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Shading behaviour in sheep: The influence of social and thermal factors

Sherwin, C.M. (1987) Shading behaviour in sheep: The influence of social and thermal factors. PhD thesis, Murdoch University.

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Abstract

Most homeotherms are believed to avoid large deviations of deep body temperature by choosing thermally preferable environments. There is evidence to suggest that this is not always true. Johnson (1987) found in two flocks of sheep that some individuals repeatedly spent time in the sun while others shaded. The animals voluntarily exposing themselves to high levels of solar radiation behaved in a fashion apparently contrary to accepted principles of homeostasis. This thesis describes investigations of the differences in shading behaviour amongst sheep and any consequences of these for their thermal status.

Three reasons for differences in shading behaviour were proposed: (1) that social differentiation within a flock enables some sheep to have priority of access to shade. (2) that sheep which remain in the sun are better able to effect heat losses when under heat load than are animals which seek shade, (3) that sheep remaining in the sun make reduced attempts to thermoregulate and instead adopt a strategy whereby body temperatures are allowed to rise.

To examine the first of these propositions, several social behaviours (leadership, dominance and aggressive tendencies) within a flock of sheep were quantified. These were compared with the amount of time each sheep spent shading. Social factors were found to be related to shade use, though thermal factors also had some influence.

To examine the second and third possibilities, a system was developed whereby deep body and skin temperatures could be recorded on freely ranging sheep. Four of these measurement systems were used to determine the thermal status of sheep at pasture. During this trial. experimental sheep did not use available shade but remained in the sun. In order to measure the effect of shading, the recording systems were therefore used again some weeks later when the same animals were confined to shade.

Because these two investigations took place several weeks apart, wool length and climatic conditions had changed and were confounding variables. To allow corrections for these, a sample of the animals was examined indoors under controlled conditions. Under simulated shaded and non-shaded conditions, the effects of wool length, air temperature and radiation load on thermoregulatory response were investigated. The observed relationships allowed predictions to be made of the probable effects of shade on the thermal status of sheep in the field.

From the results of the indoor investigations, it was possible to predict that the body temperatures and respiration rates of the free-ranging sheep staying in the sun would be significantly raised above those expected if they had sought shade. The extent of this thermal disturbance was not great, but it was unexpected since shade was readily available to the sheep. Several explanations of why the sheep did not use shade are considered. The observations made suggest that the sheep behaved in a sub-optimal manner contrary to accepted behavioural models. The results have revealed new theoretical and practical aspects of the shading behaviour of sheep.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary Studies
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Johnson, Ken
URI: http://researchrepository.murdoch.edu.au/id/eprint/53285
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