Over the past 40 years, the realism of the effects of climate change has increased. Even though the Intergovernmental Panel on Climate Change had already observed an increase in global temperature, a rise in sea levels and a decrease in snow and ice extent, an additional increase in global temperatures of between 1.1Â°C and 6.4Â°C is expected by the 21st century (IPCC, 2007). Furthermore, conditions in Africa have been predicted to get progressively hotter and more arid with climate change (Boko, et al., 2007). Take for instance the proportion of arid and semi-arid lands in Africa, a continent already under pressure from climatic stresses, is likely to increase by 5-8% by the 2080s (Boko, et al., 2007). Southern Africa, a predominantly semi-arid region, is projected to become generally warmer with an increase in very hot days exceeding 35Â°C (Davis, 2011). In the arid areas of southern Africa, however, the thermal threat is compounded, or even exceeded, by the threat of reduced water availability. Yet, it's not the increase in temperature that is most concerning; it's the rate at which it is accelerating (Davis, 2011).
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Climate is an external driver and holds dominant control over the natural distributions in most species (Kearney & Porter, 2009; Pearson & Dawson, 2003). Changes in food resources, rainfall, cloud cover, fire, habitat destruction and fragmentation are influenced and exacerbated by a change in climate, such as temperature (Huey, et al., 2012). An increase in temperature, predicted by Root et al. (2003), may shift the ranges of species either poleward or up in elevation as species move to occupy areas within their metabolic temperature tolerances. Therefore at given locations the density of species may change (Root, et al., 2003) and species that currently coexist, may progressively move apart (Thuiller, et al., 2006). As a consequence, it is expected that a fundamental change in community structures will occur as species associations shift with influxes of new species (Thuiller, et al., 2006). Therefore if we are to have a better understanding of how climate change will affect ecological organisations in the future, we need to identify and determine which species, habitats and ecosystems will be the most sensitive to climate change (Williams, et al., 2008). Factors that lead to a species sensitivity include but are not limited to: physiological limits; ecological traits such as behaviour; and genetic diversity (Williams, et al., 2008). Since spatial and temporal climate variability is predicted to increase, understanding the influence of temperature on physiological performances is critically important to predicting animal responses to climate change (Tattersall, et al., 2012; Acevedo-Whitehouse & Duffus, 2009; Fuller, et al., 2010; Huey, et al., 2012; McCarty, 2001; Thuiller, et al., 2006).
Essentially to be able to combat climate change, an animal has three possible responses. Firstly an animal would need to expand or shift its current distribution range to a more suitable habitat where the climate is within its thermal limits. However, if moving is not feasible, the second option that an animal will have is to adapt to these changing climatic conditions. This ability to adapt allows the animal to modify its behaviour, morphology, or physiology in response to altered environmental conditions. Such changes could occur in the timing of events (phenology), migration, flowering or egg laying (Root, et al., 2003; Hughes, 2000). Species, however, do have the capacity to adapt to changing environmental conditions both by phenotypic plasticity within a life span and by microevolution over a few life spans (Fuller, et al., 2010). But, the climate is changing at a relatively fast rate and large bodied species with long generation times, such as large herbivores, may not be able to adapt genetically fast enough (Berteaux, et al., 2004). If they are to survive they will have to show plasticity in the expression of the genes they currently possess. Thirdly, if the first two responses are not possible, the species will unfortunately become extirpated or extinct (Walther, et al., 2002; Pearson, et al., 2003; Fuller, et al., 2010).
Mammals are important flagships for conservation efforts and a change in mammal populations could have detrimental consequences on conservation, societal and economic factors (Berteaux, et al., 2006). The extinction risk of South African mammals from climate change is estimated to be as high as 69% by 2050, if dispersal is limited (Thomas, et al., 2004). Globally, it has already been reported that large mammal populations have been decreasing inside and outside of protected areas mainly as a consequence from anthropogenic causes (Caro & Scholte, 2007; Craigie, et al., 2010). As a result of the modern human dominated landscape, large mammals face unique challenges when securing resources (Bleich, et al., 2010), including forage and water, as they have to deal with current threats such as over exploitation, habitat loss and land transformation. A number of threats such as the ones mentioned above limit their dispersal capacity thus reducing the ability for these large bodied mammals to shift in their geographical range.
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Ungulates account for the vast majority of large herbivores currently on earth. Even-toed ungulates belong to the order Artiodactyla which evolved during the Eocene. It is suggested that, during a warming period in the Mid-Miocene Climatic Optimum, morphologic changes become evident in populations as individuals were affected by an interaction between its genetic make-up, what it ate and the environment (Barnosky, et al., 2003). So it is believed that Artiodactyla may be pre-adapted to the future hot and dry conditions predicted for large regions of Africa (Hetem, 2009). These even-toed ungulates have been evolutionarily successful and hugely speciose, with 90 extant genera (Mitchell & Lust, 2008), thus their influence stretches across nearly every biome and their indigenous range includes all zoogeographic regions except Antarctica. These artiodactyls have species-specific morphology which has constrained and shaped their adaptations to enable these species to be successful in variability in forage quality and quantity, water availability, temperature extremes, and strategies to avoid predation, insect parasitism and harassment (Klein, 2001).
Large-scale behavioural response
Theoretically an animal's first response to climate change will be to track their climatic envelope. Movement is fundamental to life and is defined as "a change in the spatial location of the whole individual in time, driven by processes that act across multiple spatial and temporal scales" (Nathan, et al., 2008). Ideally, an animal should move to an area where they will be able to maximize their energy gain in the shortest possible time (Groom & Harris, 2009; Bailey, et al., 1996). Thus they should spend most of their time in areas where the available quantity and quality of forage is highest (Bailey, et al., 1996). For example, Sinclair & Fryxell (1985) suggested that ungulates commonly move to areas of recent rainfall to take advantage of the increasing quantity and or quality of vegetation and leave areas when drought conditions prevail (Sinclair & Fryxell, 1985). This movement pattern has been observed in the Serengeti, where the annual migration of blue wildebeest (Connochaetes taurinus), zebra (Equus spp.) and Thomson's gazelle (Eudorcas thomsonii) is related to rainfall seasonality and soil type differences (Bolger, et al., 2008). Thus, the search for green forage appears to be one of the major drivers of ungulate migrations to floodplains and high-rainfall regions during the dry season (Fynn & Bonyongo, 2010). Thus, large ungulates will search for better forage and water availability over large distances suggesting that they are capable of moving to more suitable habitats. Yet, it is often considered as a response to short term goals such as reproduction, maintenance, including feeding, and survival, including escaping threats (Holyoak, et al., 2008).
But how far must an animal move to be able to find its suitable environment? For instance, Thuiller et al. (2006) predicted that the scimitar-horned oryx (Oryx dammah) will lose almost all its current habitat in the Sahara desert by 2080 as it will get too hot and will have to move thousands of kilometres, from the Sahara desert to Namibia or Botswana. However, without human intervention this distance may not be feasible. Over the last two centuries, ungulate migrations have been severely disrupted by human activities (Bolger, et al., 2008). Additionally, non-climatic factors, such as the presence of natural barriers, biotic interactions and/or anthropogenic habitat fragmentation caused by urbanisation and agriculture, in conjunction with climate change may hamper the ability of a species to move (Thuiller, et al., 2006; Levinsky, et al., 2007). Thus the ability of a species to track their bioclimatic envelope at a sufficient rate to keep up with the changing climate will largely be dependent on the dispersal characteristics of individual species (Pearson & Dawson, 2003), on anthropogenic and on natural restrictions on distributions, such as scarcity of forage and water availability, competitive interactions with other wildlife or livestock and predation (Groom & Harris, 2009). Thus, restricted access to resources makes it difficult for ungulates to follow an optimal adaptive foraging strategy, at the same time reducing predation risk, minimizing human interference and last but not least climate change. The alternative large mammals have, if unable to move the large distances required to track their bioclimatic envelopes, is to adopt an adaptation response which may be able to buffer changes in climate by changing their landscape use at a fine-scale.
Fine-scale behavioural response
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Desert ungulates use a variety of physiological, morphological, and behavioural mechanisms to deal with the conflicting challenges of maintaining body temperature within acceptable limits and minimizing water loss (Cain III, et al., 2006). If the ability of an animal to change its distribution or range is limited, then flexible behavioural processes such as activity patterns and microclimate selection may be able to buffer the effects of the changing climatic conditions (Hetem, 2009).
Mammals use a variety of behavioural strategies for avoiding an environmental heat load, for example, mammals can reduce their activity levels to minimize endogenous heat production, seek shade, or switch to nocturnal activity. In thermal heterogeneous habitats, terrestrial animals will select different microclimates in order to maximise heat gain/loss. The use of cooler microclimates may reduce heat loads and may help maintain a favourable temperature gradient between the body and the environment thereby facilitating convective heat loss, reducing the need for evaporative cooling and minimizing water loss (Cain III, et al., 2008; Hetem, et al., 2012), thus conserving water. This use of shaded microhabitat is a common behaviour especially during midday when temperatures are highest, and has been observed in a variety of ungulates. Animals from Serengeti displayed a strong linear relationship between use of shade and both ambient temperature and incidental solar radiation (Jarman & Jarman, 1973; Dunbar, 1979). Both the oryx and the sand gazelle, studied in the Mahazat as-Sayd Protected Area in west-central Saudi Arabia, selected microclimates during the heat of the day that were cooler than the temperature recorded in the sun (Hetem, et al., 2012). Desert bighorn sheep used caves as a lower temperature microhabitat in order to decrease potential thermal loads experienced at high ambient temperatures (Cain III, et al., 2008).
Some ungulates use shade-seeking behaviour during the hot dry period as an adaptation to increasing temperatures, others reduced diurnal activity and become more active at night (Owen-Smith 1998, Ruckstuhl and Neuhaus 2009, Hetem, Strauss, et al. 2012). Gemsbok (Knight, 1991) and the Arabian Oryx (Hetem, 2012), increased shade-seeking behaviour at high ambient temperatures and increased foraging activity during the night. However, species that inhabits areas where shade is unavailable, such as the black wildebeest (Maloney et al. 2005), reduced diurnal feeding activity in high ambient temperatures by standing and repositioning body orientation, and compensated by feeding mainly at night during the warm seasons. Seasonal changes in the duration or timing of daily activities can reduce heat loads and minimize evaporative water loss (Cain III, et al., 2006). In addition to the thermoregulatory benefits of restricting activities to cooler night-time periods, ungulates in some areas also increase the intake of preformed water by foraging at night (Cain III, et al., 2006). Nagy & Knight (1994) found that springbok preferred to eat leaves at night during the dry seasons as the moisture content of leaves of arid land plants can increase markedly at night as the air cools and relative humidity increases. Consequently, there is potential for rapid behavioural responses to a warming climate, without sacrificing foraging time. However, behavioural adjustments in habitat use often involve trade-offs between positive and negative factors, such as shade-seeking or the timing of activity may increase predation risk (Godvik, et al., 2009; Hebblewhite, et al., 2009). The exact trade-offs experienced by large herbivores in relationship to climate change is largely unknown and the costs of using phenotypic plasticity remains to be discovered.
These behavioural adjustments may buffer the impact of climate change (Mitchell, et al., 2008; Kearney, et al., 2009; Huey, et al., 2012), but for endotherms such behavioural adjustments may compromise homeothermy (Hetem, et al., 2012) and we may start to see changes in an animal's physiology. Body temperature is a relatively easy measure of physiology and likely to be affected by changing temperature and resources.
The next following sections will focus on the adaptations that ungulates may use to curb high environmental temperatures, which are likely to occur with climate change. In order for these endotherms to inhabit dry hot areas, they need to adopt physiological and behavioural mechanisms that will allow them to keep an internal temperature substantially different from the prevailing environment (Tattersall, et al., 2012), using thermoregulation. Thermoregulation is defined as "the process whereby animals maintain body temperature within a restricted range utilizing autonomic control mechanisms that evoke biochemical, physiological, and behavioural processes that modify heat loads internally and externally" (Tattersall, et al., 2012). This process is done by losing heat passively to the environment by means of radiation, convection and conduction if the body temperature is higher than ambient temperature (Cain III, et al., 2006). But if the temperature gradient between the animal and the environment becomes too small or when ambient temperature exceeds body temperature, an endotherm can either use evaporative cooling, or tolerate an increase in ambient temperature (Cain III, et al., 2006; Tattersall, et al., 2012).
Evaporating cooling, or better known as evaporative heat loss, can occur either by the evaporation water from the skin (through sweating/licking) or respiratory tract (panting). Evaporative cooling is the most effective method of regulating body temperature when the ambient temperature exceeds body temperature (Cain III, et al., 2006). Indeed, normally hydrated ungulates with free access to water can maintain body temperature within a fairly narrow range (Taylor 1970a, b). When hydrated, sweating and panting generally increases with an increase in ambient temperatures in order to maintain a constant body temperature. However, when dehydrated, some species (e.g., Grant's gazelle (Nanger granti), Thomson's gazelle (Eudorcas thomsonii), oryx (Oryx beisa), eland (Taurotragus oryx), and camel (Camelus dromedarius)) have the potential to reduce sweating by as much as 89% and the rate at which the animal's body temperature fluctuates increases over a wider range (Schmidt-Nielsen, et al. 1957, Taylor 1970b). This wider fluctuation in body temperature is defined as dehydration-induced hyperthermia (Cain III, et al., 2006; Angilletta, et al., 2010; Tattersall, et al., 2012). However, when dehydrated animals evaporative cooling potential is limited as they have to "trade off" osmoregulation (control of body temperature) with thermoregulation (control of body temperature). Therefore ungulates possess a limited capacity for evaporative cooling because dehydration impairs performance and could ultimately lead to death and the optimal strategy of thermoregulation in a hot environment depends on an individual's state of hydration (Angilletta, et al., 2010). In an arid ecosystem like the Kalahari where surface water is not freely available all year round and is one of the most important resources competed for (Eloff, 1984). Ungulates may have insufficient access to water and would need to use other strategies to maintain homeothermy.
As mentioned earlier, some desert adapted animals may tolerate high body temperatures (E.g., Arabian oryx (Oryx leucoryx) and sand gazelle (Gazella leptoceros)) and may be better adapted to cope with high ambient temperatures. 4 This tolerance is a process called hyperthermia which allows the body temperature to rise in conjunction with ambient temperature decreasing the thermal differential for heat gain from the environment and conserving body water. According to Tattersall et al. (2012), this strategy is fairly common for mammals. Large mammals theoretically have sufficient thermal inertia to tolerate progressive hyperthermia for the entire day, before radiating the stored heat to the night sky when ambient temperatures fall below the body temperature (Tattersall, et al., 2012). For example the dromedary camel, has the ability to store up to 6Â° C when it's dehydrated (Schmidt-Nielsen, et al., 1957). By using this method, the animal can conserve body water by reducing the evaporative water loss that is initially required for thermoregulation (Schmidt-Nielsen, et al., 1957). However, it does not come without limitations. Such hyperthermia is likely to occur with dehydration and may reflect a stress rather than adaptation strategy. Physiological performances may need to be traded-off against hyperthermia and eventually a critical body temperature will be reached and the animal will die (Tattersall, et al., 2012). It was originally thought to be adaptive, but the adaptive value has recently been questioned since hyperthermia has not been demonstrated in hydrated individuals. Whether such hyperthermia is a programmed active process, or whether it is incidental, resulting from failure of the water sources necessary for thermoregulation, remains to be investigated.
It is not only high temperatures that affect thermoregulation. When ambient temperatures are low, animals consume a large amount of energy to increase metabolic heat production required to maintain a constant body temperature (Tattersall, et al., 2012). Thus, the energetic consequences at maintaining thermoregulation at low temperatures could be relatively high especially when food is scarce (Tattersall, et al., 2012). But mammals, as resilient as they are, have developed another strategy called hypothermia as a surviving mechanism. Hypothermia is a process whereby the body temperature drops by a small amount (Tattersall, et al., 2012). This in turn decreases the energetic costs of maintain of maintaining homeothermy.