Adaptation strategies of yak to seasonally driven environmental temperatures in its natural habitat

The gradual increase of ambient temperature (TA) at high altitude can cause heat stress as an effect of climate change and may shift the traditional habitat of yak to further higher altitude. Therefore, an attempt has been made in this study to evaluate the thermo-adaptability of yaks to different seasons at high altitude. The adaptive capabilities of yaks were assessed based on different heat tolerance tests in relation to changes in rectal temperature (RT; degrees F), respiration rate (RR; breaths/min), pulse rate (PR; beats/min), and plasma heat shock protein (HSP) profile. The experiment was conducted in 24 yaks, divided into three groups based on age as calf (n = 8), adult (n = 8), and lactating cow (n = 8). Thermal adaptability was determined by temperature humidity index (THI), dairy search index (DSI), and Benezra's thermal comfort index (BTCI) along with HSP70 profile. The THI was higher (P < 0.01) in summer than winter which increased from lowest (40.87) to highest (61.03) in summer by 20 points, where yaks were under heat load beyond THI 52. The RT (100.09 +/- 0.18 degrees F), RR (21.76 +/- 0.18), and PR (59.78 +/- 0.32) increased by 23-35%, and this was correlated to the higher values of DSI exceeding 1 in calves (1.35 +/- 0.03), lactating cows (1.29 +/- 0.04), and adults (1.23 +/- 0.32) during summer in comparison to winter (0.98 +/- 0.02). The BTCI also showed values greater (P < 0.01) than 2 in calves (3.47 +/- 0.27), lactating cows (3.23 +/- 0.28), and adults (2.98 +/- 0.29) which reflected 49-75% increase in rectal temperature and respiration rate during summer. Further, heat stress was substantiated by threefold higher (P < 0.01) level of plasma HSP70 in calves (189.61 +/- 3.90 pg/ml) followed by lactating cows (168.62 +/- 3.03 pg/ml) and adults (155.33 +/- 2.30 pg/ml) against the winter average of 87.92 +/- 3.19 pg/ml. Present results revealed that yaks were experiencing heat stress in summer at an altitude of 3000 m above sea level and calves were more prone to heat stress followed by lactating cows and adults.