Living on the edge: physiological and behavioural plasticity of African antelopes along a climatic gradient

Climate change, habitat loss and fragmentation individually or synergistically force species to live in a sub-optimal condition in terms of climate and resource posing threat to fitness and survival of the species. Hence, a very pressing issue for biodiversity conservation at present is to understand if species are able to keep pace with these rapidly changing environment conditions. To persist with these changes, phenotypic plasticity of behaviour and physiology may be the most likely response for long-lived endothermic species because of their longer generation times. Therefore, the central aim of this thesis is to investigate intra- and intervariability of behavioural and physiological adaptation of range of African antelopes along spatio-temporal scales in their natural habitats. With the aim to understand the behavioural plasticity of African antelopes to the climatic stress, in Chapter 2, I investigated effect of heat stress on diurnal activity pattern of three species of antelopes of different body size and feeding types namely, eland Taurotragus oryx (≈ 420 kg; mixed feeders), blue wildebeest Connochaetes taurinus (≈180 kg; grazer) and impala Aepycerus melampus (≈ 50 kg; mixed feeder) across season and extreme climatic condition as indicated by the 10 hottest days. During summer when the heat stress was its highest, the heat stress negatively influenced diurnal activity of all the three species. However, they shifted the timing of their activity more to the early morning (eland) or late in the evening, or both (wildebeest and impala) to avoid heat stress and maximize intake of food in a season when forage is abundant. During the spring and the 10 hottest days over the entire study period, only the diurnal activity of the larger antelopes (eland and wildebeest) was negatively influenced by the heat stress whereas the smaller impala was unaffected. Therefore, these large African antelopes apparently suffer from heat stress during spring and the extreme hottest days due to their limited capacity to dissipate heat. In chapter 3, to understand possible behavioural adaptation of the largest African antelope eland against the thermal stress, I investigated the daily and seasonal selection of microhabitats based on altitude and microclimate at the southern limits of its distributional range. Eland actively selected lower altitudes with warm microclimates during the winter and the five coldest days when the ambient conditions were below its thermal neutral zone. In contrast, eland did not select higher altitudes or cooler climate when it was warm in the summer. However, selection of cooler microhabitats was only evident in the three extremely hottest days when the heat stress was close to the upper end of its thermal neutral zone. Hence, the eland was able to use diverse topography as a thermal refuge to buffer the adverse effect of both cold and very hot condition. In the fourth chapter, to study behavioural response of African antelopes to variation in food resources which is predicted to exacerbate due to climate change and habitat loss and fragmentation, I investigated adaptation of home range sizes of eland, impala (both mixed feeders) and wildebeest (a grazer) over time (seasons) and between two climatically contrasting areas in South Africa, taking Mapungubwe National Park as the core area and Asante Sana Game Reserve as the edge area. This comparative study not only showed the home range size of wildebeest in Mapungubwe was larger during the resource-poor dry season compared to the resource-rich wet season but their home range size in the core area was also a four to seven times larger in the dry season than those in the edge area. In contrast, the home range size of impala was 3-14 times larger in the edge area than those in the core area. Surprisingly, the home range size of eland neither differs across any season within study areas nor between Asante Sana and Mapungubwe, while their average year-round home range size in core area was larger than that in edge area. These results suggest that the home range size of these African antelope is most likely a response to resource quality and availability specific to the local habitat. With an attempt to investigate physiological plasticity of African antelopes over a spatiotemporal context, in Chapter 5, I compared intraspecific variation of body temperature, as measured by amplitude, of the eland, blue wildebeest and impala in the two climatically contrasting areas: one with a less seasonal pattern and a mild winter (Mapungubwe National Park) and the other with a more seasonal pattern and a long and cold winter (Asante Sana Game Reserve). The 24-hour amplitude of body temperature of both mixed feeder (eland and impala) did not differ between the study sites, regardless of season. In contrast, the grazer (wildebeest) at a less seasonal site exhibited not only a higher variability in the 24-hour amplitude of body temperature (Tb)(~4oC) but also a lower daily minimum body temperature by ~2 oC compare to the normothermic level during the dry season than the wildebeest at a seasonal site. Further, the variation in Tb amplitude were influenced both by temperature (positive effect) and rainfall (negative effect), a proxy for food availability only among the wildebeest from less seasonal site. This suggest that these physiological response of higher variability of Tb amplitude and reduced minimum Tb among the wildebeest in Mapungubwe is a response to nutritional stress rather than a response to cold climate. These behavioural (home range) and physiological (body temperature) response of African antelopes to stressful conditions are specific for species and habitats. The smallest impala, which is a mixed feeder, maintained homeothermy even though they were exposed to stressful habitats by selecting the most productive habitat, i.e., riparian habitat in Mapungubwe. In Asante Sana, impala maintained homeothermic status by extending their dry season home range size when their principle food Acacia Karoo was not available. The largest antelope (eland) maintained homeostasis by ranging over large areas to track heterogeneously distributed resources, which is only possible due to their large size and ability to cope with lower quality food. Eland in Mapungubwe had larger home range sizes compared to Asante Sana which was most probably due to the poor quality of the habitat in Mapungubwe. Interestingly, the wildebeest in Mapungubwe did not maintain homeothermy particularly in dry season. Not only their amplitude of Tb was much larger (~4oC) and Minimum Tb lowered by 2 oC compared to normothermic level, they also extended their home range size four to seven folds compared to the wildebeest in Asante Sana. This failure to maintain homeothermy and extension of home range size was due to nutritional stress and therefore these antelopes are living in a physiologically stressful environment. With the predicted increase in the frequency and intensity of drought periods in southern Africa due to climate change, wildebeest, and other grazers, will likely experience greater nutritional stress in the future. To conclude, this thesis shows importance of studying behavioural and physiological traits among a range of species along temporal and spatial scales in their natural habitats to understand the adaptive capacity, therefore sensitivity of animal species. Apparently, homeothermic mammals cannot cope well with heat stress, which negatively influence the larger ones more than the smaller ones. However, mammals can overcome these stresses by shifting time of their activity to cooler parts of the day or by selecting optimal microhabitats that minimize absorption of heat at high temperatures or that maximize the absorption of heat at low temperatures. The behavioural (larger home range size) and physiological (reduced body temperature) response of wildebeest, a grazer to dry season but not that of the mixed feeder emphasizes that grazers will become more nutritionally stressed than mixed feeders at times of low rainfall. With the predicted increase in low rainfall events in many parts of the world and changes in vegetation structure in savannas due to climate change, browsers and mixed feeders will be likely to benefit more in future than the ruminant grazers.