Physiological insight into the high-altitude adaptations in domesticated yaks (Bos grunniens) along the Qinghai-Tibetan Plateau altitudinal gradient

Abstract

The cold, hypoxic conditions of high-altitude habitats impose severe metabolic demands on domesticated yaks (Bos grunniens). Understanding how yaks cope with the combined effects of hypoxia and aerobic metabolism can provide important insights into the process of adaptive evolution. Here, hematological parameters, blood gases and blood volume, as well as enzyme activity, were determined in yaks from different altitudes. A total of 60 yaks, including 16 Tibet Alpine yaks (approximately 5100m a.s.l.), 23 Gannan yaks (3585m a.s.l.) and 21 Tianzhu white yaks (2960m a.s.l.) in the typically alpine meadows for the altitude of the Tibetan Plateau, were compared. The average red blood cell counts (RBC) value (9.34×1012/L) for the Tibet Alpine yaks was significantly higher than those of Gannan yaks (7.34×1012/L) and Tianzhu white yaks (6.77×1012/L) (P<0.05). Similar levels of significance were observed in the hematocrit (HCT), mean cell volume (MCV) and mean corpuscular hemoglobin concentration (MCHC) concentrations in Tibet Alpine yaks. Increases in hemoglobin (Hb) occurred in the Tibet Alpine yaks, with values of 13.21 and 10.67g/dl for the Gannan yaks and Tianzhu white yaks, respectively, which were lower than that of the Tibet Alpine yaks (P<0.05). Gas-tension and pH changes resulting from chronic hypoxia were evident in the reduced O2 and CO2 tensions in both venous and arterial blood and the slight but significant increase in the blood pH. The blood volume displayed a similar pattern of being greater in the Tibet Alpine yaks than in the Gannan yaks and Tianzhu white yaks. Lactate (LA) and lactate dehydrogenase (LDH) activity positively correlated with altitude (P<0.05). These results are interesting in respect to the low-oxygen environment in which yaks live and are consistent with the higher myoglobin (Mb) concentration in yak cardiac muscle, possibly permitting this type of yak muscle to be switched to aerobic metabolism when needed. These results suggest that Hb levels are not a totally conclusive explanation for the yak adaptation to high altitude, and the physiological adaptation was achieved in yaks perhaps by increases in RBC and blood oxygen affinity and decreases in MCV. In addition, enhanced enzymatic activity plays a key role in added metabolic cost and anaerobic metabolism.