Oxygenating blubber: a challenge for fat animals

Adipose tissue expansion can cause reduced blood flow and tissue hypoxia. However, seals require a thick blubber layer for survival. How blubber maintains adequate oxygenation is unknown. Here, we examine blubber depth and fatness effects on tissue oxygenation. Body condition was assessed in juvenile grey seals using morphometric data, blubber depth and photogrammetry. Systemic blood oxygen saturation (SpO2 ), heart rate and breathing rate were monitored throughout sampling. Blubber oxyhaemoglobin and deoxygenated haemoglobin were measured in dorsal flank blubber under appropriate sedation and anaesthesia using a novel non-invasive technique: Near Infrared Spectroscopy (NIRS). Simultaneously, temperature and partial pressure of oxygen (pO2) was measured at different blubber depths, with an optical, nonconsuming oxygen probe. Total blubber depth ranged between 10 mm and 28 mm (mean = 18.1 mm). Despite constant systemic blood saturation throughout sampling and among individuals (SpO2 = 89.4% ± 6.6), there were large inter-individual pO2 differences, ranging between 11 mmHg and 59 mmHg (mean = 32 mmHg). pO2 inter-site variability and variation over time were tested with no significant effect (n = 4, p > 0.05). Linear mixed effects analysis (LMM) showed that pO2 varies irrespective of the depth of the measurement, but showed a negative relationship with body fatness (n = 10, p < 0.05, R2 marginal = 0.31, R2 conditional = 0.97), consistent with findings in mice. NIRS data analysis is ongoing to investigate oxygen supply to blubber in relation to pO2. Further histological and molecular analysis are also underway to explore vascularity and consequences of hypoxia for blubber.