is 2). These levels change continually, until after 180 days of lactation the Na/K
ratio declines to 0.5 . The milk constitutes an additional drain on the salt resources
of lactating mammals, and the concentration of th e constituents exhibit ph ysiological changes. The mammary glands are embryologically homologous with the
sweat glands, wh ich, as we have seen, are subject to regul atory actions by aldosterone. The effects of corticosteroids on the composition of the milk do not appe ar
to have been examined.
The lacrymal glands secrete tears, which contain about 150 m-equiv sodium/l
and 15 m-equiv potassium /l (see THAYSEN, 1960). Excessive weeping during a p eriod of sodium deficiency would thus be inadvisable , but in most mammals this
situ ation is an unlikely on e. Our interest in these glands is mainly due to the presence of a modified lacrym al gland (orbital (salt gland ') in turtles, which functions
as a ma jor avenu e fo r salt excretion (SCHMIDT-NIELSEN, 1960), and which in th ese
reptiles may be influ enced by corticosteroid hormones (HOLMES and McBEAN ,
1964). In sheep, however, the infusion of aldosterone does not alter the lacrymal
secretion (BOTT et .d., 1966).
c) Accumulation. of Water and Salts
Water and salts are obtained:
1. In the food. Th e water content of plants is often equivalent to more than 95 %
of th eir weight, and even a carnivorous diet contains about 70 % w ater. In add ition
to such directly available fluid , the metabolism of fats, carbohydrates and proteins
result s in formation of additional water; th e yield from oxidation of on e gram of
fat is 1.1 ml, while th e same amount of carbohydrate gives 0.6 ml and protein 0.4
ml. There has been much conj ecture about th e import ance of fat reserves in assisting
animals to maintain a positi ve water balanc e. The camel's hump was probably th e
earliest recipient of such speculation, but th is has been discounted by KNUT
SCHMIDT-N IELSEN. It mu st be remembered th at the oxidation of an energy substrate requires an exchange of gases in the respiratory tr act , and thi s ma y result in
evaporative water loss. If the animals are breathing dr y air, oxidation of such substr ates will result in a net loss of water, due to satu ration of the expired air w ith
water vapour. Inspiration of more humid air will decrease thi s loss, so that in ma rine
mammals, like th e whal e, which breathe air already satur ated with water vapour,
a net gain of water from metabolism can be expected.
As alread y described th e sodium content of plants may vary considerably and
in some inland continental and alpine regions th e sodium levels may be low . This
is the prime determinant of the salt status of all herbivorous animals living in th e
area, while carnivorous spec ies sho uld obtain adequ ate sodium from the prey on
which the y feed. T he diet of other mammals ma y con sist predominantly of halophyt ic plants with a high concentration of salts such as eaten by African sand
rats (SCHMIDT-NIELSEN, 1964a) and poss ibly Au stral ian hopping mic e (MAcMILLEN and LEE, 1969).
2. In the drinking wate r. Drinking supplies th e w ater requ irement beyond th at obtained from th e food. O 'CO NNORand Po n s (1969) have emphasized th e predominant imp ortance of thi s process in regulation of a positive wa ter balance. The urin96
ratio declines to 0.5 . The milk constitutes an additional drain on the salt resources
of lactating mammals, and the concentration of th e constituents exhibit ph ysiological changes. The mammary glands are embryologically homologous with the
sweat glands, wh ich, as we have seen, are subject to regul atory actions by aldosterone. The effects of corticosteroids on the composition of the milk do not appe ar
to have been examined.
The lacrymal glands secrete tears, which contain about 150 m-equiv sodium/l
and 15 m-equiv potassium /l (see THAYSEN, 1960). Excessive weeping during a p eriod of sodium deficiency would thus be inadvisable , but in most mammals this
situ ation is an unlikely on e. Our interest in these glands is mainly due to the presence of a modified lacrym al gland (orbital (salt gland ') in turtles, which functions
as a ma jor avenu e fo r salt excretion (SCHMIDT-NIELSEN, 1960), and which in th ese
reptiles may be influ enced by corticosteroid hormones (HOLMES and McBEAN ,
1964). In sheep, however, the infusion of aldosterone does not alter the lacrymal
secretion (BOTT et .d., 1966).
c) Accumulation. of Water and Salts
Water and salts are obtained:
1. In the food. Th e water content of plants is often equivalent to more than 95 %
of th eir weight, and even a carnivorous diet contains about 70 % w ater. In add ition
to such directly available fluid , the metabolism of fats, carbohydrates and proteins
result s in formation of additional water; th e yield from oxidation of on e gram of
fat is 1.1 ml, while th e same amount of carbohydrate gives 0.6 ml and protein 0.4
ml. There has been much conj ecture about th e import ance of fat reserves in assisting
animals to maintain a positi ve water balanc e. The camel's hump was probably th e
earliest recipient of such speculation, but th is has been discounted by KNUT
SCHMIDT-N IELSEN. It mu st be remembered th at the oxidation of an energy substrate requires an exchange of gases in the respiratory tr act , and thi s ma y result in
evaporative water loss. If the animals are breathing dr y air, oxidation of such substr ates will result in a net loss of water, due to satu ration of the expired air w ith
water vapour. Inspiration of more humid air will decrease thi s loss, so that in ma rine
mammals, like th e whal e, which breathe air already satur ated with water vapour,
a net gain of water from metabolism can be expected.
As alread y described th e sodium content of plants may vary considerably and
in some inland continental and alpine regions th e sodium levels may be low . This
is the prime determinant of the salt status of all herbivorous animals living in th e
area, while carnivorous spec ies sho uld obtain adequ ate sodium from the prey on
which the y feed. T he diet of other mammals ma y con sist predominantly of halophyt ic plants with a high concentration of salts such as eaten by African sand
rats (SCHMIDT-NIELSEN, 1964a) and poss ibly Au stral ian hopping mic e (MAcMILLEN and LEE, 1969).
2. In the drinking wate r. Drinking supplies th e w ater requ irement beyond th at obtained from th e food. O 'CO NNORand Po n s (1969) have emphasized th e predominant imp ortance of thi s process in regulation of a positive wa ter balance. The urin96