Climate Change on Himalayan Yak (Bos grunniens): History
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Contributor:
S. Sapkota
,
K. P. Acharya
,
R. Laven
,
N. Acharya

Climate change is a global issue, with a wide range of ecosystems being affected by changing climatic conditions including the Himalaya. Yak are exquisitely adapted to the high-altitude conditions of the Himalaya and are thus highly likely to be affected by climate change. 

  • yak
  • climate change
  • temperature
  • heat stress
  • diseases

1. Introduction

Domestic yak (Bos (Poephagus) grunniens) and their hybrids with domestic cattle (both B. taurus taurus and B. t. indicus) are hardy animals that are most commonly found in the highland regions of Asia including the Himalaya [1][2]. Yak are usually found in areas that are 3000 to 6000 m above sea level, and are adapted to extremely cold, harsh environments with low atmospheric oxygen concentrations and high levels of solar radiation [3]. They are used for a huge range of purposes including producing milk and milk products (cheese, butter, and whey), fuel (yak dung), meat, blood, fat, hair, and skins [4]. Yak are also used as a pack of animals, being able to travel up to 15 km per day in high-altitude areas [5] and carry loads of up to 100 kg in weight [5].
Like all Bos species, yak is a strict herbivore. They can graze/browse a wide range of vegetation from grass to small shrubs, grazing short forage like sheep (using their incisor teeth and lips), and browsing longer material like cattle (using their tongues to grasp plant material) [6]. Yak usually rely on natural vegetation as their feed source as supplementary feeding (other than wilted forages) is limited in most yak herding regions, including the Himalayas [7].

2. Climate Change in the Himalaya

The Himalayan ecosystem has been significantly affected by climate change [8]. The rise in temperature has altered climate patterns, destabilized the patterns/forms of water resources [9], and increased the risk of natural hazards [10] as well as affecting the growing cycle of plants, and the migratory pattern of animals [11].

2.1. Temperature

Based on data from 478 meteorological stations across the Hindu-Kush Himalayan region collected between 1961 and 2015, there had been a significant decrease in the number of extreme cold events (cold nights, cold days, and frost days) and a concomitant increase in the number of extreme warm events (warm nights, warm days, and summer days) (see Table 1 below) [12]. Similar trends have been reported in the western Himalaya [13].
Table 1. Trends of extreme temperature indices over the Hindu-Kush Himalaya region in the period 1961–2015. Source: Sun et al. [12].
Indicator Name Definition Trend (d/10 Years)
Cold nights Days when Tmin < 10th percentile −0.977
Cold Days Days when Tmax < 10th percentile −0.511
Warm nights Days when Tmin > 90th percentile 1.695
Warm days Days when Tmax > 90th percentile 1.239
Frost days Annual count when Tmin < 0 °C −3.636
Summer days Annual count when Tmax > 25 °C 6.741
This increase in temperature is most marked in the winter season. Shrestha et al. [14] reported that, between 1982 and 2006, across multiple ecoregions in the Himalaya, there was a warming trend of 0.07 °C per year in the winter and 0.03 °C per year during the summer season. Current projection models suggest that this warming will continue; Shrestha et al. [15] estimated that, between 2020 and 2050, the average temperature across the Hindu-Kush Himalayan region will increase by 1–2 °C (and in some areas by 4–5 °C).

2.2. Precipitation

Average annual precipitation across the Himalaya increased by 163 mm (6.52 mm per year) between 1986 and 2006 [14]. This increase has been very seasonal, with an increase of 187 mm (7.48 mm per year) during the summer months but a decrease of 17 mm (−0.68 mm per year) during the winter months. Climate models suggest that precipitation will increase by an average of 5% (maximum 25%) between 2020 and 2050, with a longer and more erratic monsoon season with fewer but more intense extreme rainfall events [15]. This is likely to be particularly pronounced in the Eastern Himalaya, while the southern part will show a slightly decreasing trend in extreme rainfall events [15]. This shows that extremes of both rain and drought could potentially bring a challenge to the Himalayan ecosystem.

2.3. Vegetation

Temperature changes have meant that high-altitude, cold-adapted plant species have shifted to higher altitudes and been replaced by species that are better adapted to warmer temperatures [16]. This is evident in the shifting in the treeline by up to 0.37 m/year across the central Himalaya [17]. These changes have resulted in alterations in vegetation type, with alpine meadows and herbs being replaced by shrubs [18][19][20]. This trend is predicted to continue, with higher CO2 scenarios being associated with greater replacement of herbs by shrubs [20]. Shrub encroachment is probably the major climate-change-associated factor decreasing the availability and quality of grazing vegetation in the Himalaya [18][19].
The changes in precipitation patterns are also crucial. Variation in precipitation between years greatly influences the production of vegetation [21], with accumulated precipitation playing a key role in seasonal vegetation production in the Himalaya [21]. Although biomass reduction induced by insufficient precipitation will reverse after good rainfall/snowfall [22], the ability of the land to recover after water scarcity will gradually decrease over time [21]. The processes responsible for this decline in responsiveness include soil erosion, a decline in infiltration or moisture-holding capacity of the soil, loss of seed banks, and shrub encroachment [23]. These changes will result in a long-term continued decrease in forage biodiversity, availability, and quality, with Yang et al. [24] reporting that increasing shrub cover was associated with reduced total herbaceous forage production and with reduced crude protein intake by yak.
Extreme weather events such as floods, droughts, and high temperatures, the risks of which are increased by climate change, increase the likelihood of outbreaks of pests and other diseases in alpine vegetation, because they affect plant defence mechanisms and make them more susceptible to pests and pathogens [25].
The changes in temperature and rainfall patterns have resulted in an earlier start of the growing season and earlier bud outbreaks as well as increased germination rates [14]. The impacts are not solely beneficial. The changes in temperature and timings are especially beneficial for weed species which have become much more invasive, thus crowding out useful forage plants. Additionally, many of the forage species relied on by yak need snow cover to insulate them from the winter cold, so have been hit hard by the loss of that cover even though winter air temperatures have increased, while other species that require winter chilling for bud break may not get sufficiently low temperatures over a sufficiently long period for that to occur [26]. 

3. Possible Consequences of Climate Changes on Yak

The thermo-neutral zone of yak ranges from 5–13 °C [6]. The risk of heat stress in yak can be assessed by measuring the temperature-humidity index (THI), which combines temperature and relative humidity. The yak’s physiological focus on reducing heat loss means that, compared to cattle, the THI threshold above which yak are likely to begin experiencing heat stress is much lower (52 vs. 72 for yak and cattle, respectively) [27]. If the relative humidity is 65%, then air temperatures >13 °C will result in a THI > 52. The change in climate in the Himalaya has meant that this THI threshold is being increasingly exceeded even during winter [28], thus putting yak at higher risk of becoming heat stressed.
The pastoral nature of yak farming, especially in its traditional transhumance form, means that artificial heat stress mitigating strategies, such as providing shelters, are difficult to provide in many circumstances. This means that heat stress due to climate change could potentially have severe impacts on the health and welfare of farmed yak. However, to date, there have been no published studies that have directly evaluated the effect of this heat stress on the productivity, disease risk, and immune functions of yak. However, the results of previous studies on yak and related domestic species, especially cattle, have suggested that the increase in the number of days where the THI of yak is exceeded is likely to have negative impacts on physiology, production, immune function, and disease risk.

3.1. Climate Change and Yak Physiology

The main effects of climate change on yak physiology are likely to be principally mediated through environmental temperature. Increased temperature leads to an increase in respiration rate as that is the yak’s main method of heat dissipation [6]. If this increased temperature continues, pulse rate and, eventually, the rectal temperature will rise [29][30].
Another significant physiological change associated with environmental temperature is plasma cortisol concentration. In yak, plasma cortisol concentrations are lower in warmer months than in cooler seasons [31]. The physiological significance of this cortisol reduction is not clear, but it suggests that one of the adaptive mechanisms of the yak to prolonged elevated heat loads is decreasing adrenal cortical output as found in cattle [32][33][34]. Alongside this decrease in cortisol concentrations, lower blood glucose, and volatile fatty acid concentrations are also observed during the warm-humid seasons compared to the cold-humid seasons [30]. These changes in blood metabolites are consistent with the findings in cattle that cold exposure causes blood glucose concentrations to rise in response to increases in circulating thyroid and adrenal hormones which contribute to metabolic heat production [35].
One crucial physiological response to heat stress is the response of the reproductive system. Yak reproduction is affected by temperature, as yak are much more likely to come into oestrus during the early morning or evening than in the hotter parts of the day, and they are more likely to be seen in oestrus on overcast rather than clear days [36]. However, the underlying physiology of this effect is unclear, as there have been no published studies on reproductive physiology in yak and heat stress.
Studies in cattle may provide some guidance as to the likely effect of heat stress on reproductive physiology in yak. In cattle, heat stress has been shown to have effects across the whole of the oestrus cycle, for example, decreasing luteinising hormone concentrations and depressing the LH surge [37][38], while also stimulating premature luteinisation and increasing progesterone production [38]. Heat stress also impairs oocyte development and disrupts normal follicular function [39][40]. If these effects also occur in yak, it is unlikely that they will be simple to ameliorate once they occur.

3.2. Climate Change and Reproductive Performance

Thus, the physiological changes associated with heat stress are likely to result in reduced reproductive performance. These impacts are likely to be exacerbated by the impacts of climate change on yak nutrition. As discussed earlier, nutrition is critically associated with yak reproduction. As the grass starts growing in May, the body condition of yak begins to improve, and in June, dry females begin to exhibit oestrus peaking around July–August. Lactating yak have a delayed return to good body condition and, thus, tend to show oestrus later (September to November) than dry yak, and are more likely to be non-pregnant at the end of the breeding season [36]. The changes in forage availability and quality as a result of climate change are likely to delay the recovery of body condition in both dry and lactating yak, increasing the proportion that is non-pregnant at the end of the breeding season (especially lactating yak). It is also important to note that a reduction in forage quality and availability is also likely to impact the onset of puberty which is closely linked to nutrition [41]. Thus, climate change is likely to have long-term effects on yak fertility unless there is greatly increased use of supplementation.
It has also been suggested that abortion in yak may be related to heat stress [42], but this suggestion was based on data from one farm where ~40% of abortions occurred in May/June. This farm had a much higher rate of abortion than is normal for yak, so it is unclear how representative it is, and the conclusion was based only on the timing of the abortions, and no investigation of the cause of the abortions seems to have been undertaken. More research is needed to properly evaluate the effect of heat stress on yak fertility.

3.3. Climate Change and Productivity

The changes in forage quantity and quality described earlier are likely to have a significant negative effect on yak productivity. Yang et al. [24] reported that the changes in forage quantity and quality associated with shrub encroachment led to reduced growth in yak. Similar results are not available for milk production and reproductive performance, but the impacts are likely to be similar.
The evidence of an impact of heat stress on the milk production of yak is limited, with no long-term studies of comparative production under different temperature and humidity conditions. Shikui et al. [43] reported that over the short term, yak produced more milk (~0.1 to 1 kg/day) on cloudy, cool days (6.7 to 9.3 °C) than on the preceding and following warmer, clear sunny days (12.5–13.5 °C). This was a small study (only 19 animals) over an approximately two-week period in June, on one farm, so much more data are required to properly establish the likely impact of heat stress on milk production by yak. Furthermore, the underlying cause of these changes is unclear, but it might be related to dry matter intake, which is negatively related to THI in cattle [44], although this reduction in food intake may not be the sole reason for the reduction in milk yield. In cattle, Gao et al. [45] reported that despite similar dry matter intake, heat-stressed cows produced less milk than pair-fed cows kept in thermoneutral conditions. They [45] suggested that THI-related impacts on mammary blood flow could be responsible for some of this reduction. The cattle data thus suggest that milk production will reduce in yak suffering from heat stress but as impact on cattle is dependent on the level of productivity, with higher-yielding cows being more susceptible to heat stress [46], yak-specific data are needed to properly assess the likely range of losses.

3.4. Climate Change and Infectious Disease Occurrence

Yak are susceptible to a large number of infectious diseases, most of which are also present in local cattle or sheep [6][47]. Losses from infectious diseases can be high in individual herds [47], but it is likely that the literature is biased toward investigations in problem herds as there are few studies on the prevalence and economic impact of infectious diseases across multiple herds and multiple regions.
On first principles, it is likely that climate change is increasing the exposure of yak to disease. Firstly, increased temperatures may increase contact between yak and cattle, particularly during the winter period. As a large proportion of the infectious disease recorded in yak is likely to come from cattle [6], this increased contact is likely to increase the exposure of yak to cattle diseases. Secondly, climate change may affect vector-transmitted disease prevalence by shifting the geographical range of vectors, increasing their reproductive efficiency, and by altering vector–host interactions [48][49]. These changes could increase the incidence of diseases that are already established and/or result in the introduction of new diseases. Finally, the increased periods of nutritional stress caused by the impact of climate change on forage quality and quantity are likely to increase the susceptibility of yak to disease. Nevertheless, the lack of baseline data on disease prevalence in yak means that it is likely to be difficult to identify whether climate change is actually altering disease prevalence. Koirala et al. [50] surveyed in the Mustang district of Western Nepal, interviewing 71 households on the effects of climate change on livestock, particularly their perception of the impact of climate change on livestock disease. Although yak is common in the Mustang district, most of the respondents did not have a yak, so the survey does not provide direct evidence of changes in the perception of disease risk in yak herders. However, many of the households did have Jhopa (yak/cattle cross) and because disease transmission from cattle to yak is likely to be a key source of disease in yak, increased disease in Jhopa is likely to be reflected in yak.
The effect of climate change on the prevalence of non-parasitic diseases, such as brucellosis and foot-and-mouth disease, is even less clear with very limited data. Most analyses of climate change and yak diseases suggest that climate change will lead to increased disease (e.g., [11][51]) without providing data, while most datasets of disease prevalence, e.g., Mortenson et al. [52] are single timepoint studies which provide no data, on their own, as to disease patterns in yak. As far as the authors are aware, the only published systematic study of temporal patterns in non-parasitic disease in yak is the meta-analysis undertaken by Zhao et al. [53] who analysed the prevalence of brucellosis in Chinese yak. They reported that the “incidence of brucellosis was higher”, with the pooled prevalence being higher after 2012 than before. However, while suggestive of an increase, this study is not conclusive. Zhao et al. [53] separated their pooled prevalence over time into three categories—before 2012, 2012 to 2016, and 2016 or later—with the three pooled prevalences being 5.8, 11.5, and 7%, respectively. Thus, the changes in prevalence over time were relatively small, and there was no clear pattern of a continuous increase. Additionally, their modelling did not rule out there being no effect of time on the prevalence of brucellosis (95%CI of their regression coefficient being −0.01 to 0.205; p = 0.075). Zhao et al. [53] suggested that the National Brucellosis Control Plan which was issued in 2016 was responsible for the decrease seen after that year, but their dataset is too small to make such a conclusion, and it is too small to allow their model to distinguish between the pre-2012 prevalence and the prevalence in the 2012–2016 time period. Thus, their [53] conclusion that “in China, yak brucellosis is reviving” seems to be based more on their perception of disease risk than on their data. They [53] linked the increase in brucellosis to increased stocking density (and thus increased exposure to brucellosis). They linked the increased stocking density to degradation of grazing areas, a change which has been clearly linked to climate change [54][55]. However, overgrazing has also been shown to occur because of human activity driving changes in land use, and it is likely that if degradation of grazing is causing an increase in brucellosis in yak, then these socio-economic changes are currently more important in driving change in disease risk than climate change [11][56].

This entry is adapted from the peer-reviewed paper 10.3390/vetsci9080449

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