LITERS DRUNK PER DAY
V. Insensible Water Loss
8. Other Methods
Insensible water loss from small skin areas can be measured by covering an area with a cup containing a desiccant and determining the weight gain of the cup (Buettner, 1953). The measurement of water loss through excised skin fastened over a cup containing saline (Berenson and Burch, 1951; Butcher, 1954), seems to be a promising method for study of I.W.S of wild mammals.
I.W.r can be estimated from respiratory volume if the expired air tem
perature is accurately known.
B. D a t a on Insensible Water Loss
Figure 4 and Table I V summarize d a t a on the total I.W. of mammals ranging in weight from 15.8 gm. to 3630 kg. These d a t a have been selected for the temperature range of 1829°, in all cases below the sweating or p a n t -ing thresholds of the animals involved. Total I.W. is related to body weight according to the formula I.W. (gm./hr.) = 2.58 B0 (kg.)0-8 2 6. Additional d a t a on other species are available as follows: Spermophilopsis leptodactyluSj Citellus pygmaeus, and C. fulvus (Shtcheglova, 1952); Setonyx brachyurus (Bentley, 1960); Perognathus baileyiy P. intermedius (Chew and D a m m a n n , 1960); Neotoma lepida and N. fuscipes (Lee, 1963); Cercaertus nanus (Bartholomew and Hudson, 1962); Clethrionomys gapperi (Getz, 1962);
Citellus leueurus (Hudson, 1962); Jaculus orientalis (Kirmiz, 1962).
Small desert rodents have a much lower t h a n average I.W., apparently because they lose little water through the skin (B. Schmidt-Nielsen et aZ., 1948). T h e I.W. of white mice at 26-27° is significantly greater t h a n t h a t
FIG. 4. Relationship between body weight and insensible water loss for various mam-mals ranging in weight from 15.8 gm. [bat (Antrozous)] to 3630 kg. (elephant). Numbers refer to listing of data in Table I V . D after a number designates a desert species.
TABLE IV INSENSIBLE WATER Loss OF VARIOUS MAMMALS0 Species Body weight gm.
I.W. mg./hr. Mg./gm. Bo/hr. Ta R.H. Feeding* Source 1. Antrozous pattidus 15.8 30.0 1.90 26 Dry f Chew and White (1960) 2. Blarina b. brevicauda 25.8 326 12.6 25 Dry F Chew (1951) 3. Peromyscus criniius 22 34.3 1.56 25 Dry f the Schmidt-Nielsens (1950) 4. P. leucopus noveboracensis 20.1 108 5.23 18 63% F Chew (1951) 5. P. 1. noveboracensis 20.7 119 5.76 25 Dry f Lindeborg (1952) 6. P. 1. torniUo 29.3 97 3.31 25 Dry f Lindeborg (1952) 7. P. maniculatus sonoriensis 17.6 48.0 2.73 27 6% f Chew (1955) 8. White mouse 29.2 89.6 3.07 25 Dry f the Schmidt-Nielsens (1950) 9. White mouse 18.9 81.1 4.28 28 — — Barbour and Trace (1937) 10. White mouse 26.0 60.0 2.31 28 — f Benedict and Lee (1936) 11. Μ us musculus 33.0 51.2 1.55 25 Dry f the Schmidt-Nielsens (1950) 12. Microtus o. ochrogaster 42.6 122 2.85 28 51% F Chew (1951) 13. Perognathus sp. 25.2 39.5 1.57 25 Dry f the Schmidt-Nielsens (1950) 14. P. penicillatus eremicus 16.9 33.5 1.98 25 Dry f Lindeborg (1952) 15. Dipodomys merriami 36.1 43.7 1.21 25 Dry f the Schmidt-Nielsens (1950) 16. D. spectabilis 100.1 70.7 0.70 25 Dry f the Schmidt-Nielsens (1950) 17. Mesocricetus auratus 95.1 93.3 0.98 25 Dry f the Schmidt-Nielsens (1950) 18. White rat 102 54.4 1.82 25 Dry f the Schmidt-Nielsens (1950) 19. White rat 236 368 1.56 28 — f Greene and Luce (1931) 20. White rat 288 1585 5.52 21 — F Dicker and Nunn (1957) 21. White rat 2886 1300 4.51 21 — F Heller (1947) 22. White rat 2886 795 2.76 21 — f Heller (1949) 23. Guinea pig 570 1990 3.49 23 52%
—
Nagayama (1932) 84 Robert M. Chewkg. 24. Setonyx brachyurus 3.55 5.5 1.55 21 61
—
Bentley (1955) 25. S. brachyurus 3.65 8.71 2.39 29 49—
Bentley (1955) 26. Rabbit 2.05 1.15 0.58 20— —
Nagayama (1932) 27. Rabbit 2.05 1.45 0.72 25— —
Nagayama (1932) 28. Marmot 1.83 0.51 0.28 17—
f Benedict and Lee (1938) 29. Corriedale, Merino sheep 28 116 4.15 30 90%—
Riek et al. (1950) 30. Berkshire pigs 81.6 119 1.46 31—
— Robinson and Lee (1941c) 31. Duroc, Hampshire swine 31.7 60 1.89 21 50% F Bond et al. (1952) 32. Duroc, Hampshire swine 45.3 46 1.01 21 50% F Bond et al. (1952) 33. Duroc, Hampshire swine 148.5 68 0.47 21 50% F Bond et al. (1952) 34. Camelus dromedarius 243 99 0.36 10—
F K. Schmidt-Nielsen et al. (1957) 35. Illawarra shorthorn cow 315 288 0.92 30 35% F Robinson and Klemm (1953) lactating 36. Jersey, lactating 285 411 1.44 30 35% F Robinson and Klemm (1953) 37. Dairy shorthorn dry cow 632 276 0.44 20—
F Balch et al. (1953) 38. Jersey, lactating 390 420 1.08 21 70% F Thompson et al. (1953) 39. Jersey, dry 400 320 0.76 21 70 F Thompson et al. (1953) 40. Holstein, lactating 500 500 1.00 21 70 F Thompson et al. (1953) 41. Brown Swiss, lactating 540 678 1.26 21 70 F Thompson et al. (1953) 42. Brown Swiss, heifer 220 327 1.48 21 70 F Thompson et al. (1953) 43. Brahman, lactating 470 448 0.95 21 70 F Thompson et al. (1953) 44. Brahman, dry 470 216 0.46 21 70 F Thompson et al. (1953) 45. Brahman, heifer 270 262 0.97 21 70 F Thompson et al. (1953) 46. Steers 405 267 0.66 21 51 F Mitchell and Hamilton (1936) 47. Steers, sheared 391 258 0.66 24 53 F Mitchell and Hamilton (1936) 48. Work horse 611 1060 1.74—
— F Wittig (1938) 49. Elephas maximus 3630 926 0.26 10—
F Benedict (1936) ° Data for Fig. 4. All animals normally hydrated and not subject to heat stress. 6 Assumed weight. c F, fed during measurements; f, fasting.86 Robert Μ. Chew of the desert rodents Dipodomys merriami and Perognathus spp. if t h e humidity of the air is low, but there are no significant differences at absolute humidities above 10.0 mg. per liter absolute humidity (Chew and D a m mann, 1960). Blarina and possibly other subterranean small mammals have an exceptionally high I.W. (Chew, 1951). Marine mammals are assumed to have no skin water loss (Irving et al., 1935).
Bats, with their tremendous surface area relative to body weight, pose an interesting problem in insensible water loss. Chew and White (1960) found t h a t the pallid bat, Antrozous pallidus, at 25-27° in dry air, loses 30 mg. H O H per hour when hanging with wings folded, and 63-80 mg. when tied down with its wings extended. However, this difference is due to the low metabolic rate of the normally resting bat (0.47 ml. 02 per gram B0 per hour) as compared to t h a t of the extended-wing bat (5.0 ml. the first first hour to 0.78 ml. the seventh hour). There is no evidence t h a t extending the flight membranes in itself causes an increased evaporation.
I n a few cases I.W. has been partitioned into its two components For the following mammals the average ratio of I . W .s: I . W .r is: Peromyscus maniculatus sonoriensis 46:54 (Chew, 1955); white r a t 57:43 (Tennent,
1945); rabbit 49:51 (Nagayama, 1932); h u m a n congenitally lacking sweat glands 70:30 (Richardson, 1926); Elephas maximus 47:53 (Benedict, 1936);
lactating cows 81:19, dry B r a h m a n cows 71:29, heifers 86:14 (Kibler and Brody, 1952); lambs, below their critical temperature, approximately 50:50 (Alexander and Williams, 1962). I n white rats the ratio averaged 53:47 over a temperature range of 15°-38°, with skin I.W. tending to be proportionately less at temperatures below 33° (average 48) and more, above 33° (average 58) (Stupfel and Geloso, 1959).
C. Factors T h a t Influence Insensible Water Loss 1. Temperature and Humidity
From the work of Lutcke et al. (1957) on dogs and of Zollner et al. (1955) and Brebner et al. (1956) on humans, it seems established t h a t loss of water through the skin in these m a m m a l s is a diffusion process. As such it varies directly with the magnitude of the vapor pressure gradient between the skin surface and the immediately overlying air, AVP = V P8 — V Pa. I t has not been possible to measure V PS, but it is usually assumed as equal to t h a t of air saturated at skin temperature.
Any change in air temperature (Ta) alters skin temperature (Ts) and t h u s V PS. A change in absolute humidity of the air not only gives a new value to AVP, but also changes T8 and V P8. For dogs at Ta 18-33° and AVP 3-17 m m . Hg, I.W.S (grams per square meter per hour) = 10.00 + 0.291
(AVP — 27.43) (Lutcke et al., 1957). Dogs show an inverse relationship between I.W.S and V Pa; lactating cows and heifers show a direct relation
ship (Kibler and Brody, 1952).
When the I.W.S measurements for dogs are analyzed into classes ac
cording to Ts and rectal temperature ( Tr) , it is seen t h a t with increasing temperature there is a decrease in the resistance of the skin to passage of water vapor. The d a t a of Webb et al. (1957) on h u m a n s and t h a t of Chevillard (1935) and Chew (1951) on mice suggest t h a t at high air tem
peratures the decrease in resistance is much greater and the relationship of I.W.s to Δ V P becomes exponential. Change in skin resistance indicates t h a t the assumption of V PS as saturated a t Ts m a y be faulty. Mole (1948) and Kaufmann et al. (1955) give theoretical considerations of changing skin humidity.
Other environmental factors t h a t alter Δ V P also influence I.W. Air movement increases or decreases evaporation from a surface as it changes the boundary layer of humid air next to the surface (Leighly, 1937). Hale et al. (1958) found t h a t h u m a n I.W.8 increases 12 mg. per square meter per hour for each millimeter of mercury decrease in barometric pressure.
2. Physiological Variables
Changes in cutaneous blood flow and in heat production can be expected to alter T8 and t h u s I.W.S. When Ta is constant, the degree of excitement of the animal is the most obvious factor responsible for variation of I.W.S of Peromyscus) there is a highly significant correlation of I.W.S with CO2 production (Chew, 1955).
T h e effect of the fur depends on whether it is acting to keep metabolic heat inside the body or to insulate against gain of heat from outside. At 20-25° shearing allows a decrease in Ts and a corresponding reduction in I.W.s in rabbits (Nagayama, 1932) and steers (Mitchell and Hamilton, 1936). At air temperatures near or above body temperature, or on exposure to the sun, just the reverse occurs for sheep (Macfarlane et al., 1958a), rabbits (Johnson et al., 1958), and camels (B. Schmidt-Nielsen et al., 1956).
I n Ayrshire bull calves, I.W.S can be increased experimentally by heating t h e hypothalamus to 41.5° for 3 minutes (Ingram et al., 1961). A D H m a y influence I.W.S in humans (Mom and Clerc, 1956).
D . N a t u r e of the Skin Barrier t o Water
R o t h m a n (1954) reviews t h e literature on this subject prior to 1951.
Considerable work has been done using excised skin, which seems to be equivalent to living skin as far as I . W . is concerned (Mali, 1956; Lutcke et al., 1957). Excised h u m a n skin retains its normal low water vapor
88 Robert Μ. Chew