The neurophysiological regulation of temperament in sheep

Abstract

This thesis examines the neurophysiological underpinning mechanisms which may be pertinent for understanding temperament and its relationship to fearfulness in sheep. In the dose response experiment conducted in chapter 3, administration of diazepam (GABA agonist) and m-CPP (5-HT agonist) gave the most consistent physiological responses in the arena test and isolation box test, which aligned with extrapolated data. The interpretation of the behavioural responses was less clear. We designed a psychological challenge (the fear potentiation model) to induce fear utilizing a line of sheep selected for calm and nervous temperament. The final model consisted of two prior exposures to an isolation and dog challenge, and a final exposure to isolation only. Manipulation of the fear response was achieved with a potentiated fear state, causing the sheep to respond to the final isolation exposure with a marked reduction in the behavioural response, similar to freezing behaviour, which was still evident after a long period of time. This implies memory of the situation resulting in a behaviourally potentiated fear response. Physiologically, the isolation and dog challenge caused only a slight potentiated response. Additionally, sheep were able to perceive the dog's presence without visual or auditory cues, suggesting possible olfactory detection. We employed the use of selected pharmacological treatments to target neurotransmitter pathways to investigate their role on fearfulness and temperament whilst exposed to the challenge test, however the role of the pathways targeted (GABA and 5-HT) and influence upon temperament was not obvious. The selected pharmacological treatments did convey effects on behavioural and physiology; however this was not evident in observing differences in the temperament selection lines. The primary observation was that of a divergent behavioural response whilst the physiological response between calm and nervous was fairly inconsistent. The marked divergence in behaviour and limited divergence of the physiological response was consistent throughout the experiments using the selection line. It appears that selection has mostly resulted in common characteristics that differentiate the line behaviourally and not physiologically, whilst the physiological data for the selection line is not tightly linked to the behavioural responses during the challenge periods.