Study species

Dendropsophus sanborni is a relatively small Neotropical treefrog of the family Hylidae that exhibits a prolonged reproductive pattern with territorial males that use vegetation surrounding flooded areas as calling sites where they organize lek mating systems (Cardoso, 1981; Martins and Jim, 2003; Toledo et al., 2003; Both et al., 2008; Santos et al., 2008; Haddad et al., 2008). The species usually occurs in high abundances, which makes this hylid an excellent model for studying social organization and reproductive dynamics over time.

Table 1.

Abundance of calling males recorded by monthly auditory counts (one count per month of the number of males calling; see Materials and Methods for details) and total number of calling males captured at the Cantaclaro ranch, Itapé, Rio Claro, São Paulo, Brazil. The ratio row shows how representative our monthly samples were.

Study area and data collection

We conducted our study at the Cantaclaro ranch (22°19′30″S, 47°42′52″W, 620 m above sea level), located in the Distrito de Itapé, municipality of Rio Claro, state of São Paulo, southeast Brazil. The ranch is about 20 km northwest of Rio Claro and about 15 km southeast of municipality of Itirapina. The relief of this rural region comprises hills and valleys and was originally an ecotone between Semideciduous Seasonal Forest and Cerrado (savannah vegetation), but it is currently extensively modified by agricultural and livestock activities. According to the classification of Koeppen, the climate of the region is of the type Cwa, marked by two defined seasons: a dry cold season, usually from April to September, and a rainy hot season, usually from October to March (Zaine and Perinotto, 1996). The study area encompassed a zone located along a slow-flowing stream in a valley, where a 90-m transect was demarcated with 10 wooden stakes, numbered 0–9, spaced at 10-m intervals.

We conducted fieldwork during the course of 1 year, starting in October 2015 and ending in September 2016, with one field visit per month. To determine the length of the species' breeding season we recorded the presence or absence of calling males every month (reproductively active males). We also estimated the monthly abundance of calling males by performing one auditory count per month. These auditory counts were performed with intervals of ca. 30 d between them and always at around 22:00 (a time that has high activity of calling males), thus avoiding subsequent sampling while also standardizing data collection in the field. These counts were performed while walking along the transect at a constant speed and registering every calling male heard (Heyer et al., 2001). Our activities in the field were conducted from 18:00–01:00, depending on the activity of calling males. We captured calling males via acoustic tracking to measure snout–vent length (SVL; mm) with digital calipers (Mitutoyo, model MIP/E-102, to the nearest 0.01 mm).

We captured a large number of calling males during each expedition and along the transect (Table 1), without prioritizing any specific area, to ensure that the choice of the individuals sampled was random. In order to document the temporal distribution of individuals during the breeding season, we marked and captured males using a standardized method consisting of the ablation of digits in a sequential order (see Martof, 1953). Recaptures were randomized by following the same method employed for captures. In addition to marking, we also recorded characteristics of calling sites (height and type of perches). These methods also allowed us to determine how the sizes of calling males are organized in space, considering that social structure is based on size. After marking and measuring, we released the calling males exactly where we found them.

Given that anurans are ectothermic and have life cycles that are strongly associated with water, we selected specific climatic traits that could potentially influence or limit the reproductive activity of males of the species, with a focus on understanding the seasonal reproductive dynamics of Dendropsophus sanborni (Saenz et al., 2006; de Sá et al., 2014; Caldart et al., 2016; Plenderleith et al., 2018). During each field visit we also recorded the meteorological conditions of air and water temperature and relative air humidity using a digital thermohygrometer (Incoterm, model 7429, to the nearest 1°C and 5% relative air humidity). We complemented our meteorological data with information about the total monthly rainfall and monthly air temperature of the study region collected by the weather station of the Centro de Análise e Planejamento Ambiental (CEAPLA/IGCE) at UNESP, Rio Claro, SP, Brazil, located ca. 18 km from the study area.

Figure 1.

Number of calling male Dendropsophus sanborni per month (bars), total monthly rainfall (mm, line), and air (dashed line) and water (dotted line) temperatures (°C) per month at the Cantaclaro ranch, Itapé, Rio Claro, São Paulo, Brazil. We converted total monthly rainfall and temperatures to log scale for better visualization of the correlation of these variables with the abundance of calling males.

Correlations between abundance of calling males and environmental factors were analyzed using Spearman coefficients (rs) because abundance of calling males was not normally distributed. To test for variation in monthly mean SVL of calling males, we performed an analysis of variance (ANOVA). Likewise, we used ANOVA to test for variation in mean SVL of calling males among distinct perch heights. We checked for homogeneity of variances and normality of residuals to validate the test. We considered statistical significance at the level of 5% (Zar, 1999) and used the statistical software BioEstat 5.3 (Santos, 2007).

Seasonal reproductive activity

The breeding season of Dendropsophus sanborni in the region of Rio Claro, São Paulo, Brazil, has a duration of 7 months, beginning in September and ending in March, with a peak in January, and thus can be classified as prolonged (sensu Wells, 1977; Wells, 2010). Toledo et al. (2003) recorded the same period of activity for D. sanborni in another locality in the municipality of Rio Claro. Although it seems D. sanborni always exhibits a prolonged reproductive pattern, its period of reproductive activity can vary among different areas of its occurrence. For example, Santos et al. (2008) reported reproductive activity for D. sanborni from August to May in the municipality of Santa Maria, state of Rio Grande do Sul, South Brazil, with sporadic vocalizations during hotter nights in July. Near Santa Maria, Ximenez and Tozetti (2015) recorded reproductive activity between August and March. Our study indicates that the length of the breeding season is associated with abiotic factors, such as rainfall, air and water temperatures, and relative air humidity. Thus, the variation observed in the length of the breeding season of D. sanborni among locations most likely follows variation related to the distinctive characteristics of each locality, particularly variation associated with climate and microhabitat.

We observed that the beginning and end of the breeding season of Dendropsophus sanborni coincide with the beginning and end of the rainy season, respectively (as defined by Koeppen; see Zaine and Perinotto, 1996). The seasonal distribution of anuran species activity can be influenced by several interrelated factors that are subject to daily, monthly, and annual variation (such as maximum and minimum temperatures, rainfall, and hydroperiod; e.g., Kopp and Eterovick, 2006; Hartel et al., 2007; Gally and Zina, 2013; Akmentins et al., 2015; Hurme, 2015; Luna-Gómez et al., 2017; Leivas et al., 2018). Thus, interactions of these factors may be responsible for variation in the reproductive periods of anurans. For D. sanborni, we observed that in addition to rainfall, other climatic traits influenced reproductive activity, including those for the region obtained from the meteorological station (mean minimum air temperature, minimum monthly air temperature, and maximum monthly air temperature), as well as those we measured directly during the nights of our field visits (air temperature, water temperature, and relative air humidity). The colder and drier season clearly limited the activity of the species, whereas the hotter and rainier season provided abiotic conditions favorable for breeding activity (see Wells, 1977; Shahriza et al., 2010; Van Sluys and Guido-Castro, 2011; Waldez et al., 2011; Schalk and Saenz, 2016).

Temporal and spatial social structure

Larger calling males were not detected at the beginning of the species' breeding season, at the end, nor in November, and variation in body size in the chorus over months was not statistically significant. The absence of larger males at the beginning and the end of the season is most likely due to still unideal climatic conditions for reproduction. Our results demonstrated statistically that males of all sizes (large, medium, and small) called throughout the entire rainy season, suggesting that a single, marked moment for breeding recruitment of younger (smaller) males and a temporally ordered social spatial structure based on body size do not occur in Dendropsophus sanborni reproduction.

Males of Dendropsophus sanborni called on perches 1–70 cm in height and exhibited preference for intermediate heights, which might represent a trade-off between propagation of vocalizations and exposure to sound-oriented predators (see Zuk and Kolluru, 1998; Amézquita and Hödl, 2004; Jacobs and Bastian, 2017). However, variation in body size of calling males among perch heights was not statistically significant, indicating that the vertical spatial organization of males is not based on body size. Further, the majority of males were observed using the same type of vegetation as calling sites, demonstrating that the use of calling sites is not based on size either. Overall, the microhabitats most preferred by males for calling sites likely reflect the best habitat characteristics and the most appropriate conditions for the reproductive mode of the species (e.g., perch height, perch type, water and soil distances; see Haddad and Prado, 2005; Dias et al., 2014; Cunha and Napoli, 2016; Mageski et al., 2016). Dendropsophus sanborni has a reproductive mode characterized by floating oviposition in aquatic vegetation, using lentic or lotic and permanent or temporary waterbodies (Cardoso, 1981; Haddad and Prado, 2005; Both et al., 2008), traits that are clearly linked to the microhabitats used by males as calling sites as recorded in the present study.

Dendropsophus sanborni males have been reported in the literature to be territorial, with intrasexual disputes primarily with the emission of advertisement calls and secondly with physical aggression (see Cardoso, 1981; Martins and Jim, 2003). According to Cardoso (1981), D. sanborni exhibits intense competition, with calling males spaced by short distances in the chorus and with frequent physical fights. Although we did not measure the distances among males, we also did not observe physical combat during the breeding season. Our data indicate that competition for territories was not intense and also that calling males did not resort to fights. Physical combat is generally avoided and used only as a last option during territorial disputes, since fights are energetically costly (and deviate energy from reproduction) and cause physical wear, injuries, and increased exposure to predators (Robertson, 1986; Giasson and Haddad, 2006; Bastos et al., 2011; Morais et al., 2012; Pereira and Maneyro, 2016; Oitaven et al., 2017).

Our recaptures of marked calling males agree with the literature and indicate some degree of site fidelity, but they provide only minimal support (since recaptures were very rare) for the hypothesis that males occupy the same territories throughout the breeding season. Also, our three recaptured calling males encompassed all of our size classes (small, medium, and large), suggesting that endurance of calling males is not associated with body size in D. sanborni, but it is also difficult to make any strong statement about the relationship between endurance and body size with few recaptures.

Our low recapture rate also suggests a high frequency of replacement of calling males throughout the rainy season, thereby significantly changing the composition of the chorus monthly. Given that our study was carried out in a 90-m transect along a slow-flowing stream, and that connections with other areas suitable for reproduction of Dendropsophus sanborni are present, a high rate of male replacement could be explained by migration and dispersion events, with the males that we sampled actually representing a subpopulation of D. sanborni. According to Pellet et al. (2007) and Wagner et al. (2011), superpopulations may be common for anurans, with individuals moving among subpopulations. Migration, dispersion, and time of permanence in each subpopulation are usually affected by competition, availability of breeding resources, and quality of microhabitats. Alternatively, our replacement hypothesis is supported by the fact that males of prolonged breeding anurans do not necessarily remain reproductively active for an entire breeding season (Grafe and Meuche, 2005; Castellano et al., 2009). Finally, other ecological mechanisms such as predation might also have acted as a selector of males, affecting the social structure of D. sanborni (see Ryan et al., 1982; Benard, 2007).

It is well documented that body size is an important trait for sexual selection and for defining territory (see Robertson, 1986; Wells, 2010; Dittrich et al., 2018), but our study did not show any spatial organization based on this trait. Usually, larger males are more successful in physical combat, and thus are more able to occupy better locations during best periods of the breeding season (Berec, 2017; Clause, 2017; Dias et al., 2017; Ospina-L. et al., 2017). Endurance favors those who remain reproductively active for longer periods of time and larger males may also spend more time as components of breeding choruses and, consequently, increase their reproductive successes (Halliday and Tejedo, 1995; Friedl and Klump, 2005; Sullivan and Kwiatkowski, 2007; Castellano et al., 2009; Keogh et al., 2012; Basto-Riascos et al., 2017; Ospina-L. et al., 2017). de Sá et al. (2014) observed size-organized temporal and spatial social structure for calling males, along with breeding recruitment of younger males, in Boana albopunctata (Spix, 1924) studied at the same locality as the present study. These authors observed that larger males of B. albopunctata initiated reproductive activity when resources were still scarce and, as months passed, larger males gradually ceased their activity, thus providing space for smaller males to breed. Therefore, for Dendropsophus sanborni we expected to observe that larger males would occupy better positions in time and space during the breeding season and would possess territories for longer periods of time, engage in combat, and occupy perches as calling sites mostly over the water and at intermediate heights. We also expected some recruitment of younger individuals towards the end of the breeding season. Together, these reproductive characteristics would indicate strong territorial behavior among males with size-driven advantages; however, we observed the opposite in D. sanborni.

While our first hypothesis, that the seasonal dynamics of Dendropsophus sanborni are conditioned by abiotic factors, was supported, our second hypothesis, that calling males are organized spatially and temporally based on body size, was unexpectedly refuted. We know that social organization in anurans is, in general, dependent on a combination of endogenous and exogenous factors (Sullivan and Kwiatkowski, 2007; López et al., 2011; Llusia et al., 2013; Steen et al., 2013). For example, McCaffery and Lips (2013) observed that the availability of breeding sites for Espadarana prosoblepon (Boettger, 1892) limited population abundance, whereas Kluge (1981), studying reproductive resources in Boana rosenbergi (Boulenger, 1898), observed that population abundance influenced male territorial behavior. Given these findings, we believe that the non-size-determined spatiotemporal structure and absence of strong territoriality observed for calling males of D. sanborni in our study are likely a consequence of the relatively small size of individuals of the species combined with the high abundance of vegetation used as calling sites in the studied area. We were most likely watching a scenario of low intrasexual competition among calling males. Alternatively, studies have indicated that for some lek-breeding anurans there are no reproductive advantages for larger males and that their reproductive strategies favor traits and behaviors other than body size and physical combat (Kluge, 1981; Gerhardt et al., 1987; Pröhl, 2003; Smith and Roberts, 2003; Friedl and Klump, 2005; Martin et al., 2011; Jacobs et al., 2016; Zhu et al., 2016). Therefore, we encourage future studies on D. sanborni to make more frequent observations per month, and directly in the field, in order to better understand the territoriality of calling males. Such efforts should focus on better detecting frequencies of physical combat, time spent by males in the chorus, potential migrations and dispersions, as well as the relationships between these behaviors and body size. Furthermore, studies including neighboring subpopulations might also help to better understand the relationship between spatiotemporal social organization and body size of calling males of D. sanborni.