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Summary The two environments produced differences in the adaptations of the laboratory group to prolonged walking on the treadmill. The rate of sweating was twice as great after 10 days and 2½ times as great after 6 weeks in Mississippi as in the northern experiments. Body temperature remained within comfortable limits in the North but in most of the men it rose to intolerable levels in Mississippi. The marked elevation in the body temperatures of the subjects in the Mississippi experiments was due to limited radiation and evaporation imposed by the hot, humid environment. Heart rate followed closely the changes in rectal temperature until limiting values were approached. Some of the men were more efficient and performed the task with lower heat production in the hot environment than in the cool one. There was a progressive decline in efficiency during work which tended to be more rapid the greater the elevation of body temperature. The negro sharecroppers who went through the experiments in the hot Mississippi laboratory maintained lower body temperatures than any other group studied including those in the northern experiments. They remained cooler than the Negroes in the North because they were more efficient and were favored by a higher ratio of body surface to weight. The white share-croppers who were studied in Mississippi were less successful in regulating body temperature than the negro sharecroppers only because they were less efficient. When walking in the sun with skin exposed the negro sharecroppers were superior in temperature regulation to partially acclimatized white men of the laboratory staff. During work the white sharecroppers had a higher rate of sweating than the Negroes—this was associated with their greater elevation of rectal and skin temperatures. Negroes are capable of much higher rates of sweating as evidenced by data on two negro servants in Mississippi who underwent marked elevation of temperature during the experiments. Sweat produced by the negro sharecroppers during work contained lower concentrations of total nitrogen, ammonia and urea, and higher concentrations of chloride than the sweat of white men. Both negro and white sharecroppers drank water more freely than the laboratory group. Mean values of R.Q. for the various groups during work in both environments were characterized by their close similarity. Blood sugar values were also very similar. Blood lactate was moderately lower in the northern experiments than in the South.

During the summer of 1962 the early phase of acclimatization to high altitude was studied in 6 of those who participated in the international high-altitude expedition to Chile in 1935. Ages of the 6 ranged from 58 to 71. Two also had taken part in a high-altitude study in 1929. In their earlier years these subjects had had an increase in hemoglobin concentration beginning with their arrival at high altitude. This response has been well established, especially by the Pike's Peak party led by Haldane and the work of Hurtado and associates in Peru. On the other hand, 5 of the 6 in the 1962 party exhibited a decrease in hemoglobin concentration during the first few days. The greatest decrease was observed in the oldest subject. His hemoglobin was 88% of his sea level value after 9 days at altitude and remained below his sea level value for another week. No observations were made on blood volume; hence, we can only speculate regarding possible related changes.

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.

1. 1. Electrocardiogram of four species of lizards, Dipsosaurus dorsalis, Crotaphytus wislizeni, Cnemidophorus tigris and Sceloporus occidentalis, was investigated. 2. 2. The effects of high and low body temperatures and intraperitoneal injection of the aqueous extract of the leaves of the plant Gymnema sylvestre on the cardiac electrical activity of D. dorsalis and S. occidentalis were recorded. 3. 3. Heart rate at 28–30°C averaged 68,75,105 and 67 beats/min in C. tigris, C. wislizeni, S. occidentalis and D. dorsalis respectively. 4. 4. Heart rate increases as body temperature, Tb, increases and vice versa. It is concluded that lizards are not only sensitive to the actual value of Tb, but also to the rate of change in Tb. 5. 5. The electrocardiogram (EKG) is recognizable mainly by the prominent QRS complex. Occasionally a negative QRS wave was recorded. 6. 6. The duration of the QRS was about 30 msec and its amplitude obtained by subcutaneous electrode placements were between 0·2 and 0·3 meV. 7. 7. A decrease in Tb below 25dgC was followed by a gradual decline in the QRS amplitude, but it increased when Tb rose from 25 to 30°C and a further rise in Tb above 35°C had a slight effect. 8. 8. The injection of the extraction of G. sylvestre caused an increase in heart rate and in respiratory frequency as an initial response. This was followed by changes in the EKG characteristics and finally a decrease in heart rate.

1. 1. For four species of rodent, man, and burro, we measured the rates of evaporation required to maintain homeothermy during exposure to desert heat. 2. 2. Required rates of evaporation for laboratory mice, laboratory rats and men were exponentially related to body weight. Kangaroo rats required a significantly lower amount of evaporation than the laboratory mice and rats. 3. 3. During walks in the open desert, man and burro required a two to three-fold increase in evaporation. 4. 4. When the metabolic cost of walking is expressed as a proportion of resting metabolism where unity equals one met, the evaporative requirement is directly related to the met with a slope of 0.33% body weight evaporated/hr/met. 5. 5. For man, the extreme hourly evaporation rate is 2.9% of the body weight. From this we calculated a limiting value for steady state exercise in desert heat that is about one-half the value sustained by a marathon runner in cool weather.

The burro ( Equus asinus ) can withstand deprivation of both food and water for up to 48 hours even in summer heat. This ability depends in part on his capacious cecum and colon where food is stored and is digested by bacterial action for hours after his last meal. In eight such periods of deprivation maximum temperatures ranged from 35 to 45°C and estimated increases in osmotic pressure in two burros ranged up to 17%. Given free choice of hay and water at the end of deprivation they chose hay always when estimated increase in osmotic pressure was less than 10%. In one case the second 24-hour period included an eight-hour walk. In this case the estimated increase in osmotic pressure was 17% in both and both chose water first. Their ability to eat dry hay after long periods of dehydration is essential to their survival in hot deserts where they can graze for 24 hours before returning to a water source.

Tolerance for sustained activity in the desert at about 40 degrees C was assessed on high school students, mostly athletically oriented and scholastically superior. The 14 males compared with the 12 females had an aerobic capacity greater by about one-half and a percentage of body fat smaller by about one-half. Each sex attained about the same percentage of aerobic capacity in their maximal sustained effort. This involved an increase in metabolic rate of 3 to 5 fold in females and 6 to 8 fold in males. In maximal sustained effort responses of males and females were alike in respect to rectal and skin temperatures and heart rate. At a rate at which nearly all walked for one hour, 100 m/min, there were no significant differences in metabolic rate, sweat rate nor in composition of sweat. Running at 120 m/min required maximal effort by most females; their maximal sweat rates ranged from 7.4 to 14.2 ml/m2.min. Most males were able to run at 160 m/min for one-half hor to one hour; their maximal sweat rates ranged from 11.3 to 14.6 m/m2.min. Superior capacity of males over females for sustained exercise in desert heat is related to their higher aerobic capacity and not to a difference in capacity for thermoregulation.

Vital capacity (VC) rarely may decrease 35-60% in healthy mountain climbers associated with high-altitude pulmonary edema (HAPE). In the age range 58-71 yr, five of six men during a week or more on White Mt. in 1962 had decreases in VC from 20 to 32% without frank symptoms of HAPE, Dill, one of the five, had decreases in VC again on White Mt. in 1977 and 1978. Yet none of 11 young climbers on White Mt. studied by Hultgren (personal communication) had a significant decrease in VC. Dill's arterial O2 saturation at age 87 at 485 Torr was about 79% in rest and 74% when VO2 was 0.74 ml/min.kg. His aerobic capacity at age 87 yr was 18 ml/min.kg at 695 Torr and 15 at 485 Torr.

Cardiac output and stroke volume were measured in two environments and at metabolic rates ranging from rest to the maximum rate that could be sustained for 25 minutes. One environment was indoors at about 23 degrees C, the other outdoors in desert sunshine and low water vapor pressure. The age range of the one female and four of the male subjects was from 19 to 40; the fifth male subject was 85 years old. Cardiac output was the same in the two environments; stroke volume was less at higher metabolic rates in the heat. The cardiac output for the old man was about one-tenth less and stroke volume about 20 ml less than that observed for the same work 50 years earlier.

The physiological adjustments of the men to both aerobic (5.6 km/h, up 9% grade), and maximal treadmill work were first determined when they were healthy college students, ages 18–22 yr. They were restudied at ages 40–44, and again at ages 49–53 yr. In the aerobic walk VO2 increased in proportion to weight gain, but efficiency did not change with age. The men who had gained most weight showed the greatest elevations of heart rate, blood lactate and “ventilatory equivalent,” and reductions of the “O2 pulse” in the walk. At age 50 mean “O2 pulse” in exhausting work had declined 13%, even though mean maximal heart rate had declined 15 beats/min from values observed in youth. At ages 40–44 yr mean VO2 max had declined 25% to values 12% lower than mean values reported in 1938 for men of this age. Eight of the men imporved an average of 11% in VO2 max between ages 40–44 and 49–53 yr; mean VO2 max of the others continued to decline with age. The five men who improved most had increased their participation in vigorous activities (tennis, squash, skiing, jogging, etc.) and had quit or reduced their smoking.

Aerobic capacity (VO2 max) and body fat were measured indoors and VO2 was measured at about 36 to 42 degrees C in desert walks or runs in 69 adults aged 17 to 88 years. Eleven were athletic youths, and many of the older adults had participated in jogging programs. Body fat increased and VO2 max decreased with age, although there were notable exceptions. Rates of walking and running were planned to require about 40% of VO2 max. Midway in each walk VO2 (ml O2/horizontal m . kg) served as a measure of skill; skill was high in 10 of 11 youths who did many walks or runs. Each older adult did three walks; skill improved. In four groups with some persons aged 50 and older with body fat up to 40%, skill in their third walk matched that of youths. The least fit women and the least fit men did not attain that level of skill.