Anatomy and pathology of adrenal glands in common dolphins : influence of life history and contaminant burdens : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University, Albany, New Zealand

Abstract

Adrenal glands play a central role in stress physiology, immune regulation, and endocrine homeostasis, yet their gross morphology, cellular architecture, and pathology remain poorly characterised in delphinids. This study investigated adrenal gland structure, cellular composition, and pathology in common dolphins (Delphinus delphis) from New Zealand waters. Ontogeny and cumulative trace element burden were examined as predictors of architecture to establish baseline morphometric and histological reference values in this population, and to investigate contaminant-associated structural remodelling. Adrenal glands were collected postmortem (2008-2025) from 103 stranded or incidentally bycaught dolphins. Gross morphometrics, digital planimetric histological analysis, and pathological assessment were performed. Hepatic and renal trace element concentrations were determined by inductively coupled plasma mass spectrometry. Linear mixed-effects models examined predictors of adrenal composition and trace element associations with glandular architecture. Age was the dominant predictor of gross adrenal weight and length, with glands increasing approximately 0.20 g and 0.80 mm per annum, respectively. Bilateral comparisons revealed significant directional asymmetry at the gross level, with left glands heavier and right glands longer, while proportional zonal composition was bilaterally equivalent. Mean cortex-to-medulla ratio was 2.15 ± 0.74, with females exhibiting significantly higher ratios than males, driven by greater cortical and lower medullary proportions. Capsule proportion increased significantly with age. Prominent medullary band presence was associated with greater medullary proportion, with band prominence corresponding to measurable differences in medullary cross-sectional area. Adrenal pathology was identified in 24.8% of assessed glands, including one case of medullary neoplasia, suggestive of a pheochromocytoma. Pathology prevalence increased with age and sexual maturity. Bioaccumulative trace element burden (selenium, cadmium, mercury) was significantly associated with greater capsular tissue and reduced cortical proportion, while essential metal status showed opposing directional associations. Mercury-to-selenium molar ratios predicted capsular expansion independently of life history variables. These associations suggested that bioaccumulative burden drives capsular expansion alongside cortical reduction, a structural pattern not replicated by stress chronicity, disease classification, or pathology presence, which were non-significant predictors of adrenal composition. This study established the first adrenal morphometric baseline for New Zealand common dolphins and provided novel evidence that bioaccumulative metal burden was associated with structural adrenal remodelling.