in a desert without drinking water. The kangaroo rat belongs to the famil y Heterom yidae which is confined to the new world; it lives in the deserts of the western
United States and parts of Mexico. Its ability to survive and thrive on a diet of air
dried seeds and vegetation, without requiring water to drink, is the result of several
adaptations that have subsequently been shown to exist in other rodents occupying
comparable ecological situations elsewhere. The principal adaptation allowing the
kangaroo rat to live under such conditions is its small evaporative water loss. Rodents do not sweat or pant, and thus have "a limited ability to undertake thermal
cooling; the y may allow their bod y temperature to rise somewhat in hot conditions
(SCHMIDT-NIELSEN, 1964b). Obligatory losses of water that take place by evaporation from the respiratory tract, are reduced in the kangaroo rat and white rat,
by utilizing a countercurrent heat exchange mechanism in the nasal passages to cool
the expired air (JACKSON and SCHMIDT-NIELSEN, 1964). Such cooling reduces its
content of water vapour. In hot situations, which can result in a critical rise of body
temperature, many rodents may spread saliva over their fur in order to facilitate
thermal cooling. However, this is not usually necessary as species that live in deserts
tend to avoid extremes of temperature by sheltering in protected situations such
as burrows. Rodents, especially those living in arid areas, conserve additional water
by forming a highl y concentrated urine and relativel y dr y faeces. Not all desert
rodents can, however, achieve a positive water balance while eating a dry diet, and
some must obtain food containing a substantial amount of free water. The pack
rat, Neotoma, like the kangaroo rat, lives in the deserts of the western United States
but eats cactus plants, while the North African sand rat , Psammomys obesus, subsists on succulent halophytic vegetation (SCHMIDT-NIELSEN, 1964a, b).
Neurohypophysial peptides, arginine-vasopressin and oxytocin, have been
identified in the four species of rodents that have been examined, including the
white rat, Rattus no ruegicus, the kangaroo rat, Dipodomys merriami, and the
guinea pig, Cavia poreel/us (see SAWYER, 1968). Arginine-vasopressin has also been
identified in five strains of mice (Mus musculus), while in the peru strain, lysinevasopressin has been found (STEWART, 1968). Neurohypophysial peptides containing lysine instead of arginine at position 8 in the molecule (see Table 2.4) have hitherto been found only in the Suiformes (pigs, peccaries and hippopotami).
The neural lobe of rats can be destroyed by cutting the supraopticohypophysial tract and allowing the peripheral tissue to degenerate. Such animals
form large volumes of dilute urine which they are unable to concentrate (diabetes
insipidus). VALTIN and his colleagues (1962) have identified a strain (Brattleboro)
of rats with hereditary diabetes insipidus due to the specific absence of vasopressin,
although oxytocin is still present (SAWYER, VALTIN, and SOKOL, 1964). Vasopressin
has been identified in the blood of the white rat, as well as in that of the kangaroo
rat (AMES and VAN DYKE, 1952), the mouse and the guinea pig (J . HELLER, 1961).
Rats on a normal diet, with drinking water available, have low circulating levels
of vasopressin (5 .u Ulml peripheral plasma), but this increases 10-fold after 24
hours without drinking water. The circulating levels of vasopressin in normalkangaroo rats, which do not require drinking water, are far higher than in deh ydrated
white rats. Injection of vasopressin into hydrated white rats and mice reduces the
urine volume and increases its concentration, and such an antidiuresis has also been
shown in the kangaroo rat (COLE, CHESTER JONES, and BELLAMY, 1963). The prima
105
United States and parts of Mexico. Its ability to survive and thrive on a diet of air
dried seeds and vegetation, without requiring water to drink, is the result of several
adaptations that have subsequently been shown to exist in other rodents occupying
comparable ecological situations elsewhere. The principal adaptation allowing the
kangaroo rat to live under such conditions is its small evaporative water loss. Rodents do not sweat or pant, and thus have "a limited ability to undertake thermal
cooling; the y may allow their bod y temperature to rise somewhat in hot conditions
(SCHMIDT-NIELSEN, 1964b). Obligatory losses of water that take place by evaporation from the respiratory tract, are reduced in the kangaroo rat and white rat,
by utilizing a countercurrent heat exchange mechanism in the nasal passages to cool
the expired air (JACKSON and SCHMIDT-NIELSEN, 1964). Such cooling reduces its
content of water vapour. In hot situations, which can result in a critical rise of body
temperature, many rodents may spread saliva over their fur in order to facilitate
thermal cooling. However, this is not usually necessary as species that live in deserts
tend to avoid extremes of temperature by sheltering in protected situations such
as burrows. Rodents, especially those living in arid areas, conserve additional water
by forming a highl y concentrated urine and relativel y dr y faeces. Not all desert
rodents can, however, achieve a positive water balance while eating a dry diet, and
some must obtain food containing a substantial amount of free water. The pack
rat, Neotoma, like the kangaroo rat, lives in the deserts of the western United States
but eats cactus plants, while the North African sand rat , Psammomys obesus, subsists on succulent halophytic vegetation (SCHMIDT-NIELSEN, 1964a, b).
Neurohypophysial peptides, arginine-vasopressin and oxytocin, have been
identified in the four species of rodents that have been examined, including the
white rat, Rattus no ruegicus, the kangaroo rat, Dipodomys merriami, and the
guinea pig, Cavia poreel/us (see SAWYER, 1968). Arginine-vasopressin has also been
identified in five strains of mice (Mus musculus), while in the peru strain, lysinevasopressin has been found (STEWART, 1968). Neurohypophysial peptides containing lysine instead of arginine at position 8 in the molecule (see Table 2.4) have hitherto been found only in the Suiformes (pigs, peccaries and hippopotami).
The neural lobe of rats can be destroyed by cutting the supraopticohypophysial tract and allowing the peripheral tissue to degenerate. Such animals
form large volumes of dilute urine which they are unable to concentrate (diabetes
insipidus). VALTIN and his colleagues (1962) have identified a strain (Brattleboro)
of rats with hereditary diabetes insipidus due to the specific absence of vasopressin,
although oxytocin is still present (SAWYER, VALTIN, and SOKOL, 1964). Vasopressin
has been identified in the blood of the white rat, as well as in that of the kangaroo
rat (AMES and VAN DYKE, 1952), the mouse and the guinea pig (J . HELLER, 1961).
Rats on a normal diet, with drinking water available, have low circulating levels
of vasopressin (5 .u Ulml peripheral plasma), but this increases 10-fold after 24
hours without drinking water. The circulating levels of vasopressin in normalkangaroo rats, which do not require drinking water, are far higher than in deh ydrated
white rats. Injection of vasopressin into hydrated white rats and mice reduces the
urine volume and increases its concentration, and such an antidiuresis has also been
shown in the kangaroo rat (COLE, CHESTER JONES, and BELLAMY, 1963). The prima
105