Metabolic responses to hyperthermia in two small desert mammals, the Pygmy rock mouse, Petromyscus Collinus and the Namaqua rock mouse, Aethomys Namaquensis.
Mowoe, Metoboroghene Oluwaseyi.
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The negative consequence of recent climate change on the Earth’s biodiversity has become more evident in recent years. Some animals, due to insularity or habitat fragmentation, are unable to shift their ranges altitudinally and latitudinally. Vulnerable species need to rely on behavioural and, more importantly, physiological responses in order to persist through present climatic changes. It has therefore become more obvious that physiological responses of individuals need to be incorporated into predictive models of the responses of mammals to accelerated climate change. The primary purpose of this study was to test the ‘Hyperthermic Daily Torpor’ hypothesis proposed recently by Lovegrove et al., (in press). The hypothesis suggests that, based on albeit limited evidence, some small mammals may be capable of hyperthermia induced hypometabolism equivalent to that experienced during torpor and hibernation in response to cold temperatures. These authors argue that such hyperthermic hypometabolism should reduce the risk of entry into pathological hyperthermia and also reduce the rate of water loss driven by heat-induced evaporative cooling. The reaction norms of desert mammals have been selected to be adaptive over a wide range of climatic conditions due to the unpredictability of their habitat. Thus, they are good models for testing the reaction norms that may be expressed in response to accelerated climate change. We therefore tested our hypothesis using two presumably heat-adapted desert rodents; the Namaqua rock mouse, Aethomys namaquensis, and the pygmy rock mouse, Petromyscus collinus, as model species. We used indirect respirometry to measure metabolic rate at high ambient temperatures. We progressively exposed the animals to high temperatures to induce thermal tolerance and thus minimize the risks of lethal hyperthermia. We also measured subcutaneous and core temperatures, using temperature-sensitive PIT tags (BioTherm Identipet) and modified iButtons (Maxim Integrated), respectively. A. namaquensis displayed the capacity for hyperthermia-induced hypometabolism (Q10 79 = 1.27 ± 1.61) whereas the P. collinus did not (Q10 = 2.45 ± 1.41). The implications of such a physiological response in A. namaquensis are crucial in terms of its capacity to minimize the risks of lethal, pathological hyperthermia. Recent models of endothermic responses to global warming based on ectothermic models predict a dichotomy in the thermoregulatory responses of mammals to high temperatures. This study, to our knowledge, provides some of the first data on these interspecific variations in the thermoregulatory responses of mammals to high temperatures. However, the different physiological responses to hyperthermia between these two species cannot be meaningfully interpreted without phylogenetically independent comparisons with other species, that is, a more expansive interspecific analysis. Nonetheless, we provide some autecological sketches to assist in future multivariate interspecific analyses. Physiological differences between captive or captive-bred and free-ranging mammals preclude the extrapolation of our findings to free-ranging mammals. It is almost impossible to collect MR data in the field, although a few authors have successfully done so, and it is often not feasible to collect Tb data in small free-ranging mammals. Most studies have therefore made use of externally-mounted temperature-sensitive data loggers in order to collect Tskin data as a proxy for Tcore data in free-ranging mammals. However, misleading gradients between Tskin and Tcore can occur if data loggers are placed too close to major-heat producing tissues and the effects of the external environment on these data loggers may result in large Tskin – Tcore gradients. The second objective of this thesis therefore was to test the validity of using subcutaneous temperatures (Tsub) from subcutaneously injected temperature-sensitive PIT tags as a proxy for Tcore using the Namaqua rock mouse, Aethomys namaquensis. We found that the difference between Tcore and Tsub was minimal (~ 0.34˚C) within the thermoneutral zone (TNZ) with slight, non-significant, differences outside the TNZ. There was a tendency for Tsub to underestimate Tcore below thermoneutrality and overestimate it above thermoneutrality. We attributed these differences to the various heat loss and heat gain mechanisms activated in response to heat and cold stress in order to maintain a setpoint Tb. Nevertheless, we found that the Tcore – Tskin differential never exceeded 1.59˚C above the wide 108 range of Tas (5˚ – 41˚C) measured. Thus, we can conclude that subcutaneous temperatures provide a reasonably reliable proxy for core temperature in small mammals.