In previous comparative studies of buffaloes and temperate cattle, a greater increase in rectal temperature (RT) and skin temperature (ST), and a greater decrease in haematocrit (Ht) have been observed in buffaloes than in temperate cattle with an increase in ambient temperature (AT). Our series of previous experiments suggested that great changes in RT, ST and Ht are induced in buffaloes by a marked increase in blood flow from the body core to the surface, which accelerates dissipation of heat from the skin surface. On the basis of these suggestions, the present study was undertaken to compare fluctuations in RT, ST and Ht between buffaloes and tropical cattle. Fluctuations in the aforementioned parameters, particularly RT and Ht, were greater in buffaloes than in cattle. Moreover, the correlation for RT or Ht v. AT was significant for buffaloes (r=0·33 and −0·37, respectively) but not for cattle. The correlation coefficient for ST v. AT was significant in both species, but was greater in buffaloes (r=0·63) than in cattle (r=0·56). These results demonstrate that with changes in ambient temperature, RT, ST and Ht fluctuate much more in buffaloes than in tropical cattle, as found previously for temperate cattle. Therefore, the distinctive thermoregulatory responses of buffalo are confirmed as being specific to this species.
From previous findings that the physiological responses of buffaloes to hot conditions were characterized by an increase in blood volume, a decrease in hematocrit (Ht) and an increase in skin temperature, we hypothesized that an increase in blood volume in buffaloes contributes to heat transportation from the body core (rectum) to the surface (skin). As a result, the rectal-skin temperature difference in buffaloes was small with the decrease in Ht value and was large with the increase in Ht value. From these results, it was concluded that an increase in blood volume in buffaloes was rapidly caused by the exposure to hot conditions and contributed to reduce the rectal-skin temperature difference and to increase the temperature difference between skin and environment. These characteristics of buffaloes may be advantageous to increase sensible heat loss from the skin surface during a wallowing.
The vasopressin (VP) and aldosterone responses of swamp buffaloes and Friesian cattle were compared during 3 days of ad libitum water availability and 3 days of water deprivation. During the period of hydration, buffaloes showed significantly higher VP concentrations (2.31±0.09 v. 1.23±0.07 pg/ml), in spite of larger water excretion rate through urine, relative to cattle. During the period of dehydration, although VP increased progressively in both species, the concentration in the buffaloes (66.7±18.6 pg/ml) was five times higher than that of the cattle (12.5±3.2 pg/ml) on Day 3. Even though the rate of urine excretion in the buffaloes decreased markedly during water deprivation, the total volume of urine excretion was still greater than that of the cattle. Haematocrit, serum osmotic pressure and total protein in both species increased significantly during water deprivation, but the rate of the increase was greater in buffaloes than in cattle. There was no correlation between sodium or potassium excretion and aldosterone concentration in either species. These results demonstrated that the VP elevation in buffaloes was largely attributable to the greater decrease of plasma volume caused by the greater water excretion. The lower retentivity of plasma volume probably means that buffaloes have a greater dependence on external water rather than to conserve internal water, because of the evolutionary adaptation of buffaloes to wet environments.
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