Discussion
Males of Phyllodytes luteolus presented advertisement calls with
a higher dominant frequency in natural environments when compared to
highway environments, and there is no evidence that chorus noise alters
the acoustic parameters of this species. Therefore, P. luteolusdoes not seem to be affected by traffic noise, which may explain its
presence in anthropized areas. In addition, we found a positive
relationship between advertisement call intensity and male body size.
Contrary to what was expected by our hypothesis (1), males ofPhyllodytes luteolus from natural areas presented higher dominant
frequencies of the advertisement call compared to individuals from
environments with road noise. Although some anuran species can increase
the dominant frequency of calls to avoid overlapping with the noise
frequency (e.g., Scinax nasicus , Leon et al., 2019; Litoria
ewingii , Parris et al., 2009; Rana clamitans and Rana
pipiens , Cunnington & Fahrig 2010, and Amolops torrents , Zhao
et al., 2018), others can decrease the dominant frequency (e.g.,Boana bischoffi , Caorsi et al., 2017). In an experimental study,
Caorsi et al. (2017) observed that Boana leptolineata did not
change the dominant frequency when exposed to different levels of
anthropic noise. Like B. leptolineata , males of Phyllodytes
luteolu s have calls with an average dominant frequency (3.46 kHz) above
the mean energy concentration of road noise (below 1 kHz) (Cunnington &
Fahrig, 2010; Warren et al., 2006). Therefore, species that have calls
with higher frequencies have little or no acoustic overlap with
anthropic noise. In addition, the frequency of ambient noise in highway
areas was very low and may not represent sufficient noise to disruptP. luteolus vocalizations. The dominant frequency of the call is
usually a static property (little intra-individual variation) as it is
related to morphological characteristics of males (Gerhardt, 1991;
Köhler et al., 2017), serving as an indicator of the quality of males
(Bastos et al., 2011; Gingras et al., 2013), being an important
parameter for the sexual selection system of a population (specific
recognition and mate selection by females). Thus, dominant frequency
values may be stabilized across populations (Friedl, 2006, Andreani et
al., 2020). Future studies should investigate possible causes for the
difference in call frequency of different anuran populations, especially
those exposed to anthropic noise.
The advertisement call of P. luteolus has a narrow bandwidth in
which the energy distributions around the dominant frequency are short.
Some studies suggest that acoustic signals with higher bandwidths may be
less detectable against noise since the signal energy is spread over a
wide range of frequencies (Parris et al., 2009; Warren et al., 2006),
therefore P. luteolus would have an advantage in noisier
environments. Many altered habitats present a constant noise pattern in
certain frequency bands and for a long period of time, acting as a
selection pressure (Warren et al., 2006). In this sense, more studies
need to be carried out to assess how different types of noise, and/or
even anthropogenic vibrations (e.g., Caorsi et al., 2020) can influence
the reproductive behavior of anuran amphibians.
There was no influence of road noise or situation (with or without
chorus) on the temporal parameters of the advertisement call of P.
luteolus , also not corroborating our hypothesis (II). The advertisement
call of P. luteolus has a long duration (around 5 sec.) and a
series of identical notes without frequency modulation. The temporal
structure of the call can influence the active space of the signal when
propagated in the environment, increasing the fidelity of the
transmitted information, and, therefore, decreasing the probability of
acoustic overlap (Ey & Fischer, 2009; Warren et al., 2006), an
attribute that is advantageous in environments with high noise levels.
In general, the social context in which males are inserted in the
reproductive site influences the change in call parameters (Bosch & De
La Riva, 2004; Toledo et al., 2015). Furthermore, males of P.
luteolus begin to vocalize after the period of greatest road traffic
(between 5 and 7 pm), and therefore, the lower flow of vehicles at night
may not represent a source of noise that modifies the acoustic
parameters of the species. It is necessary to investigate whether other
factors are more important to determine changes in the acoustic
parameters of P. luteolus , such as the density of conspecific
individuals in the choirs. Experimental studies on the immediate
response of males are important for a better understanding of the
behavioral strategies adopted by P. luteolus on the balance of
costs and benefits determined by changes in acoustic parameters when
they are subjected to different noise levels.
We found a positive relationship between advertisement call intensity
and male body size. In general, morphological characteristics of males
can determine the temporal parameters of the call (Kohler et al., 2017;
Wells, 2007). For example, larger males have larger lungs and,
therefore, greater energy reserves, being able to emit calls with
greater intensity (Wells, 2007; Wells & Schwartz, 2006). Possibly, even
in noisy scenarios, larger males are able to increase the intensities of
their calls to avoid signal masking and promote greater signal
detection, localization and discrimination (Halfwerk et al., 2016; Yi &
Sheridan, 2019). Therefore, the intensity of the call is an important
parameter for sexual selection (Gerhardt & Huber, 2002; Wells, 2007)
since the sounds with higher intensities can propagate over long
distances, potentially attracting a greater number of females and thus
obtaining greater reproductive success (Bastos et al., 2011; Kime, 2000;
Penna & Solís, 1998).
The Yellow Heart-tongued Frog (Phyllodytes luteolus ) has
advantageous acoustic characteristics in conditions where anthropogenic
noise is present. Therefore, even in noisy environments, individuals of
this species manage to recognize and discriminate in the conspecific
chorus. This fact may explain the successful occupation of areas outside
its original range, having already been reported as an invasive species
(Forti et al., 2017). Invasive species may impose limits on reproductive
success and survival of other frog species that live in the same
habitat. For example, males of P. luteolus calling in the same
frequency range as the species Ischnocnema sp. The acoustic noise
caused by the vocalization of an invasive species can harm the acoustic
communication system in native species as it can influence the ability
of females to locate males in the reproductive environment (Forti et
al., 2017). This is the first study evaluating the influence of sound
noise on a bromeliad anuran. Future studies may increase our knowledge
on the effects of anthropogenic sound and serve as a subsidy for
conservation actions, especially those aimed at acoustic monitoring in
noisy environments.