Repeatability of baseline corticosterone and short-term corticosterone stress responses, and their correlation with testosterone and body condition in a terrestrial breeding anuran (Platymantis vitiana)

Abstract

Repeatability of physiological response variables, such as the stress hormone corticosterone, across numerous sampling occasions is an important assumption for their use as predictors of behaviour, reproduction and fitness in animals. Very few studies have actually tested this assumption in free-living animals under uncontrolled natural conditions. Non-invasive urine sampling and standard capture handling protocol have enabled the rapid quantification of baseline corticosterone and short-term corticosterone stress responses in anuran amphibians. In this study, established non-invasive methods were used to monitor physiological stress and urinary testosterone levels in male individuals of the terrestrial breeding Fijian ground frog (Platymantis vitiana). Adult male frogs (n=20) were sampled at nighttime on three repeated occasions at intervals of 14days during their annual breeding season on Viwa Island, Fiji. All frogs expressed urinary corticosterone metabolite responses to the capture and handling stressor, with some frogs showing consistently higher urinary corticosterone responses than others. Ranks of corticosterone values at 0, 4 and 8h, and the corrected rank were highly significant (r=0.75–0.99) between the three repeated sampling occasions. Statistical repeatabilities were high for baseline corticosterone (r=0.973) and for corticosterone values at 2h (r=0.862), 4h (r=0.861), 6h (r=0.820) and 8h (r=0.926), and also for the total (inclusive of baseline corticosterone values) and the corrected integrated responses (index of the acute response) [r=0.867 and r=0.870]. Urinary testosterone levels also showed high statistical repeatability (r=0.78). Furthermore, variation in baseline and short-term corticosterone stress responses was greater between individuals than within individuals. Baseline urinary corticosterone was significantly negatively correlated with the corrected integrated corticosterone response (r=−0.3, p<0.001) but non-significantly with body-condition (r=−0.04) and baseline urinary testosterone (r=−0.07). In contrast, the corrected integrated corticosterone response was positively correlated (non-significantly) with baseline urinary testosterone (r=+0.04) and body-condition (r=+0.08). Urinary testosterone levels and body-condition were significantly negatively correlated (r=−0.23, p<0.001). The results suggest that male frogs with higher levels of testosterone could have depleted energy reserve during the breeding period. The acute corticosterone responses help in replenishing energy that is needed for breeding and survival. The results also provide some support to the ‘cort-fitness’ hypothesis as highlighted by the negative correlation between baseline corticosterone and body-condition. It is most likely that the acute corticosterone response is adaptive and linked positively with reproductive fitness and survival in male anurans.