Introduction

Acoustic communication is widely used by animals to transmit information through sounds. These sounds are produced by an emitter (source) and are propagated in the environment, causing some response in the receivers (Kime, 2000; Wells, 2000). In environments where acoustic signals propagate, transmission can be hampered by other different sounds, causing stress, irritability, reduced fitness, in addition to being associated with other risk situations (Grenat et al., 2019; Leon et al., 2019; Troianowski et al., 2017). There are three main types of sound noise that interfere with the transmission and detection of the species’ acoustic signal: abiotic (environmental), such as the presence of winds, rains, streams and ocean tides (Caldart et al., 2016); biotic, produced by intra and interspecific individuals that can form dense social groups (Lengagne, 2008); and anthropic, which are related to acoustic pollution caused by humans, such as the flow of automobiles on highways, civil construction machinery, air transport, ships and boats (Cunnington & Fahrig, 2010, 2013). The interference caused by these noises can negatively influence the fitness of individuals, and consequently affect populations and communities (Hanna et al., 2014; Hoskin & Goosem, 2016). Over the last few decades, there has been an increase in studies on the effects of noise on the acoustic communication of organisms (Gomes et al., 2022; Grenat et al., 2019). One of the groups of animals most affected by human noise are anuran amphibians, which use acoustic signals as their main form of communication (Gomes et al., 2022; Wells, 2007).
During the breeding period, most anurans form dense aggregations in water bodies (Wells, 2007). Communication between frogs occurs mainly through the emission of different types of vocalizations (Toledo et al., 2015), however, the most emitted acoustic signal is the advertisement call, which has the main function of attracting reproductive partners and delimiting territories (Guerra et al., 2018; Toledo et al., 2015; Wells, 2007). The calls of conspecific individuals (and also of other species) can represent biotic sound noises that interfere in the local acoustic space. Thus, males in vocalization activity must avoid the overlapping of these acoustic signals (e.g., temporal and spectral parameters) in some way (Bittencourt et al., 2016; Herrera-Montes & Aide, 2011). Dense choruses of males in vocalizing activity may also lead to limitations in the ability of females to choose reproductive partners (Wollerman & Wiley, 2002).
In addition to biotic noises, more attention has recently been directed to anthropic noises. This type of noise alters the conditions of the acoustic environment of many habitats, creating new environmental pressures that directly affect many animals that communicate acoustically, including frogs (Barber et al., 2010; Desrochers & Proulx, 2017; Knight & Swaddle, 2011; Sabah et al., 2017), birds (Bermúdez-Cuamatzin et al., 2009; Herrera-Montes & Aide, 2011; Slabbekoorn & Ripmeester, 2008) and marine mammals (Melcón et al., 2012; Moore & Clarke, 2002; Stocker, 2002). Among the anthropic noises, highways are considered the biggest source of noise pollution, producing sounds with high energies concentrated in low frequencies (<5 kHz) (Warren et al., 2006). The urban expansion, and consequently the road network, not only decreases the availability of habitats but also increases the amount of human noise, causing negative effects on the transmission and reception of sound between conspecifics (Bittencourt et al., 2016; Sun & Narins, 2005), and may even reduce the chances of survival of individuals (Gomes et al., 2022; Herrera-Montes & Aide, 2011). However, the species show several solutions to solve the problems in the communication limitation imposed by the noises, such as, changing the temporal and spectral acoustic parameters of the calls to reduce the noise masking effect (Cunnington & Fahrig, 2010, 2013; Grenat et al., 2019).
Among the strategies used by anurans to reduce or avoid the overlap between biotic and anthropic noises on their calls, there are changes in amplitude (Halfwerk et al., 2016; Parris et al., 2009; Yi & Sheridan, 2019), frequency (Caorsi et al., 2017; Cunnington & Fahrig, 2010), duration (Zhao et al., 2021) and emission rate (Hanna et al., 2014; Kaiser & Hammers, 2009; Legett et al., 2020). These changes can be advantageous when the individuals are under external influences, since the acoustic signals indicate the physical condition of the individuals. Therefore, they must be transmitted in the best possible way in the environment (Cunnington & Fahrig, 2010; Kime, 2000), just as the acoustic adaptation hypothesis predicts (Goutte et al., 2018; Morton, 1975). Thus, changes in parameters of the call may indicate an adaptation in response to noise, but they may generate additional fitness costs, negatively affecting survival and reproductive success (Herrera-Montes & Aide, 2011). It is often difficult to find evidence that suggests that changes in the calls of individuals observed in nature are caused by a single factor (Grenat et al., 2019), as variations and/or adjustments in acoustic parameters can be influenced by the environment (abiotic factors; (Kime et al., 2000)), size of choirs (social factors; (Gambale & Bastos, 2014; Morais et al., 2012)) and/or level of human noise (Caorsi et al., 2017). Therefore, there may be confounding factors when trying to explain changes in behavior if the study does not consider the multiple biological and environmental aspects to which individuals are exposed.
Biotic factors (body size, weight, predation and abundance of males in vocalization activity) and abiotic factors (temperature, humidity and vegetation heterogeneity) influence the anurans vocalizations in different ways. For example, body size influences the spectral structure (frequency) of the call, so that larger individuals present calls with lower frequencies (Kohler et al., 2017). Thus, acoustic signals provide reliable information about male body size (Bastos et al., 2011; Morais et al., 2012). The number of individuals in the chorus influences the intensity of the call as males increase sound pressure to promote greater attractiveness (Bastos & Haddad, 2002; Morais et al., 2012). As frogs are ectothermic animals, temperature influences the metabolic rate, reflecting changes in the temporal parameters of calls, such as duration and emission rate (Bastos & Haddad, 2002; Furtado et al., 2016). All these aspects must be considered in bioacoustics studies to avoid bias in the interpretation of results.
Since human activities have impacted the behavior of amphibians in different ways, in this work we evaluated whether the call of a Yellow Heart-tongued Frog species is affected by noise pollution produced by car traffic on highways and by the noise of conspecifics in the chorus. We hypothesized that (1) males exposed to anthropic noise (road traffic) will present a higher dominant frequency of the advertisement call to decrease or avoid signal masking, and that (2) males that vocalize in conspecific choruses with higher density of individuals will present higher values in the temporal parameters of the call (e.g., longer call duration and intensity) to increase the efficiency in signal transmission (and reduce or avoid overlapping of the call) in the environment. For this, we compared the acoustic parameters of advertisement calls of males of Phyllodytes luteolus(Wied-Neuwied, 1821) from natural and urban environments, and in the presence of loud and quiet choruses. Phyllodytes luteolus is an excellent model organism to test these hypotheses because it is a common species, forms reproductive choruses, uses acoustic signals as the main form of communication and is found in bromeliads in natural and urban environments (Forti et al., 2017; Salles & Silva-Soares, 2010).