The adaptive strategies of yaks to live in the Asian highlands

Abstract

The yak (Bos grunniens), an indigenous herbivore raised at altitudes between 3,000 and 5,000 m above sea level, is closely linked to more than 40 ethnic communities and plays a vital role in the ecological stability, livelihood security, socio-economic development, and ethnic cultural traditions in the Asian highlands. They provide the highlanders with meat, milk, fibres, leather and dung (fuel). They are also used as pack animals to transport goods, for travel and ploughing, and are important in many religious and traditional ceremonies. The Asian highlands are known for an extremely, harsh environment, namely low air temperature and oxygen content and high ultraviolet light and winds. Pasture availability fluctuates greatly, with sparse pasture of poor quality over the long seven-month cold winter. After long-term natural and artificial selections, yaks have adapted excellently to the harsh conditions: 1) by genomics, with positively selected genes involved in hypoxia response and energy metabolism; 2) anatomically, including a short tongue with a weak sense of taste, and large lung and heart; 3) physiologically, by insensitivity to hypoxic pulmonary vasoconstriction, maintaining foetal haemoglobin throughout life, and low heart rate and heat production in the cold season; 4) behaviourlly, by efficient grazing and selecting forbs with high nutritional contents; 5) by low nitrogen and energy requirements for maintenance and low methane emission and nitrogen excretion, namely, 'Low-Carbon' and 'Nitrogen-Saving' traits; 6) by harboring unique rumen microbiota with a distinct maturation pattern, that has co-evolved with host metabolism. This review aims to provide an overview of the comprehensive adaptive strategies of the yak to the severe conditions of the highlands. A better understanding of these strategies that yaks employ to adapt to the harsh environment could be used in improving their production, breeding and management, and gaining benefits in ecosystem service and a more resilient livelihood to climate change in the Asian highlands.

Keywords: Adaptive strategy; Gastrointestinal microbiome; Grazing behaviour; Nutrient metabolism; Yak.

Fig. 1

The yak is crucial for livelihoods, social life, socio-economic standings and ethnic traditions for pastoralists. (A) Yaks provide milk, meat, hides, transport, and dung fuel for pastoralists, and are also important in traditional sports and ethic culture. (B) Yaks have an impact on the socio-economic development, as yak production accounts for more than 50% of the total output of animal husbandry in the Tibet Autonomous Region of China.

Fig. 2

The ‘Nitrogen-Saving’ strategy of yaks to cope with the harsh conditions of the Asian highlands. Compared with cattle, yaks require less nitrogen for maintenance and excrete less urinary nitrogen, digest and retain nitrogen to a greater extent, synthesize more urea by the liver and recycle more urea nitrogen to the rumen, use more recycled urea nitrogen by ruminal bacteria and synthesize more ruminal microbial protein when consuming the same diet and nitrogen intake, especially at low nitrogen intake.

Fig. 3

The ‘Low-Carbon’ strategy of yaks to cope with the harsh conditions of the Asian highlands. Compared with cattle, yaks have a lower fasting heat production, produce more ruminal total short chain fatty acids, emit less enteric methane, digest gross energy to a greater extent, use energy more efficiently and reduce metabolic rate with a decrease in air temperature to conserve energy.

Fig. 4

The strategies of yaks that allow them to inhabit the Asian highlands. Yaks are raised at altitudes between 3000 and 5000 m above sea level across the Asian highlands. They have adapted well to the extremely harsh environment by evolving distinct genomic, anatomical, physiological, nutritional metabolic and grazing behavioural traits and by possessing unique gastrointestinal microbiota to cope with the sparse vegetation of low quality and the severe environmental conditions.