Summary
Trade-offs shaping behavioral variation are often influenced by the environment. We investigated the role that the environment plays in mediating trade-offs using a widespread frog with a conspicuous mating display, Pseudacris crucifer. We first demonstrated, using playback and desiccation experiments, that calling site selection involves a trade-off between sound transmission and desiccation. We then determined the influence of local environmental conditions on the intensity of the trade-off by examining range-wide behavioral and environmental data. We showed that the benefit of improved call transmission is positively influenced by vegetation density and ground cover. Behavioral data are consistent with this relationship: sites with a greater transmission benefit have increased prevalence of arboreally calling males. We also found that the prevalence of arboreal calling behavior increases with relative humidity and air temperature, suggesting an influence of these environmental variables on the desiccation cost of arboreal calling. This study provides a clear example of the role of the environment in mediating trade-off intensities and shaping critical behavioral traits.
Local environment mediates the intensity of a trade-off associated with arboreal calling behavior in a treefrog. Combining observational and experimental approaches, we show that arboreal calling behavior increases the transmission of a mating call while potentially subjecting individuals to a rate of desiccation six times greater than terrestrial calling. Local environmental conditions influence both the benefit and the cost of this trade-off, subjecting different populations to varying trade-off intensities and shaping arboreal calling behavior.
Methodology
Variation in arboreal calling benefit
We performed call playbacks at 15 sites across the species’ range to test for variation in the benefit of arboreal calling. We created a playback loop consisting of five single calls (i.e., “peeps”) from each of 18 males from across the species range. Calls were standardized to an amplitude of 87 dB based on the mean amplitude from Brenowitz et al. (1984), using Audacity (Version 2.1.1; http://www.audacityteam.org), and were broadcast at 87 dB using a Braven (Irvine, CA) BRV-1 speaker at ground level and 1.2 m aboveground; 1.2 m was chosen as it was the highest perch height that we had observed a calling individual in our range-wide surveys at the time of study design. The playback was re-recorded using a Marantz (Marantz America, Mahwah, NJ) PMD-660 with a Sennheiser, Wedemark, Germany (Point Claire, PQ, Canada) ME67 directional microphone at 2- and 10-m distances to investigate possible differences between short-range propagation and long-range propagation. Playbacks were always recorded at ground level as we have rarely observed females in arboreal perches.
Cost of arboreal calling behavior
We tested the effects of perch height on EWL and body temperature using plaster models deployed in a small marsh at the Queen’s University Biological Station (Chaffey’s Lock, ON, Canada; 44.571121, −76. 330742) over two nights with similar weather conditions (10 and 11 June 2017). This palustrine marsh is dominated by Typha and Salix spp. and is known to have recurring P. crucifer breeding assemblages (Klaus 2012).
We made models that were approximately the mass of a sexually mature male spring peeper (1.5 g based on data from 489 frogs; Cairns NA, unpublished data) and of similar surface area (8.3 cm2) using the formula described in McClanahan and Baldwin (1969). We used the silicone mold Smooth-On (Macungie, PA) 133 Downloaded from https://academic.oup.com/beheco/article-abstract/31/1/132/5582220 by Queen's University user on 02 June 2020 Behavioral Ecology OOMOO-30 to create a rectangular mold of the model shape that fit these parameters (length: 2 cm, width: 1.03 cm, height: 0.7 cm). Unpainted plaster of Paris has been shown to be suitable for creating models of wet-skinned amphibians as they experience rates of EWL and surface temperature changes similar to live animals (Tracy et al. 2007; Peterman et al. 2013). Following Peterman et al. (2013), we mixed four parts plaster of Paris with three parts of water, and poured the solution into the molds. The plaster dried for at least 1 h before removal. We did not paint the plaster as this alters the evaporative properties of the model (Peterman et al. 2013). We cured the plaster models in an incubator at 65 °C for at least 4 h, then labeled the models with individual identifiers using a permanent marker. We weighed each model twice to the nearest 0.01 g using a portable electronic scale (Durascale, MyWeigh, Phoenix, AZ) and averaged the measurements to determine the dry mass. Before deployment in the field, we soaked the models in water for 4 h and weighed them twice to determine the average starting wet mass.
We initially included three treatments for our models: 1.2 m above the ground, 0.05 m above the ground, and on the ground with 30% of the model in water, mimicking observations of calling male spring peepers. However, the treatment with the models placed partially in water was excluded from subsequent analyses as the models disintegrated over the course of the night, skewing the results of their net loss of mass. We set-up twenty-five 2-m tall stakes along the periphery of the water’s edge where the density and stature of vegetation was approximately consistent among stakes. We used small finishing nails driven into the stakes parallel to the ground to serve as stands for the models. We deployed the models at 8 PM local time (GMT-4) and collected them at midnight, similar to the timing of chorusing activity observed at the study site. The surface temperature of the models was taken before removal from the perch using a laser thermometer (Raytek Minitemp MT6, Santa Cruz, CA) and the models were immediately weighed to determine their final wet mass.