Adiposity Profiling in Killer Whales (Orcinus orca): A Different Perspective for Body Condition Analysis

Cetacean blubber is a multifunctional vital organ that serves as a diagnostic medium for assessing nutritional status, life history, and general health at both macro- and microscopic scales. In this study, we provide the first histological characterisation of adipocyte morphology in killer whales (Orcinus orca) using full-depth blubber samples collected postmortem from 25 individuals (16 females, 9 males) from strandings in New Zealand. Specifically, we assessed the relationships between adipocyte area (AA), adipocyte index (AI), and biological covariates. Additionally, we tested the application of a novel blubber grading system to explore the impact of histological slide characteristics on adiposity metrics, confirming that decomposition state did not systematically bias morphological data. Linear mixed-effects models identified distinct stratification across the blubber profile. Adipocyte area was predicted by both total body length and blubber thickness, indicating that blubber expansion is mediated by body size and cellular hypertrophy. In contrast, the AI was primarily driven by nutritional status. A significant interaction between cell size and depth revealed that the inner layer is the most dynamic stratum, indicating superior architectural plasticity. To improve diagnostic resolution, we further developed an integrated health ratio (AA/AI). This metric was significantly lower in emaciated individuals, as it accounts for the concurrent reduction in lipid stores and increase in structural fibre density. Our findings establish a baseline for killer whale blubber physiology and demonstrate that with rigorous quality control, stranded specimens yield reliable morphometric data for the assessment of body condition in cetaceans.