temporal scales in habitat use studies. C. cor remains an intriguing species for exploring questions connecting behavior and ecology from evolutionary or conservation perspectives.
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