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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