habitat. Evaporation of water then made its initial appearance as a vertebrate problem. In addition, there arose the novel necessity to replenish the body wat er Qeriodicall y from the spo radi c sources available in a terrestrial environment. The second major event to influence tetrapod water metabolism, was the adoption of
homoiothermy by the birds and mammals (or possibly an ancestor of these) .
Potential evaporation from the body surface is increased at the relativel y high, and
sustained body temperatures of such animals, while th e added metabolic need for
respiratory gas exchanges also increases evaporative water loss from the respiratory
tract. Heat loss accompanying evaporation of water is utilized for cooling in hot
environmental conditions, when additional evaporation is facilitated by the secretion of sweat or by panting. Viviparity also increases the needs for water and salts,
especially in placental mammals which retain the young in utero until they are relatively well developed. The secretion of milk to nourish the young places yet another
periodic osmoregulatory burden on the mother, but has obvious advantages for
the offspring. The mammals thus have the usual tetrapod problems with respect
to their water and salt metabolism, and a few additional ones as part of the pri ce
that they must pay for their unique physiology. However, the high metabolic rate
of mammals allows them to travel considerable distances for food and water, while
their viviparity and abilit y to suckle their young, foster reproduction in situations
in which it could otherwise be difficult, both nutritionally and osmotically. It
should be remembered that while the high rate of metabolism in mammals increases
their requirements for osmoregulation, such adjustments may be facilitated by the
increased energy produced by the cells of such animals.
The relative abundance or scarcity of salts may limit the distribution of mammals. An excess of water or salts in the food can be tolerated by some species, but
not others. Marine mammals like whales and seals live in a hyperosmotic salt solution, equivalent in concentration to 3.3% sodium chloride. Man y marine mammals eat invertebrates that are isoosmotic to sea-water. Such mammals do not appear to drink sea-water normally, but when feeding on invertebrates take in large
quantities of salts, far more than most other mammals could tolerate with the limited quantities of osmotically free water that are available. Most terrestrial mammals
cannot survive if provided with drinking water that contains 2 to 3 % sodium chloride, but there are exceptions. The North African sand rat, Psammomys obesus,
normally eats halophytic plants with a salt concentration higher than that of seawater (SCHMIDT-NIELSEN, 1964a), while the western harvest mouse, Reitbrodontomys r. balicoetes, of the United States eats similar plants and can drink sea-water
(HAINES, 1964). Other species may drink sea-water sporadically, and this has been
shown among the Australian marsupials, including the Tammar wallaby, Macropus
eugenii (KINNEAR, PUROHIT, and MAIN, 1968). In man y areas of the world there
is a deficiency of sodium in the soil, a condition that appears most often in the
interior of continental areas and high mountainous regions. The vegetation reflects
the low sodium content of the soil, so that herbivorous animals may have restricted
amounts of salt available. Thus the herbage in grazing areas of central Australia
may onl y contain 1 to 3 m-equiv/kg dry weight of sodium, compared to 100 to
350 m-equiv/kg dry weight in coastal areas (DENTON, 1965). The popularity of
'salt licks' among mammals in such regions is well known.
90
homoiothermy by the birds and mammals (or possibly an ancestor of these) .
Potential evaporation from the body surface is increased at the relativel y high, and
sustained body temperatures of such animals, while th e added metabolic need for
respiratory gas exchanges also increases evaporative water loss from the respiratory
tract. Heat loss accompanying evaporation of water is utilized for cooling in hot
environmental conditions, when additional evaporation is facilitated by the secretion of sweat or by panting. Viviparity also increases the needs for water and salts,
especially in placental mammals which retain the young in utero until they are relatively well developed. The secretion of milk to nourish the young places yet another
periodic osmoregulatory burden on the mother, but has obvious advantages for
the offspring. The mammals thus have the usual tetrapod problems with respect
to their water and salt metabolism, and a few additional ones as part of the pri ce
that they must pay for their unique physiology. However, the high metabolic rate
of mammals allows them to travel considerable distances for food and water, while
their viviparity and abilit y to suckle their young, foster reproduction in situations
in which it could otherwise be difficult, both nutritionally and osmotically. It
should be remembered that while the high rate of metabolism in mammals increases
their requirements for osmoregulation, such adjustments may be facilitated by the
increased energy produced by the cells of such animals.
The relative abundance or scarcity of salts may limit the distribution of mammals. An excess of water or salts in the food can be tolerated by some species, but
not others. Marine mammals like whales and seals live in a hyperosmotic salt solution, equivalent in concentration to 3.3% sodium chloride. Man y marine mammals eat invertebrates that are isoosmotic to sea-water. Such mammals do not appear to drink sea-water normally, but when feeding on invertebrates take in large
quantities of salts, far more than most other mammals could tolerate with the limited quantities of osmotically free water that are available. Most terrestrial mammals
cannot survive if provided with drinking water that contains 2 to 3 % sodium chloride, but there are exceptions. The North African sand rat, Psammomys obesus,
normally eats halophytic plants with a salt concentration higher than that of seawater (SCHMIDT-NIELSEN, 1964a), while the western harvest mouse, Reitbrodontomys r. balicoetes, of the United States eats similar plants and can drink sea-water
(HAINES, 1964). Other species may drink sea-water sporadically, and this has been
shown among the Australian marsupials, including the Tammar wallaby, Macropus
eugenii (KINNEAR, PUROHIT, and MAIN, 1968). In man y areas of the world there
is a deficiency of sodium in the soil, a condition that appears most often in the
interior of continental areas and high mountainous regions. The vegetation reflects
the low sodium content of the soil, so that herbivorous animals may have restricted
amounts of salt available. Thus the herbage in grazing areas of central Australia
may onl y contain 1 to 3 m-equiv/kg dry weight of sodium, compared to 100 to
350 m-equiv/kg dry weight in coastal areas (DENTON, 1965). The popularity of
'salt licks' among mammals in such regions is well known.
90