Blubber Cortisol: A Potential Tool for Assessing Stress Response in Free-Ranging Dolphins without Effects due to Sampling

Abstract

When paired with dart biopsying, quantifying cortisol in blubber tissue may provide an index of relative stress levels (i.e., activation of the hypothalamus-pituitary-adrenal axis) in free-ranging cetacean populations while minimizing the effects of the act of sampling. To validate this approach, cortisol was extracted from blubber samples collected from beach-stranded and bycaught short-beaked common dolphins using a modified blubber steroid isolation technique and measured via commercially available enzyme immunoassays. The measurements exhibited appropriate quality characteristics when analyzed via a bootstraped stepwise parallelism analysis (observed/expected = 1.03, 95%CI: 99.6 – 1.08) and showed no evidence of matrix interference with increasing sample size across typical biopsy tissue masses (75–150mg; r2 = 0.012, p = 0.78, slope = 0.022ngcortisol deviation/ultissue extract added). The relationships between blubber cortisol and eight potential cofactors namely, 1) fatality type (e.g., stranded or bycaught), 2) specimen condition (state of decomposition), 3) total body length, 4) sex, 5) sexual maturity state, 6) pregnancy status, 7) lactation state, and 8) adrenal mass, were assessed using a Bayesian generalized linear model averaging technique. Fatality type was the only factor correlated with blubber cortisol, and the magnitude of the effect size was substantial: beach-stranded individuals had on average 6.1-fold higher cortisol levels than those of bycaught individuals. Because of the difference in conditions surrounding these two fatality types, we interpret this relationship as evidence that blubber cortisol is indicative of stress response. We found no evidence of seasonal variation or a relationship between cortisol and the remaining cofactors.

Low reproductive success rates of common bottlenose dolphins Tursiops truncatus in the northern Gulf of Mexico following the Deepwater Horizon disaster (2010-2015)

“…The analysis was conducted within a Bayesian frame work to allow for better propagation of the measured uncertainty into the subsequent analyses. All blubber progesterone measurements were log transformed prior to analysis to minimize the heteroscedasticity that is common in hormone measurements (Kellar et al 2009(Kellar et al , 2015. The output generated estimates of the parameters for the best logistic regression fit and the 95% Bayesian credibility envelope around the re sulting function.…”

Cortisol levels in beluga whales (Delphinapterus leucas): Setting a benchmark for Marine Protected Area monitoring

“…Middle and outer blubber tissues are also ideal tissue when considering storage degradation factors of heat, light, and oxygen exposure that the inner blubber and sample edges are readily exposed to (Trana et al 2015); the use of the outer layer would allow for live biopsy collections. Lastly, these layers are known to reflect long-term storage of other compounds, such as fatty acids, vitamins, and contaminants, and have been suggested as an ideal tissue to reflect resting or chronic health (Kellar et al 2015). The design of a monitoring program must consider factors that influence cortisol levels, such as individual biometrics, method of sample collection, the selection of tissue, sample storage prior to analysis, the extraction procedure and instrumentation used that will also preclude data comparison with other studies.…”

“…Cortisol has been used as an indicator of stress response and overall population health for a wide range of mammals (Sheriff et al 2011;Atkinson et al 2015). It has been quantified in many matrices such as blood (serum/plasma), urine, feces and hair, as well as in blubber and the blow from cetaceans (St. Aubin et al 2001;Schmitt et al 2010;Macbeth et al 2012;Palme et al 2013;Thompson et al 2014;Kellar et al 2015;Trana et al 2015). Cortisol has also provided insight into stress associated with contaminants, such as persistent organic pollutants (POPs) that include PCBs and PBDEs (Verboven et al 2010;Bechshoft et al 2012b).…”

“…For instance, cortisol levels can reflect diurnal and seasonal cycles as well as size, sex, and age of individuals (Rosen and Kumagai 2008;Myers et al 2010;Kellar et al 2015). Endogenous compounds such as vitamins A and E have also been shown to interact with glucocorticoid homeostasis and functioning; both vitamins appear to diminish glucocorticoid stress responses in organisms and thus antagonize the hypothalamic-pituitary-adrenal (HPA) axis.…”

Daily salivary cortisol levels in response to stress factors in captive common bottlenose dolphins (Tursiops truncatus): a potential welfare indicator

“…The HPA axis is a key element of the stress response and, when activated, the resultant hormonal cascade increases the levels of glucocorticoids (GCs) in the bloodstream (Möstl and Palme 2002, Mormède and others 2007, Romero and Butler 2007). Cortisol, the major GC in cetaceans, can be measured in a number of tissues and secretions such as serum or plasma, saliva, milk, ocular secretions, urine and faeces (Suzuki and others 1998, Pedernera-Romano and others 2006, Amaral 2010, Hunt and others 2013) and more recently demonstrated in blubber (Kellar and others 2015), baleen plates (Hunt and others 2014b, Tallo-Parra and others 2015), skin (Bechshoft and others 2015) and exhalations (Hogg and others 2009, Hunt and others 2014a). Circadian and seasonal cortisol fluctuations make it difficult to establish baseline levels to assess acute stress factors (Suzuki and others 2003).…”