The auditory evoked response as an indicator of stress in free-ranging animals

Technology that allows remote monitoring of animals in the field, while freely behaving, would remove many of the problems imposed upon stress studies by experimenter intervention. The acquisition of data from free-ranging animals is difficult, but has significant advantages for studies of stress, nutrition, and behaviour in domestic animals. To perform this remote monitoring function I developed the Free-Range Physiological Monitor. The FRPM. The FRPM is a state-of-the-art ambulatory physiological monitor, designed to enable researchers to record aspects of the stress response simultaneously and on a number of free-ranging animals. The FRPM incorporates a powerful microprocessor to analyse the recorded data on the animal itself, and can operate indefinitely in the field by solar recharging of its internal batteries. It incorporates a high-speed radio modem with a range of up to 2km to allow the transmission of measurements in real-time to a base computer and to communicate with the researcher. In its current configuration the FRPM enables the assessment of central nervous system activity through the recording of both the electroencephalogram and the Auditory Evoked Response (AER), allows the monitoring of cardiac (heart) activity through the electrocardiogram (ECG), and provides support for measurement of additional stress parameters such as temperature, respiration rate, and physical activity. The long-range and extended duration capability, and the types of information that can be measured, combine to make the FRPM a unique tool for the welfare assessment of free-ranging animals. The potential of the technology for application in animal welfare research is widespread. Following development of the FRPM it was considered necessary to validate its design in an experimental setting. As an initial application, the use of the AER as a potential indicator of stress in freely behaving sheep was investigated. This application was considered “high-risk” in that although there was no unequivocal evidence of such a link, there was considered sufficient evidence in the literature to justify a study into the possibility. This application also provided an ideal environment to test the capabilities and reliability of the FRPM. As the AER originates from the brain itself, it was hypothesised that any change in the operation of the brain, such as that associated with stress, might cause the AER to be modified. To test this hypothesis 12 Romney-cross sheep were individually instrumented with the FRPM, and AER responses recorded as each animal was exposed to potential stressors. The AER results were complemented by heartrate, also recorded by the FRPM, and behaviour observations which helped to provide additional insight into the animals’ response to the stressors. In total, each animal was exposed to four separate stressors. These comprised of Control (no stressor), Distraction (a moving, noisy water sculpture), Isolation (separation from the flock), and Fear (exposure to a dog) stressors. Based on the data acquired in this study, the use of the AER as an indicator of stress appears to be valid, however a larger study would be required to establish a robust relationship. Waveform sequences acquired from individual animals subjected to stressors showed considerable modification during exposure to the stressor and, in several cases, suggested evidence of recovery-type trends upon stressor application and removal. However, this was not the case for all animals in the experimental group, neither was it the case with the acquired heartrate and behavioral data. These results suggest that different subjects can show quite individual responses to the same stressor, making the creation of a general model for changes in the AER as a quantifier of stress difficult. However, the AER may be a useful measure within an individual and when combined with more conventional measures such as heartrate and behaviour, which in themselves can be difficult to interpret, the AER should provide additional insight into the stress responses of freely-behaving animals