Partial pressure of oxygen in adipose tissue and its relationship with fatness in a natural animal model of extreme fat deposition, the grey seal

Excessive adiposity is associated with altered oxygen tension and comorbidities in humans. In contrast, marine mammals have high adiposity with no apparent detrimental effects. However, partial pressure of oxygen (Po 2 ) in their subcutaneous adipose tissue (blubber) and its relationship with fatness have not been reported. We measured Po 2 and temperature at different blubber depths in 12 healthy juvenile grey seals. Fatness was estimated from blubber thickness and morphometric parameters. Simultaneously, we monitored breathing pattern; heart rate and arterial blood saturation with a pulse oximeter; and relative changes in total hemoglobin, deoxyhemoglobin, and oxyhemoglobin in blubber capillaries using near‐infrared spectroscopy (NIRS) as proxies for local oxygenation changes. Blubber Po 2 ranged from 14.5 to 71.4 mmHg (39.2 ± 14.1 mmHg), which is similar to values reported in other species. Blubber Po 2 was strongly and negatively associated with fatness (LME: p  < 0.0001, R 2 marginal = 0.53, R 2 conditional = 0.64, n  = 10), but not with blubber depth. No other parameters explained variability in Po 2 , suggesting arterial blood and local oxygen delivery did not vary within and between measurements. The fall in blubber Po 2 with increased fatness in seals is consistent with other animal models of rapid fat deposition. However, the Po 2  levels at which blubber becomes hypoxic and consequences of low blubber Po 2 for its health and function, particularly in very fat individuals, remain unknown. How seals avoid detrimental effects of low oxygen tension in adipose tissue, despite their high and fluctuating adiposity, is a fruitful avenue to explore.