Trading heat and hops for water: Dehydration effects on locomotor performance, thermal limits, and thermoregulatory behavior of a terrestrial toad

Abstract

Due to their highly permeable skin and ectothermy, terrestrial amphibians are challenged by compromises between water balance and body temperature regulation. The way in which such compromises are accommodated, under a range of temperatures and dehydration levels, impacts importantly the behavior and ecology of amphibians. Thus, using the terrestrial toad Rhinella schneideri as a model organism, the goals of this study were twofold. First, we determined how the thermal sensitivity of a centrally relevant trait-locomotion-was affected by dehydration. Secondly, we examined the effects of the same levels of dehydration on thermal preference and thermal tolerance. As dehydration becomes more severe, the optimal temperature for locomotor performance was lowered and performance breadth narrower. Similarly, dehydration was accompanied by a decrease in the thermal tolerance range. Such a decrease was caused by both an increase in the critical minimal temperature and a decrease in the thermal maximal temperature, with the latter changing more markedly. In general, our results show that the negative effects of dehydration on behavioral performance and thermal tolerance are, at least partially, counteracted by concurrent adjustments in thermal preference. We discuss some of the potential implications of this observation for the conservation of anuran amphibians.

Keywords: Bufonidae; critical temperatures; optimal temperatures; preferred body temperatures; tolerance range.

Figure 1

The toad Rhinella schneideri, a large‐bodied terrestrial anuran widely distributed in South America. Photograph by Lucas S. Almeida

Figure 2

Influence of dehydration level on the preferred body temperatures (T _pref), of Rhinella schneideri. The line inside the box plot, the borders, and the whiskers represent, respectively, the median, the second and third interquartile range, and the range of the data; dots are outliers. Different letters above boxplots indicate significant differences

Figure 3

Absolute locomotor performance of Rhinella schneideri, at different levels of dehydration, at five different temperatures. Each symbol represents the mean and the associated whiskers the confidence interval (95%)

Figure 4

Thermal performance curves of Rhinella schneideri at four different levels of dehydration (a = 100%; b = 90%; c = 80%; d = 70%). The red vertical line represents the optimal temperature, the gray area represents the thermal performance breadth (80% of maximal performance), the blue solid vertical line represents the mean T _pref, and the blue dotted lines represent the T _pref interquartile range (25th and 75th)

Figure 5

Optimal temperature for locomotor performance (T _o) of Rhinella schneideri at different levels of dehydration. The line inside the box plot, the borders, and the whiskers represent, respectively, the median, the second and third interquartile range, and the range of the data; points are outliers. Different letters above boxplots indicate significant differences

Figure 6

Upper B80 (blue) and lower B80 (red) values for Rhinella schneideri at four different dehydration levels. The dots represent the mean and the whiskers the confidence interval (95%). Different letters above boxplots indicate significant differences

Figure 7

Critical thermal minimum (a) and maximum (b) of Rhinella schneideri at different levels of dehydration. The line inside the box plot, the borders, and the whiskers represent, respectively, the median, the second and third interquartile range, and the range of the data; dots are outliers. Different letters above boxplots indicate significant differences

Figure 8

Mean tolerance range of Rhinella schneideri at different levels of dehydration. The whiskers and letters above the bars represent the standard deviation and significant differences, respectively