Acoustic parameters of Phyllodytes luteolus in conditions of low and high noise from vehicle traffic

Acoustic communication in animals can be affected by multiple biotic (intra and interspecific) and abiotic (e.g., wind and rain) natural noises. In addition, human beings produce additional novel sources of noise, which can reduce or inhibit the reception of acoustic signals by conspecifics, leading to behavioral changes. In this study, we investigated whether sound of conspecifics and road noise additively affect the acoustic parameters of the advertisement call of males of a Yellow Heart-tongued Frog (Phyllodytes luteolus). We hypothesized that males that vocalize in larger choruses (many males) and in areas close to highways (anthropic noise) will increase their temporal and spectral acoustic parameters, respectively, to avoid acoustic signal masking. We recorded the vocalizations of 38 males in environments close (N=18) to and distant (N=20) from highways in different social contexts (many or few individuals in the chorus). Contrary to our expectation, the results indicated that individuals exposed to road noise had lower dominant frequency calls than those from natural areas, and that the density of males in the chorus had no influence on the acoustic parameters. Furthermore, we found a positive relationship between body size and intensity, indicating that larger individuals can emit calls that can reach greater distances. The advertisement call of Phyllodytes luteolus has a high dominant frequency, with little overlap with the frequency of anthropic noises (roads), which may explain its presence and reproductive success of this species in bromeliads from urbanized areas.

Methods
Field activities were carried at five sampling sites in the municipality of Ilhéus, Bahia, Brazil. All areas have bromeliads used as reproductive sites by Phyllodytes luteolus. Three sites with the presence of vehicle traffic noise were located next to the BA-001 highway (between 3 and 10 meters away from the highway), in the stretches between Olivença and Acuípe villages, in the municipality of Ilhéus. The sampling sites were defined as Natural (no traffic noise) or Highways (with traffic noise). For each recorded individual, we obtained the social situation in which the male was inserted, being: with chorus (four or more conspecific individuals in vocalization activity within a three-meter radius – CC) or without chorus (four or less conspecific individuals vocalizing within a radius of three meters – SC). Males of Phyllodytes luteolus in vocalization activity were found through visual and auditory searches between 19:00 and 24:00 hrs. After viewing the male, the advertisement calls were recorded using a Sennheiser ME66 directional microphone coupled to a Marantz PMD660 hand recorder at a distance of 0.5 meters from the vocalizing male. As P. luteolus males have a long silent interval between calls, each male was recorded for ten minutes, in order to obtain as many calls as possible. The acoustic parameters were extracted using Raven Pro 1.6 software. Three to five calls were analyzed for each individual. The acoustic parameters extracted were: call duration (s), number of notes, number of pulses per call, pulse duration (s), rise time (s), interval between calls (s), repetition rate (calls/min.) and dominant frequency (Hz). The acoustic terminologies followed Köhler et al. (2017). The sound pressure level (SPL; intensity) was obtained for three to five advertisement calls from each individual using a Minipa MSL-1301 decibel meter (range: 30-130dB; 125ms, fast) positioned directly in front of and 0.5 meters away from the male in vocalization activity. Shortly after the recordings were finished, the males were captured for measurement of the snout-vent length (SVL) with a Western caliper (accuracy of 0.1 mm). Air temperature and humidity were recorded using a thermo-hygrometer (TechnoLine WS 9440, 0ºC precision).