Communication signals are found across taxa in various modalities (Moulton 1968; Gerhardt and Huber 2002; Schaefer 2010) and used in a variety of contexts including species recognition, mate attraction, and territorial defense (Gerhardt 1992; Seyfarth and Cheney 2003; Kaplan 2014; Rubio et al. 2024a). Due to their key roles in mate attraction and species recognition, acoustic signals are often under strong selective pressure, exhibit rapid evolutionary change, and can be used to predict patterns of species diversification (Jones and Others 1997; Seddon et al. 2008; Wilkins, M. R., Seddon, N., & Safran, R. J. 2013). Acoustic signals and any associated preferences are influenced by several sources of selection, such as ecology, morphology, genetics, and sensory perception of the species ultimately leading to signal diversification (Ryan 1986; Ryan et al. 1992; Hoy 1992; Derryberry 2009; Xu and Shaw 2021).
The perception of acoustic signals, or hearing, is associated with sensitivity to signal properties and the ability to discern various acoustic signals from background environmental noise. In the context of intraspecific communication, the evolution of sensory systems are generally expected to be accompanied by the evolution of communication signals, allowing for a potential match between acoustic signals and a receiver’s perception that is predicted to increase signal efficiency in noisy environments (Capranica and Moffat 1983; Boake 1991). The matched filter hypothesis suggests that the sensitivity of signal receivers to acoustic signals will evolve to be finely tuned to match the spectral distribution of the sender’s signal and maximize the signal-to-noise ratio (Capranica and Moffat 1983; Yang et al. 2019b; Cobo-Cuan and Narins 2020). This implies that preferences for spectral properties, such as dominant frequency, may be modulated by individual differences in perception. This hypothesis has been well supported in anurans, where female inner ear sensitivity is often tuned to the spectral characteristics of male advertisement calls, helping to distinguish conspecific calls from background noise (Narins et al. 2004; Feng et al. 2006; Moreno-Gómez et al. 2013; Zhao et al. 2017) and heterospecific calls (Luther 2009; Siegert et al. 2013; Goutte et al. 2017; Yang et al. 2019b). Additionally, sex differences in hearing sensitivity have also been identified, suggesting that male inner ear sensitivity may also be tuned to the conspecific calls to aid in the establishment of territory (Narins and Capranica 1976, 1980; Shen et al. 2011; Zhu et al. 2017).
There are exceptions to the matched filter hypothesis, meaning hearing sensitivity is not tuned to match the acoustic signals of that species (Zhao et al. 2017; Hoke et al. 2022). For example, perception and signaling can evolve separately under different selective regimes, such as the perceptual system evolving under high background noise and causing preferences for higher frequencies to avoid masking (Zhao et al. 2017). In addition, behavioral characteristics, such as the territoriality of a species, can lead to a mismatch between signal and perception. In one example of this, divergent lineages of Phyllobates possess a mismatch between signals and perception that allows for the flexibility to recognize both conspecific and heterospecific signals that may belong to potential competitors (González-Santoro et al. 2023).
Genetic mechanisms underlying the development and maintenance of hearing have been explored in mammals (Bowl et al. 2017; Marcovich and Holt 2020; Trigila et al. 2021), birds (Lomax et al. 2001; Sadanandan et al. 2023), insects (Wiley et al. 2012; Warren and Eberl 2024), and anurans (Serrano et al. 2001; Mangiamele and Burmeister 2008). However, few studies had addressed these mechanisms in relation to ecological divergence in acoustic signaling. Recently, Linderoth et al. (2023) identified significant divergence in poison frogs in two putatively hearing-related genes: Synaptojanin1 (synj1) and Apoptosis Inducing Factor Mitochondria Associated 1 (aifm1). In other taxa, mutations in synj1 are associated with inner hair cell dysfunction and the highly similar gene Synaptojanin2 is associated with hearing loss (McDermott et al. 2007; Trapani et al. 2009; Uthaiah and Hudspeth 2010; Manji et al. 2011; Martelletti et al. 2020). With respect to the aifm1 gene, mutations are associated with auditory neuropathy as well as hearing loss and deafness in humans (Rinaldi et al. 2012; Zong et al. 2015; Elrharchi et al. 2020; Kawarai et al. 2020; Wang et al. 2020). Experimental knock-ins of an aifm1 mutation associated with auditory neuropathy show progressive hearing loss and abnormal hair cell mitochondria morphology (Shi et al. 2024). While the functions of these genes have been characterized in mammals, their roles in anuran hearing, particularly in an ecological context, have not been investigated.
Neotropical poison frogs (family Dendrobatidae) are an interesting system for studying divergence in acoustic signaling and perception. Call diversification in poison frogs is thought to have increased due to aposematism reducing predation, which in turn allowed acoustic signals to become exaggerated through sexual selection (Santos et al. 2014). In frogs, calls are essential for species recognition, and all measured call traits show a strong phylogenetic signal within poison frogs, suggesting that they have contributed significantly to the diversification within this group (Erdtmann and Amézquita 2009). It has been suggested that advertisement calls are used over long distances for mate attraction until visual signals are able to be used over short distances, at which point multimodal signaling will occur in both mating and agonistic interactions (Ursprung et al. 2009; Dreher and Pröhl 2014; Mayer et al. 2014). For instance, the strawberry poison frog (Oophaga pumilio) mates assortatively by color in the wild (Summers et al. 1999; Gade et al. 2016; Yang et al. 2019a). However, when presented with both local and non-local male advertisement calls, female O. pumilio prefer local male calls, regardless of the color model presented (Dreher and Pröhl 2014). This suggests that acoustic signals in this species are of primary importance for mediating mating interactions when multimodal signals are present (Dreher and Pröhl 2014).
The mimic poison frog, Ranitomeya imitator, has undergone a Müllerian mimetic radiation resulting in four distinct color morphs (striped, spotted, banded, Varadero red-headed) with three known transition zones between them (striped-banded, striped-spotted, and striped-varadero) in northern Peru (Fig. 1). Incipient speciation is hypothesized in R. imitator based on divergence in color morphs within the striped-varadero transition zone (Twomey et al. 2014, 2016). There is limited evidence of assortative mating by color morph, specifically within the striped-varadero transition zone as well as the allopatric banded populations (Twomey et al. 2014, 2016). A recent study found that male aggression is not associated with the visual phenotypes of color morph and body size (Bieri et al. 2024). This suggests other modalities, such as acoustic signals, may be important for mediating interactions in R. imitator. Ranitomeya imitator acoustic signals show significant variation with respect to color morph in three call traits: note length, pulse rate, and dominant frequency (Twomey et al. 2015), though the drivers and function underlying this variation are unknown. A significant amount of work has been put into unraveling the genetic mechanisms and function of visual signaling in R. imitator (Twomey et al. 2020; Linderoth et al. 2023; Rubio et al. 2024c, b; Stuckert et al. 2024), however, no studies have attempted to investigate the genetic mechanisms associated with acoustic signaling and hearing perception in this system.
Linderoth et al. (2023) found that the hearing-related genes synj1 and aifm1 show significant divergence between the striped and banded morphs of R. imitator. The matched filter hypothesis implies that hearing and perception—which are affected by genes such as synj1 and aifm1—may play a role in the species recognition and signal divergence of R. imitator. A mismatched relationship between acoustic signals and perception may arise because signal and perception may diverge at different rates depending on the strength of natural and sexual selection (Gerhardt 1994; Wilczynski et al. 2001). In this study we investigated whether R. imitator call traits vary across their range and with respect to color morph. Further, we investigated the genotypic differences between populations and whether these differences are associated with call traits. We predicted that allelic variants of synj1 and aifm1 would be associated with call traits if they are playing a role in the acoustic signal perception of R. imitator.