M. K. Yousef

Tritiated Water-Turnover Rate in Rodents: Desert and Mountain

pret. The discovery of the usefulness of tritiated water (TOH) in measurement of total body water (Pace et al. 1947) has offered a practical method for the study of daily water exchange. This technique has been verified in man (Pinson and Langham 1957) and in several other mammalian species (Reid, Balch, and Glascock 1958; Richmond, Langham, and Trujillo 1962; Holleman and Dieterich 1973; Yousef 1973). Using TOH, one can measure the rate of body water utilized per day including water imbibed, food water, and metabolic water.

Physiological and ecological studies on the chisel-toothed kangaroo rat, dipodomys microps☆

Abstract 1. 1. The chisel-toothed kangaroo rat, Dipodomys microps, is common to abundant and active throughout the year in the high desert (1200–1800 m) of southern Nevada. 2. 2. There is a wide range in above ground air temperature (mean monthly minimum 6°C, mean monthly maximum 40°C) throughout the year while mean burrow temperatures vary between 5 and 26°C. 3. 3. The thermoneutral zone for D. microps extends from 27 to 32°C. 4. 4. Below the lower critical temperature the rate of oxygen consumption (VO2) increases at the rate of 0·1 ml/g per hr for every decrease of VC. Above the upper critical temperature, VO2, increases approximately 40 per cent. 5. 5. The pulmo-cutaneous water loss increases with increasing ambient temperature. 6. 6. Body temperatures of field and laboratory animals varied between 32 and 38°C in relation to activity level. The lethal body temperature is about 40°C. Body temperature begins to rise at ambient temperature of about 33°C. 7. 7. Daily cycles of hibernation occasionally occur under laboratory conditions or may be induced readily by food restriction. Successful hibernation may occur over increasingly longer daily periods. These daily periods continued throughout the experimental period of 1 week accompanied by weight losses of up to 28 per cent of the original weight. 8. 8. In conclusion, the narrow thermoneutral zone, lower upper critical temperature and accompanying hyperthermia suggest that this kangaroo rat is more cold-adapted than other species of kangaroo rat. In addition, D. microps appears to enter daily hibernation more readily than other kangaroo rats. These physiological features appear to be important adaptations for the higher elevations in the Mohave and the Great Basin deserts.

Thermoregulatory responses in the plains pocket gopher, Geomys bursarius.

Abstract 1. Thermoregulation was studied in the Plains pocket gopher, Geomys bursarius. 2. The thermoneutral zone, TNZ, for G. bursarius extends from 30 to 33°C with a resting VO2 of 0·70 ml O2/g per hr. 3. Below the lower critical temperature VO2 increased but was variable due to labile and reduced rectal temperature, Tre. Above the upper critical temperature VO2 increased approximately 27%. 4. Calculated thermal conductance expressed in ml O2/g per hr. C decreased progressively as ambient temperature, Ta, was reduced. 5. The pulmo-cutaneous water loss increased with increasing Ta. 6. The Tre of animals in TNZ averaged 35°C. As Ta decreased Tre progressively declined reaching an average of 26·5°C at a Ta of 10°C. Above TNZ, Tre increased to a high of 39°C at Ta of 38°C. 7. Data on G. bursarius was compared with other pocket gophers studied. These comparisons suggest that pocket gophers in general deviate from strict homiothermy and utilize labile Tre in varying degrees as an adaptive mechanism for survival in stressful environments.

Physiological studies on the rock pocket mouse, Perognathus intermedius.

Abstract 1. 1. Metabolism, thermoregulatory patterns, pulmocutaneous water loss and hibernation cycles were studied in the rock pocket mouse, Perognathus intermedius . 2. 2. The thermoneutral zone for P. intermedius was not evident when measurements of rate of oxygen consumption, V O 2 , were made in April. However, the zone was from about 33 to 36°C when measurements were made in January. 3. 3. The average V O 2 was about 23 per cent higher in January than in April. 4. 4. At T a below 30°C, V O 2 increased at a rate of 0·2 and 0·22 ml/g. hr for every 1°C decrease during January and April respectively. The rate of increase in V O 2 in January was about 13 percent lower as compared to the rate of increase in April. 5. 5. At ambient temperature T a above 37°C, V O 2 increased during January and showed no appreciable change during April even at a T a above 40°C. 6. 6. The T re is labile even at T a of 30°C. T re ranges between 33 and 36°C, and changes in T re appear to be related to time of day and T a . Lethal T re is about 41°C, and the rate of hyperthermia is less rapid than in other species of heteromyids. 7. 7. The pulmocutaneous water loss increases with rising T a . Water loss is similar in P. intermedius to other species of Perognathus at T a of 20–30°C, but at T a above 30°C, P. intermedius evaporated more than P. californicus . 8. 8. Daily hibernation occurs spontaneously at T a below the lower critical temperature and can be induced by food restriction.

Daily cycles of hibernation in the kangaroo rat, Dipodomys merriami

Abstract Thirty-six kangaroo rats were captured in the Mojave Desert near Las Vegas. Eight rats, group A, were kept at room temperature, 25 ± 2 °C. Ten, Group B, were kept at 13–15 °C and fed ad libitum. Eighteen, group C, were kept at 13–15 °C and fed only 2 g of dry oats daily. Rate of O2 consumption, VO2, food intake, rectal temperature, Tre., body weight and composition were measured in all groups. Group B showed a significant increase in VO2, and food intake and no change in Tre body weight as compared to group A. After 2 days on the limited food intake group C began to lose weight, and Tre then began to fluctuate ranging between 14 and 36 °C; the animals exhibited hibernation when the Tre was low. Eight of the 18 rats of group C reached Tre values of 14 °C or below; only one of these survived. The lowest Tre in the other 10 was 15 °C; all survived. Chemical analysis of the homogenized rats showed a significant decrease in body fat in group C to average 1.0% contrasted with 7.8% in group B and 3.7% in group A. The VO2 ranged from 1.9 ml/g hr at a Tre of 36.5 to 0.2 ml/g hr at a Tre of 15 °C. In conclusion D. merriami utilizes hibernation as an effective adaptive mechanism to carry out their essential body functions when on a limited food intake. It seems that maintaining a level of more than 1.5% of body fat is essential for successful arousal from hibernation.

Thermoregulatory responses of diabetic rats

1. Thermal responses and skin microcirculation were measured in streptozotocin-induced diabetic (SD) rats during acute and chronic exposure to ambient (Ta) temperatures ranging from about 5 to 35 degrees C. 2. At 28 degrees C, SD rats had higher rate of oxygen consumptions (VO2), tail skin blood flow (SKBF), but lower rectal temperatures (Tre) than saline-injected controls. 3. Chronic exposure of the SD rats to 35 and 5 degrees C caused a sharp rise and decline in Tre, respectively. 4. At 35 degrees C, hyperthermia in the SD rats was associated with greater increase in VO2 than controls, but changes in SKBF were similar in both groups. 5. At 5 degrees C, VO2 changed similarly in both the SD and control rats, but vasoconstriction was greater in the controls. 6. The data suggest that hypothermia in SD rats may be associated with impairment of vasoconstriction and hyperthermia may be related to an increase VO2 not accompanied by greater vasodilation.

Responses of Merriam's Kangaroo Rat to Heat

Kangaroo rats (Dipodomys) which live in the desert of the southwestern United States represent an outstanding example of adaptation to desert environment (Schmidt-Nielsen 1964a, pp. 150-178). Schmidt-Nielsen and Schmidt-Nielsen (1950) concluded that a primary factor in adaptation of Dipodomys merriami to desert environment is that it avoids high temperatures by remaining in burrows during the day. This view is supported by Carpenter (1966), who found that Dipodomys is seldom subject to high ambient temperature. However, Dawson (1955) has suggested that the presence of D. merriami in shallow burrows may indicate greater heat resistance than is possessed by other species of Dipodomys. He found that D. merriami and the diurnal antelope ground squirrel, Citellus leucurus frequently are exposed to high temperatures. The objective of our study was to examine selected physiological responses of winterand summer-acclimatized rats to heat, with the hope of interpreting the conflicting findings mentioned above.

Thermoregulatory responses of desert wood rats, neotoma lepida

A study of thermoregulatory response of N. lepida over a range of ambient temperatures showed the following: 1. 1. The thermoneutral zone for this species was 31–35° C and minimal resting VO2 was 0.60 ml O2/g per hr. 2. 2. The wood rat, N. lepida displayed a labile rectal temperature along with a low M/C ratio which is of adaptive significance to its survival in a desert environment. 3. 3. Evaporative water loss was higher than reported for other species. 4. 4. When acutely exposed to severe cold (−20°C) or heat (43°C), this species does not maintain homeothermy.

Energetic Cost of Running in the Antelope Ground Squirrel Ammospermophilus leucurus

The energetic cost of running in small mammals has been of interest to various laboratories in recent years (Pasquis, Lacaisse, and Dejours 1970; Taylor, Schmidt-Nielsen, and Raab 1970; Yousef et al. 1970; Wunder 1970). This is due to the fact that fast locomotion is essential for survival of animals in their natural habitats. Animals must have the ability to search for food and to escape their prey. Exposure to cold and heat is known to alter the resting energy expenditure of small and large mammals (Yousef, Hahn, and Johnson 1968; Yousef and Dill 1970, 1971). However, the combined action of temperature and exercise on thermogenesis seems to vary from species to species. It is generally accepted that heat produced by exercise substitutes for shivering in man but not in small laboratory animals (Jansky 1965) or small wild animals (Hart and Heroux 1955; Yousef et al. 1970). Jansky (1965) suggested that differences between species do not depend on body weight or on their phylogeny. With the exception of our recent report (Yousef et al. 1970), there are no data on the influence of temperature on working heat production of desert mammals. On the contrary, several reports have been published on laboratory mammals and nondesert wild animals (Hart 1952; Hart and Heroux 1955;

Physiological and ecological studies on Citellus lateralis

1. 1. Thermoregulation, ecological distribution and seasonal activity of the golden-mantled ground squirrel, Citellus lateralis, were examined in coniferous habitats above the desert in the Spring Range of southern Nevada. 2. 2. Lower elevational limits for C. lateralis are about 1800 m and the upper limits are approximately 3500 m. 3. 3. Ground squirrels are seasonally active from 8 to 9 months of the year depending upon the altitude. 4. 4. The extent of snow pack and cold air temperature modify the length of the hibernation period. 5. 5. The thermoneutral zone for C. lateralis extends from 27 to 35°C. 6. 6. Below the lower critical temperature, the rate of oxygen consumption (O2) increases at the rate of 0·1 ml/g per hr for every decrease of 1°C. Above the upper critical temperature, O2 increases 25–30 per cent approximately. 7. 7. The pulmo-cutaneous water loss increases with increasing ambient temperature. 8. 8. The rectal temperature of normally active animals varied between 25·8 and 38·7°C. The lethal temperature does not differ from most other small mammals. Hyperthermia set in at an ambient temperature of 35°C or above. 9. 9. In conclusion, the wide zone of thermoneutrality, tolerance to a reasonable degree of hyperthermia and water deprivation offer an explanation to the expansion of C. lateralis habitat into the arid edge of the high desert.