Evaporative cooling in fowls

Part I1. Reasons are given for expressing the evaporative loss of the resting fowl in the following way:where C is a constant, f(tr) is a function of rectal temperature, ps is the vapour pressure of a saturated atmosphere at the temperature of the evaporative surfaces, and pa is the vapour pressure of the ambient air.2. It was found that for practical physiological purposes the rectal temperature may be taken as that of evaporation, from which ps, is deduced. The real temperature of evaporation is probably a few degrees below this, as would be expected.3. It is concluded from this, that atmospheric humidity hinders the evaporative cooling of fowls less than that of men. Therefore, a farmer cannot estimate the comfort of his fowls in hot weather from his own subjective impressions. The experiments also explain theoretically, why evaporative coolers for poultry houses are successful in practice.PART II1. The evaporative loss over a wide range of rectal temperature is plotted in Fig. 1. By applying formula (ii) it was possible to prepare curves for 28 and 5 mm. Hg atmospheric vapour pressure. These vapour pressures are near the extremes of absolute humidity, which fowls naturally encounter in hot climates. Brown Leghorns were used, and it is pointed out that the results with other breeds might not be quite the same.2. It was calculated that part of the increase in evaporative cooling in hyperthermia is due to the rise in temperature of evaporation. At high rectal temperatures its importance is comparable with that of increased pulmonary ventilation.3. There was no significant sex difference in the relation between evaporative loss and rectal temperature (Fig. 2).4. The maximum possible evaporative cooling of fowls in equilibrium with their environment was 41 Kg.cal./sq.m./hr. at a humidity of 28 mm. Hg atmospheric vapour pressure and 64 Kg.cal./sq.m./hr. at 5 mm. This is much less than the maximum attained by man and somewhat less than that of the dog. Although evaporation per unit surface area was less than in the cow, it was much the same in relation to metabolic requirements.5. Next, certain discrepancies are considered in the relation between rectal temperature and evaporative loss (Tables 3–5). At rectal temperatures near the panting threshold evaporation was found to be greater at high air temperatures and during the night. For the night tests fragmentary data only were available. These findings are tentatively attributed to a lower normal body temperature, which lowered the panting threshold.