Thermoregulation And Heat Balance In Some East African Herbivores
Finch, Virginia A
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This thesis is an attempt to understand some aspects of the nature of the thermal physiology and physics of energy exchanges of animals studied in near-natural field conditions. Three indigenous species of herbivores, eland, hartebeest and zebu cattle and one exotic breed, Ayrshire ,cattle, were crio sen for comparison of thermoregulatory responses to solar radiation. Elal1d,zebu, and Ayrshire use sweating primarily and hartebeest use panting primarily as a means of evaporative cooling. All four types of herbivores have been studied extensively in the climatic chamber. The experimental site for the study was isfsemi-arid bush cOQntry within a moderate temperature zone (max. air temperature, 320C), however solar radiation reached values of1.44 cal/cm2.min. Section 1 When animals were maintained in the sun all ,day evaporative water losses increased steadily reaching maxim values concurrently with the''Period of maximum solar radiation. The effect of solm::radiation on cutaneous moisture vaporization was a doubling of maximum rates r8corded in the l~boratory. Changes in both cutaneous moisture vaporization and respiratory rate were significantly correlated with changes in skin temperature as measured on the back, but not with changes in rectal temperature. Correla tion between skin temperature and solar intensity was fo:md to be high. As skin temperature in the hydrated eland, zebu and hartebeest represented a mean value of 80% of the variability of evap- orative moisture loss and rectal temperature represented 25%, it appeared that skL~ temperature was the more important input to the thermo- regulatory center. During dehydration for the zebu and eland, skin temperature remained the more important factor affecting 0vaporative water losses, although there was a tendency for the effects of rectal temperature to increase. In the hartebeest, rectal temperature became the :.,...i. more important component affecting respiratory rate. From the sharply distinguishable differences between insensible and sensible water loss in relation to skin temperare, critical skin t~mperature for the onset of evaporative moisture loss was-determined. The onset of sweating and panting.,occurred sIrnult.aneously for the hydrated zebu and eland at.skin temperature of approximately 310C for both species, while the critical SkDl temperature for onset of panting in the ha..rtebeest was apprOXl• maG.i,e 1y330C'. During dehydration there was a delay in onset of evaporative water losses in both the zebu and eland until a higher critical skin temperature between 35 and In the hartebeest during dehydration, onset of panting was more closely related to rectal temperature, the critical temperature being 39.50C. In the hartebeest, .comparison of the coefficients of diffusion when hydrated and dehydrated showed that the rate of water loss was significantly lower during dehydration. This result was not found for the zebu and eland. During dehydration cutaneous moisture vaporization was reduced by a mean value of 40% in all species whereas respiratory rate was retained or increased over hydration. Mean daily rectal teBlJierature,on the whole, was not higher than during hydration, and there was a reduction in mean daily rectal temperature from hydration on the last day of dehydration in all species. In these experiments in a moderate temperature zone, a lowering of metabolic heat production could serve to reduce the total heat load, thereby reducing the 'necessity to loose heat '. evaRorative means and maintaining by.' the normal body temperature as measured during hydration. _ ... t>'_ Section 2 The effect of an imposed climatic stress of a hot, dry environment on sweat gland secretion, respiratory rate, and body temperature of a non-arid adapted cattle breed was investigated over a period of two years. This was done by transferring two pure-bred Ayrshire steers from a cooler, more moist environment to a hotter, more dry environment. Initially cut aneous evaporative water losses were 1.5 times greater, respiratory rate 2.5 to 3 times greater, and rectal temperature 10C higher than that of resident arid-adapted zebu. During the last period of measurement, rectal temperature in the Ayrshire was reduced to values similar to the zebu, while respiration rates and cutaneous vapor.j.z;ationa,lthough reduced from the initial period, remained somewhat higher than the zebu. These reductions suggested an adjustment of metabolic heat production so that lower evapora- tive water losses were suf'fLci.errto maintain a lower body temperature. Fiye months after the introduction of the Ayrshire cattle to the hotter environment, cutan- ....... eous molstu_:rzveaporization decreased to one-fifth its original v.a.l.,u"e An intravenous injection of adrenaline failed to stimulate sweat gLand secre- tion. T.he reduc.tion,incutaneous vaporization was thought to be associated with a combi.na.t Lon of the low nutritional level of the available pasture and probable depressed food intake from climatic heat stress which may have adversely affected follicle growth and thereby reduced sweat gland activity. When grazing conditions improved, cutaneous evaporation increased to values somewhat similar to the zebu. ... ~- Section 3 The energy exchange between animals and the radiant environment was assessed by estimating each flux of radiant energy absorbed and each mode of heat dissipated in the field under conditions of intense solar radiation for the eland, hartebeest and zebu. The calculation of energy exchanges were based on formulas which treated the animal's shape as an open-ended cylinder. The mean effective absorptance of shortwave radiation by the fur coat was 0.65 in the hartebeest, 0.(5 in the eland, and between 0.78 and 0.81 in the zebu, while absorptance of 19n~vave radiation was considered to be unity. The largest single component of the environmental heat loag.was Longwave radiation emitted from the ground. The totaJ.:Longwave radiation from the ground and atmosphere contributed 58% to the absorbed radiation, the remainder being due to absorbed shortwave radiation. Since the net longwave radiative heat flow was away from the animal's body and toward the environment, the real heat stress on the animal was from shortwave radiation. Metabolic heat production contributed 8 to 11% to the total heat gain, the remaining heat gain being due to absorbed environmental radiation. Of the heat absorbed, 58 to 60% in the eland and zebu and 68% in the hartebeest was reradiated from the fur of the animal's coat. Heat dissipa- tion through respiratory evaporation was between 4 and 5% in the eland and zebu and 8% in the harte- beest of the total heat loss. ~ecause cutaneous evaporation varied over the different skin regions, preliminary mapping trials were performed. As determined from.",, .~ mean of four measurements around each animal's circumference, heat dissipation through cutaneous water loss was approximately 20% in the zebu and eland and 5% in the hartebeest of the total heat loss. Heat storage was small in all species, and rectal temperature during the period of measure- ment (0900 to 1500 hr) under resting metabolic conditions rose a mean value of 0.50C in all species. ConvectiDnal heat loss amounted to 14 to l_ 6</0< in the zebu and eland and "18% in the hartebeest of the total heat dissipation. In the zebu and eland, whose rectal temperatLce was approximately 20C higher than mean skin temper- ature, there was a gradient of heat flow from the core to the skin surface. In the hartebeest, mean skin temperature was 0.5o C higher than rectal temperature, providing a small gradient of heat flow inward to the animal's core. In all $pecies, mean fur surface temperature was higher than mean skin temperature resulting in a gradient of heat flow toward the animal's skin. Mter heat loss from the fur through convection and re radiation, 17 to 20% in the eland and zebu and 5% in the hartebeest of absorbed radiation was available for conductance to the a~imal's body. In the case of the eland and zebu, cutaneous moisture loss dissipated all heat absorbed as well as a portion of the metabolic heat. An attempt .).VJasmade to assess the importance of coat structure as a mechanism for reducing the severity of solar radiation. Error in the system resulted in inconclusive evidence about heat flow through the coat. Howeve!, there was some evidence to suggest that heat flow was least through the t~icker: more dense coat of the hartebeest and greatest through the thinner, more sparce coat of ..... the el.and, The degre~ ot''accuracy of the heat balance measurements were evaluated by analyzing the errors in the system. .In rel, ation to the heat b2~ance est Lmat Lon (Eq_uation3.2.7) the mean percentage error between heat loss and heat gain was approx- xxiii imately 3%; however, in relation to net radia tion exchanges and metabolic heat production the error was between 17 and 47%. A large portion of the error may be associated with error in calculating convectional heat loss. Comparison of the use of the cylinder as a model with radiation profile of sheep suggested that little error was associated with the cylinder model.