Selected and shared hematological responses to apnea in elite human free divers and northern elephant seals (Mirounga angustirostris)

Despite elite human free divers achieving incredible feats in competitive free diving, there has yet to be a study that compares consummate divers, (i.e. northern elephant seals) to highly conditioned free divers (i.e., elite competitive free-diving humans). Herein, we compare these two diving models and suggest that hematological traits detected in seals reflect species-specific specializations, while hematological traits shared between the two species are fundamental mammalian characteristics. Arterial blood samples were analyzed in elite human free divers (n = 14) during a single, maximal volitional apnea and in juvenile northern elephant seals (n = 3) during rest-associated apnea. Humans and elephant seals had comparable apnea durations (∼6.5 min) and end-apneic arterial Po₂ [humans: 40.4 ± 3.0 mmHg (means ± SE); seals: 27.1 ± 5.9 mmHg; P = 0.2]. Despite similar increases in arterial Pco₂ (humans: 33 ± 5%; seals: 16.3 ± 5%; P = 0.2), only humans experienced reductions in pH from baseline (humans: 7.45 ± 0.01; seals: 7.39 ± 0.02) to end apnea (humans: 7.37 ± 0.01; seals: 7.38 ± 0.02; P < 0.0001). Hemoglobin P₅₀ was greater in humans compared to elephant seals (29.9 ± 1.5 and 28.7 ± 0.6 mmHg, respectively; P = 0.046). Elephant seals overall had higher carboxyhemoglobin (COHb) levels (5.9 ± 2.6%) compared to humans (0.8 ± 1.2%; P < 0.0001); however, following apnea, COHb was reduced in seals (baseline: 6.1 ± 0.3%; end apnea: 5.6 ± 0.3%) and was slightly elevated in humans (baseline: 0.7 ± 0.1%; end apnea: 0.9 ± 0.1%; P < 0.0002, both comparisons). Our data indicate that during static apnea, seals have reduced hemoglobin P₅₀, greater pH buffering, and increased COHb levels. The differences in hemoglobin P₅₀ are likely due to the differences in the physiological environment between the two species during apnea, whereas enhanced pH buffering and higher COHb may represent traits selected for in elephant seals.NEW & NOTEWORTHY This study uses similar methods and protocols in elite human free divers and northern elephant seals. Using highly conditioned divers (elite free-diving humans) and highly adapted divers (northern elephant seals), we explored which hematological traits are fundamentally mammalian and which may have been selected for. We found differences in P₅₀, which may be due to different physiological environments between species, while elevated pH buffering and carbon monoxide levels might have been selected for in seals.