in water loss. Similarly, SCHMIDT-NIELSEN and his collaborators (1967) have shown
that the camel reduces its oxygen consumption when dehydrated. Such a decrease
in metabolism is not invariably seen in dehydrated mammals. Thus the East African
waterbuck, Kobus defassa ugandae, fails to change its rate of oxygen consumption
when deprived of water (TAYLOR, SPINAGE, and LYMAN, 1969). Largely as a result
of the high rate of accompanying evaporative water loss these antelope withstand
water deprivation poorly, even when compared with domestic Hereford cattle. It
seems possible that the ability to reduce the metabolic rate under such conditions
constitutes a physiological reaction that could adapt the animals more readily to
the changed circumstances. The thyroid gland influences the metabolic rate in
mammals, but it is not known whether it is concerned with the changes that are
observed in dehydrated animals. Such a possibility should be simple to test.
1') Urinary . Urinary water losses occur either as a homeostatic response like excreting a dietary excess of fluid, or as an unavoidable consequence of the necessity
to excrete solutes. Excess water may be taken in, especially by herbivorous species
eating large amounts of succulent food. Solutes that must be excreted may represent
an excess which is taken in with the food and water, or be metabolic solutes, principally urea in mammals.
The magnitude of the renal water loss varies and depends on the diet, the environmental conditions and the ability of the animal to concentrate its urine. Diets
with a high salt content or that are rich in protein, necessitating additional urea
excretion, increase urinary water loss. The proportion of the total water loss that
is represented by the urine will vary, depending mainly on the environmental conditions that influence the extrarenal loss. In normal circumstances urinary water
loss is less than 50% of the total water loss, and may be lower than 10%, especially
in hot environments where evaporative loss is high. Mammals and birds, in contrast
to other vertebrates, can form a urine which is hyperosmotic to their body fluids
- the more concentrated, the greater the saving in water. The maximal ability to
concentrate the urine varies considerably in different mammals; the mountain beaver, Aplodontia rufa, can secrete urine with a maximum concentration of about
500 m-Osmole/l , (DICKER and EGGLETON, 1964) while the North African sand
rat, Psammomys obesus, can form urine over 6000 m-osmole/l, (SCHMIDT-NIELSEN,
1964a) and Australian hopping mice, Notomys alexis, 9000 m-osmole/l. (MAcMILLEN and LEE, 1969). Neurohypophysial hormone normally initiates the formation
of a hyperosmotic urine.
d) Faecal. The faeces of mammals usually contain 50 to 60% water, but this may
be as high as 85% in grazing cattle . The faeces of camels with water to drink, contain
about 55% water, but after a day without water this level drops to about 45%
(SCHMIDT-NIELSEN, 1964a). It is unknown whether such changes involve special
regulatory mechanisms mediated by hormones.
92
that the camel reduces its oxygen consumption when dehydrated. Such a decrease
in metabolism is not invariably seen in dehydrated mammals. Thus the East African
waterbuck, Kobus defassa ugandae, fails to change its rate of oxygen consumption
when deprived of water (TAYLOR, SPINAGE, and LYMAN, 1969). Largely as a result
of the high rate of accompanying evaporative water loss these antelope withstand
water deprivation poorly, even when compared with domestic Hereford cattle. It
seems possible that the ability to reduce the metabolic rate under such conditions
constitutes a physiological reaction that could adapt the animals more readily to
the changed circumstances. The thyroid gland influences the metabolic rate in
mammals, but it is not known whether it is concerned with the changes that are
observed in dehydrated animals. Such a possibility should be simple to test.
1') Urinary . Urinary water losses occur either as a homeostatic response like excreting a dietary excess of fluid, or as an unavoidable consequence of the necessity
to excrete solutes. Excess water may be taken in, especially by herbivorous species
eating large amounts of succulent food. Solutes that must be excreted may represent
an excess which is taken in with the food and water, or be metabolic solutes, principally urea in mammals.
The magnitude of the renal water loss varies and depends on the diet, the environmental conditions and the ability of the animal to concentrate its urine. Diets
with a high salt content or that are rich in protein, necessitating additional urea
excretion, increase urinary water loss. The proportion of the total water loss that
is represented by the urine will vary, depending mainly on the environmental conditions that influence the extrarenal loss. In normal circumstances urinary water
loss is less than 50% of the total water loss, and may be lower than 10%, especially
in hot environments where evaporative loss is high. Mammals and birds, in contrast
to other vertebrates, can form a urine which is hyperosmotic to their body fluids
- the more concentrated, the greater the saving in water. The maximal ability to
concentrate the urine varies considerably in different mammals; the mountain beaver, Aplodontia rufa, can secrete urine with a maximum concentration of about
500 m-Osmole/l , (DICKER and EGGLETON, 1964) while the North African sand
rat, Psammomys obesus, can form urine over 6000 m-osmole/l, (SCHMIDT-NIELSEN,
1964a) and Australian hopping mice, Notomys alexis, 9000 m-osmole/l. (MAcMILLEN and LEE, 1969). Neurohypophysial hormone normally initiates the formation
of a hyperosmotic urine.
d) Faecal. The faeces of mammals usually contain 50 to 60% water, but this may
be as high as 85% in grazing cattle . The faeces of camels with water to drink, contain
about 55% water, but after a day without water this level drops to about 45%
(SCHMIDT-NIELSEN, 1964a). It is unknown whether such changes involve special
regulatory mechanisms mediated by hormones.
92