Water‐borne and plasma corticosterone are not correlated in spotted salamanders

Abstract

Water‐borne hormone measurement is a noninvasive method suitable for amphibians of all sizes that are otherwise difficult to sample. For this method, containment‐water is assayed for hormones released by the animal. Originally developed in fish, the method has expanded to amphibians, but requires additional species‐specific validations. We wanted to determine physiological relevance of water‐borne corticosterone in spotted salamanders (Ambystoma maculatum) by comparing concentrations to those taken using established corticosterone sampling methods, such as plasma. Using a mixture of field and laboratory studies, we compared water‐borne corticosterone levels to other traditional methods of sampling corticosterone for spotted salamander larvae, metamorphs, and adults. Despite multiple attempts, and detecting differences between age groups, we found no correlations between water‐borne and plasma corticosterone levels in any age group. Water‐borne sampling measures a rate of release; whereas plasma is the concentration circulating in the blood. The unique units of measurement may inherently prevent correlations between the two. These two methods may also require different interpretations of the data and the physiological meaning. We also note caveats with the method, including how to account for differences in body size and life history stages. Collectively, our results illustrate the importance of careful validation of water‐borne hormone levels in each species in order to understand its physiological significance.

Androgen responsiveness to simulated territorial intrusions inAllobates femoralismales: evidence supporting the challenge hypothesis in a territorial frog

“…The positive correlation between plasma and water-borne androgens in A. femorali s is in line with that found in other species e.g. fishes and amphibians (Baugh et al, 2018; Gabor et al, 2016, 2013; Kidd et al, 2010; but see Millikin et al, 2019 for non-correlation between water-borne and plasma corticosterone in spotted salamanders). Water-borne sampling has enormous advantages for estimating hormonal concentrations with little manipulation of the research animals (Narayan, 2013).…”

Exposure to glucocorticoids alters life history strategies in a facultatively paedomorphic salamander

“…Our highest CORT dose produced whole‐body content levels of 2961.38 ± 321.25 pg/g (mean ± SE ), which were similar to whole‐body content found in larval A. jeffersonianum exposed to confinement protocol (~3000 pg/g) by Chambers et al (2011). Other studies on Ambystoma CORT content mostly considered only larger adults and only measured blood plasma CORT content, which ranged from approximately 200–45000 pg/ml (Carr & Norris, 1988; Cooperman et al, 2004; Homan et al, 2003a; 2003b; Houck et al, 1996; Millikin et al, 2019). However, Baugh et al (2018) showed there is a fairly tight correlation ( r = .81) between blood plasma and whole‐body CORT content (in frogs), but that whole‐body CORT levels measured approximately 80%–85% lower than plasma CORT.…”

Evaluating corticosterone as a biomarker for amphibians exposed to increased salinity and ambient corticosterone

“…Waterborne CORT has been physiologically and biologically validated for many fish and amphibian species ( Scott and Ellis, 2007 , Narayan, 2013 , Narayan et al , 2019 ). For physiological validation, studies often compare waterborne to plasma or whole-body CORT to ensure concentrations in water correlate with endogenous concentrations of CORT ( Scott and Ellis, 2007 , Gabor et al , 2013a , Millikin et al , 2019 ). However, when animals are too small to obtain plasma, extracting CORT from other tissues is necessary ( Millikin et al , 2019 , Hall et al , 2020 ).…”

“…For physiological validation, studies often compare waterborne to plasma or whole-body CORT to ensure concentrations in water correlate with endogenous concentrations of CORT ( Scott and Ellis, 2007 , Gabor et al , 2013a , Millikin et al , 2019 ). However, when animals are too small to obtain plasma, extracting CORT from other tissues is necessary ( Millikin et al , 2019 , Hall et al , 2020 ). CORT is synthesized in the interrenal gland in amphibians ( Johnston et al , 1967 ) and recovery of CORT is generally greater from interrenal glands than whole body (EJC, unpublished data).…”

“…Expected and observed values were linear for each comparison ( P < 0.001 and R 2 > 0.984; Supplementary Table 1 ). Recovery was calculated by dividing observed by expected concentrations ( Millikin et al , 2019 ) and minimum recovery ranged from 79–128%. Sensitivity of plates ranged from 8.81–42.95 pg/ml and all samples had higher concentrations than sensitivity of respective plates.…”