Analyzing acoustic interactions in natural bullfrog (Rana catesbeiana) choruses.

During their breeding season, male anurans of many species form aggregations or choruses in which they vocally advertise their presence, possession of a territory and willingness to mate. These choruses can be quite dense, both spatially and acoustically. This density imposes significant perceptual demands on the chorus residents. Males need to regulate the timing of their own calls to minimize interference or masking by the calls of neighbors and to facilitate efficient broadcasting of their calls to recipient females. Field recordings and playback experiments have identified particular strategies males adopt to solve this task, with synchrony of calls or alternation of calls between neighbors being the most common (reviews: Gerhardt & Huber, 2002; Wells & Schwartz, 2007). An individual male within a chorus can also acquire important information about the identity and location of other chorus members by listening to their calls (Boatright-Horowitz et al., 2000; Davis, 1987). Analysis of interactions between chorusing males suggests that males space themselves within choruses and respond to each other by means of certain behavioral “rules” (Boatright-Horowitz et al., 2000; Greenfield & Rand, 2000). During their spring/summer breeding season, male bullfrogs (Rana catesbeiana) form nightly choruses in ponds or lakes and broadcast advertisement calls, both to attract females for mating and to advertise their presence to rival males. The structure of these choruses is typically quite stable, with individual male frogs occupying essentially the same locations over periods of days, weeks or even months (Boatright-Horowitz et al., 2000; Howard, 1980; Ryan, 1980). The possession of stable, well-defended territories and the prolonged breeding season facilitates familiarity among neighboring males. To a large extent, interactions between males are acoustically mediated. Each male produces advertisement calls periodically, but there are considerable between-male differences in both temporal and spectral properties of these calls, including differences in call rate, fundamental frequency and note duration (Bee & Gerhardt, 2001; Bee, 2004; Simmons, 2004). Within an active chorus, however, it is not always possible to distinguish calls of individuals by spectral or temporal properties alone because calls of multiple bullfrogs can occur simultaneously or with significant overlap in time. Moreover, successive notes from the calls of the same individual vary in envelope modulation (Suggs & Simmons, 2005), which produces additional spectral cues that may be difficult to segregate from those in notes of neighboring males. An alternative or supplemental means of individual identification is to identify the sources of calls using information about the relative spatial locations of males in the chorus. Some individuals are located in close proximity while others are spaced further apart, so that any given male receives an assortment of calls from other males in different directions and at different distances (Boatright-Horowitz et al., 2000). The use of both kinds of information (acoustic cues and spatial location) can provide the means of reliably distinguishing individual callers, and then describing their acoustic interactions with other callers. Our understanding of the structure and dynamics of frog choruses has been limited, however, by technical aspects involved in first recording and then sorting and identifying calls of individual males in a dense, noisy chorus in such a manner that all of the relevant information can be obtained. Much of our knowledge of vocal interactions between chorusing male frogs is based on responses to sound playbacks by individual focal males (often separated from other chorusing males), or on recordings of natural vocal interactions between small groups (two through five) of callers within a larger chorus (e.g., Arak, 1983; Brush & Narins, 1989; Klump & Gerhardt, 1992; Rosen & Lemon, 1974; Schwartz, 1987). Much of this work relies on the use of single microphones for localizing and identifying calling males. While multi-channel recording and call monitoring systems have been described (Brush & Narins, 1989; Grafe, 1996; Schwartz et al., 2002), they have not as yet been widely adopted, even though such techniques offer the ability to analyze choruses over large spatial and temporal scales. Grafe (1997) monitored chorusing behavior of male painted reed frogs (Hyperolius marmoratus) during female phonotaxis using an array of four widely-spaced microphones. Locations of calling males were derived by triangulation based on arrival time differences of vocalizations at pairs of microphones. The focus of this study was on female preferences and not on chorusing dynamics, so vocal interactions between calling males were not analyzed in detail. The array used by Grafe (1997) is similar to those developed for analyses of songbird vocal behavior (McGregor, Dabelsteen, Clark, Bower, Tavares, & Holland, 1997; Merrill, Burt, Fristrup, & Vehrencamp, 2006). While these techniques are promising, recordings and interpretations of vocal interactions within dense, natural choruses taken as a whole and over long chorusing times remain relatively rare, both in anurans and in other chorusing animals (D’Spain & Batchelor, 2006; Greenfield & Snedden, 2003). In this paper, we describe a technique developed for acoustic surveying that we adapted for the task of identifying individual male bullfrogs in a large active chorus. Our technique is based on audio recordings obtained using a pair of cube-shaped acoustic sensors, each containing multiple closely-spaced microphones, placed in two separate locations around the chorusing site. Data recorded by the sensors are subsequently processed by a computational model of the peripheral auditory system (Mountain, Anderson, Bresnahan, Brughera, Deligeorges, Hubbard, Lancia, & Vajda, 2007) that provides acoustic analysis of recorded sounds as well as estimates of sound source direction around each cube. Because information about the acoustic characteristics of bullfrog vocalizations and about the location of individual callers can be derived from the same records, this is a promising methodology to fully characterize acoustic interactions in noisy environments.