Hyperthermia induced oxidative damage in two wild caught African rodents, the diurnal four striped-field mouse Rhabdomys dilectus and the nocturnal Namaqua rock mouse Micaelamys namaquensis

Abstract

This thesis set out to investigate how heat stress may affect the oxidative balance and influence the activity patterns of animals. The use of wild animals has been scarce in testing the effects of heat stress on the oxidative balance of animals, especially under a simulated heat wave condition. Animals are expected to differ in their oxidative stress in response to heat stress, depending on their thermotolerance and the life history trait of activity is tested here as an indirect measure of thermotolerance. Therefore, two rodent species within the same family were used for experiments, one with a nocturnal activity rhythm and the other with a crepuscular/diurnal activity rhythm. Animals were tested firstly in a laboratory whole-body hyperthermia experiment, followed by a 3-day heat stress test with extreme temperatures. Following these experiments, animals were subjected to a simulated heat wave to determine how ecologically relevant temperatures may influence not just the oxidative balance of individuals, but also their activity patterns and water drinking in the absence of a microclimate. In order to determine the oxidative balance of individuals under control and heat treatment conditions, markers of oxidative damage and antioxidant defense were used. Markers for lipid and protein oxidation and a marker of enzymatic antioxidant activity and total non-enzymatic antioxidant activity were investigated in the liver, kidney, brain and heart. During the simulated heat wave, activity was measured with video recordings, with the videos analysed for water drinking behaviour as counts and time spent drinking water. During the whole-body hyperthermia experiment without the extreme temperatures, both species demonstrated no significant changes in liver oxidative markers except for high oxidative damage in the kidney for R. dilectus and high oxidative damage in the brain for M. namaquensis. During the 3-day acclimation experiment with extreme temperatures, both species were oxidatively stressed in the heart, with significant oxidative damage in the kidney for R. dilectus and significant oxidative damage in the liver for M. namaquensis. During the simulated heat wave, both species showed no significant changes in liver oxidative balance. The kidney was oxidatively stressed for M. namaquensis, with both species exhibiting signs of oxidative stress in the brain. Animals did not shift their activity during the heat wave, but rhythms demonstrated signs of disruption during the simulated heat wave. Activity increased during the day for both species during the heat wave. This was due to animals being uncomfortable and drinking more water. For the nocturnal species, the frequency of drinking water increased during the day and during the night during the simulated heat wave. The crepuscular species only increased in the amount of time spent drinking water during the day during the simulated heat wave. Overall, the nocturnal species was more susceptible to heat stress inducing either increased oxidative damage or reduced antioxidant defense compared to the crepuscular species, but this was tissue specific and also dependant on the experimental condition. It is recommended that future studies directly measure the reduced: oxidised state of individuals in addition to markers of defense and damage.