Short-term thermoregulatory responses of horses to brief changes in ambient temperature
The first objective of this thesis is to study thermoregulation in standing horses, especially the short-term physiological responses to brief changes in the thermal environment. My hypothesis is that the characteristics of the thermoregulatory system reflects the evolutionary background of horses. Horses evolved on the steppe, where they lived in herds and fled when in danger. As homeothermic animals, they have to maintain an almost constant body core temperature by losing excess heat from a constantly ongoing internal heat production despite changes in the thermal environment. On the steppe, temperature, radiation and wind can vary greatly, which affects the means of heat loss. In case of flight, the horse has to lose the heat load induced by muscular work so as to restore the body temperature. A second objective is to measure the thermal properties and heat losses of horses to improve the management and housing of horses. The study was performed in a climatic chamber on five horse at six different temperature levels: -3°C, 6°C, l5°C, 20°C, 30°C and 37°C, The results, discussed in the frame story, are presented in four articles: I. Dissipation of heat from standing horses exposed to ambient temperatures between - 3°C and 37°C. II. Thermal insulance of peripheral tissue and coat in horses. III. Effects of short-term changes in ambient air temperature or altered insulation in horses. IV. Climatic energy demand of horses. I found that horses increase evaporative heat loss (50 - 450 W m^-2) by sweating and by increasing respiratory rate in warm and hot environments. This is a mechanism, that is evolved to handle the exercised-induced heat load. The evaporative heat loss is regulated by both body core and skin temperature. The tissue thermal insulance (0.015 - 0.100 m2 K W^-1) is an evolutionary compromise to provide insulation but not to be limiting to the processes of sweating. The coat, being a thin insulating layer of hair, is probably the best physical compromise for allowing the skin to lose heat by evaporation whilst providing protection from heavy solar loads during the day and radiative heat loss at night. However, in cold environments the insulation may not be sufficient to maintain body temperature. Since horses evolved to graze for most of the day, they can compensate for an excessive heat loss by regulating the feed intake. In cold climates, the horse needs extra feed, called climatic energy demand, which is estimated to 2.78 W m^-2 per degree Celcius below the lower critical temperature. The thermoneutral zone for the horses in the study is estimated to range from 5°C to 25°C. The fact that horses live in a herd is an advantage since it enables the herd members to protect and help each other to handle different thermal environments, e. g. by standing close together to limit the exposed area. In conclusion my synthesis of the study is that the thermoreglatory system of the horse has characteristics that are advantageous for an animal that has been evolved to be able to flee when in danger and to live freely on the steppe.
horse; body temperature; heart rate; respiratory rate; altered insulation; thermoregulation; thermal property; thermal insulance; thermal resistance; heat loss; latent heat loss; heat dissipation; evaporative heat loss; non-evaporative heat loss; sensible heat loss
Rapport - Sveriges lantbruksuniversitet, Institutionen för lantbruksteknik
Publisher: Institutionen för lantbruksteknik, Sveriges lantbruksuniversitet
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