Camp.
Biochem.
Physiol.
Vol.
IOiA, No.4,
pp. 693-699,
1992
Printed inGreatBritain
0
03W9629/92 ES.00 + 0.00 1992 Pergamon Press pk
HEAT EXCHANGE BY THE PINNA OF THE AFRICAN ELEPHANT (LOXODONTA AFRICANA) POLLYK.PHILLIPS* and JA~EDWARDH~~ Department of Physiology and Biophysics, University of Illinois, Urbana, IL 61801, U.S.A. Telephone: (217) 333-1735; Fax: (217) 333-1133 (Received 2 August 1991)
Abstract-1. Surface temperatures of the pinnae of four female African elephants were measured at ambient temperatures between 14 and 32°C using infrared thermography. Instantaneous heat losses calculated using those values ranged from 10.67 to 76.2 W under the observed conditions. 2. Using a value of 17 k&/kg/day, those heat losses account for 0.65-4.64% of the animals’ standard metabolic rates, considering one side of one ear only. 3. A model of heat flow across a flat vertical plate was constructed and compared to the actual values. Up to 100% of an African elephant’s heat Ioss needs can be met by movement of its pinnae and by vasodilation. 4. Thermography indicates that the temperature distribution pattern across the pinna changes with amnbient temperature and that areas of specialized motor control exist.
~RODU~ION
Homeotherms appear to have adapted to new or changing environments by varying the size and shape of their bodies and extremities. These physical changes facilitated heat conservation or dissipation as dictated by ambient conditions. Larger animals need to develop means of dealing with the great amounts of heat that they produce. The African elephant (Loxodonra africana), the largest land mammal, has accordingly developed the largest the~o~~lato~ organ known in any animal, the pinna or external ear, which it uses as a radiator-convector. The extent to which the elephant uses its ears in thermoregulation has been disputed. In his classic study of elephant physiology, Benedict (1936) claimed that a large, hairless animal such as the elephant would have no need for a special heat regulating mechanism in its ears and if it did possess it, it would be a singuiar provision in nature. Others, however, have indicated that the pinna of an elephant’s ear is the main external organ connected with body temperature regulation (Sikes, 1971). The combined surface area of both sides of both ears of an African elephant is about 20% of its total surface area (Wright, 1984). The ears are sensitive, thin, and easily frozen (Benedict, 1936). The extensive vascular network of subcutaneous vessels lying to the medial side of the ear cartilage (Sikes, 1971) combined with the high surface to volume ratio and large surface area of the ear indicate this area has a role in temperature regulation. Further, the almost constant motion of the pinnae exposes the medial side and corresponding vessels to air currents and could increase heat loss from the ear as well as from the fanned body surface (Wright, 1984). *To whom all correspondence should be addressed. cm4
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Sikes (1971) states that evidence has been found which indicates that the degree of dilation of the ear vessels can be controlled, although she does not explain what that evidence is. Benedict et al. (1921) note wide and rapid changes in ear temperatures which suggests large fluctuation in blood flow even at the ear tips. They also found the protected, medial side of the ear to be warmer than the front, with marked differences in small areas. A 4000 kg elephant needs to maintain a heat loss of 4.65 kW or more while moving and feeding (Wright, 1984), which indicates that the elephant not only creates a great deal of heat, but must have an effective means for controlling heat flow. The pinna appears to suit that purpose very well. Infrared thermography provides a non-invasive measure of pinna temperature to estimate the African elephant’s use of its pinnae for regulation of body temperature and heat loss. Converting the surface temperatures to a model of heat flow and comparing this to metabolic rates may clarify the role of the pinnae in the thermoregulation of the largest land animal. MATERlAISANDMETHODS The four female elephants used in this study were made available through the cooperation and generosity of Brookfield Zoo, Brookfield, IL, U.S.A. and Mr Bruce Brewer, curator of mammals. The vital statistics of the elephants, listed in Tabk 1, were obtained through the assistance of Mr Brewer and zoo personnel. Infrared scans of the elephants, either under overcast conditions or indoors, were taken on 5 days of varying ambient temperature using an Inframetrics model 525 infrared (i.r.) scanner and electronics/control unit. When using this system thermal radiation is detected by the scanner across a band of wavelengths from 8 to 12 pm at which energy absorption by the atmosphere is at a minimum. Radiation from points in the field of view is compared to a
693
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POLLYK. ~H~LLIPSand JAMES EDWARDHEATH
Table 1. Vital statistics of the Brookfield Zoo elephants Ear Ear Weight width Height length Elephant (kg) (m) (en0 (em) 3764 88.9 Affie 2.56 137.0 68.6 Mame 2000 2.31 127.0 13.1 2000 2.29 M’toto 109.0 1136 63.5 Christy 1.80 106.7
liquid nitrogen internal standard temperature of - 196°C. The imaging system then creates a black and white picture at the rate of 10 frames per set by electronically converting the radiation at each point to light intensity. The images produced were recorded for later analysis using standard videotape equipment. The standard thermogram produced by the Inframetrics 525 scanner includes a calibrated gray scale across the bottom of the picture (Fig. 1). Constant temperature contours am intensified over the normal image when the scanner is in the isotherm mode. in this mode, temperatures of two or more areas can be determined relative to one another. A IO-point scale is displayed to the left of the screen with a cursor which indicates the difference as a fraction of the temperature range. The range can be varied but, in most instances, a range of either 10 or 20” was used. The range size is indicated by the size of the vertical bar within the displayed IO-point scale. When an object of known temperature is included within the field of view, it becomes a constant temperature reference body for any other objects in that field of view. Ambient and reference temperatures were obtained by use of a copper/constantan thermocouple and a Bailey Instruments Company BAT-12 model thermometer.
Surface temperatures were determined by analysis of single frames on a DEC PDP-11 computer with a video frame analysis package. When using this system, single frames are frozen and stored on computer diskette. The intensity value of each pixel within that frame is digitized by the computer. Plotting of pixel intensity vs position along the calibrated gray scale enables minimum and maximum temperature to be read. From this information and the temperature of the known reference body, the computer calculates a calibration curve of temperature versus pixel intensity. The examined temperature range corresponds to a linear 2/3 section of this curve, falling midway between the two extremes. The reference body also falls within the linear portion of the curve. The computer is able to set the limits of each isotherm within a specified area of the picture, then sums the total number of pixels included within each isotherm. The average temperature of each isotherm, along with the pixel tally is calculated then printed out. Mean surface temperature of each pinna was determined by use of this information in a separate program. Alternatively, the following equation can be used: T,=cT,
; , 0 t
where T, is average temperature of an isotherm, I’, is pixel tally of that isotherm, and P, is total number of pixels in the analysed image (Mohler, 1987). Heat exchange across the elephants’ pinnae was calculated using the mean surface temperatures acquired through pixel talley and the above equation. In one case, specific sections were found to vary greatly in temperature from the remaining portion of the ear. In that instance, surface temperature and thus heat loss was determined
Fig. 1. Standard thermogram produced by the Inframetrics system showing M’toto at T. = 23°C. The calibrated gray scale is visible across the bottom of the picture. A 10” range is indicated by the bar in the IO-point scale at the left. This scale and cursor identify computer enhanced isothermais and can be used to determine temperature of two or more points relative to one another. Mean pinna surface temperature is 27.54”C.
Elephant heat exchange separately for each section and then totaled. The surface temperature calculated in this method was not significantly different than that calculated by using mean surface temperature. The elephants’ pinnae did not contact any solid objects during filming. Radiative heat flux in essentially bareskinned animals such as the elephant is governed by skin temperature (Cena, 1973). Both natural and forced convection must be considered since the continuous flapping of the elephants’ ears creates wind of significant velocity, yet the presence of this force does not eliminate natural convection (Mitchell, 1973). Finally, although African elephants can theoretically meet up to 75% of their heat loss requirements by trans-epidermal evaporation (Wright and Luck, 1984), it was not the intent nor within the realm of this study to determine heat loss through that avenue. To estimate the use the African elephant makes of its ears for heat loss, a model of heat flow across a flat plate was constructed. Since Mame was one of the medium sized elephants, and was observed at most tem~ratures, the model was constructed using her ear size. The elephants’ theoretical heat losses were calculated at various wind velocities and ambient temperatures. Heat exchange was calculated for radiation, natural convection and forced convection, then summed for a total. Radiative heat loss was calculated as:
where T, and T, represent surface and ambient temperatures in Kelvin (converted by K = “C + 273.14), R represents the area in m* (Gates, 1962), (r represents the Stefan-Boltzman Constant, 5.673 x lo-’ Wjm*/K, and p represents emissivity for which the standard value for biological tissue is 0.96 (Mohler, 1987). Both natural and forced convection can be calculated from the same equation:
(Gates, 1980). The difference in calcutating the heat flux due to the two types of convection occurs in the determination of the heat transfer coefficient (h,) which is defined as:
h,=y, in which k is the thermal conductivity of air, 25.7 x 10-3W/m,YC, D represents the critical dimension of the object in meters, and Nu is the Nusselt number for that object (Gates, 1980). The Nusselt number used for the elephant ear in still air was that of a vertical plate in still air under turbulent conditions: Nu = 0.13 (Gr x Pr)*‘r where Pr is Prandtl’s number, considered constant at 0.72 over the range of environmental conditions encountered by animals (Gates, 1980). Gr represents Grashof’s number in still air as determined by the following equation: Gr = (15.4 x I07)(T,-
T,) D’.
The Nussdt number for the elephant ear in forced convertion corresponds to the value for a vertical plate under turbulent flow: Nu = 0.032 Re”,s where the Reynold’s number (Re) is further defined as follows:
695
with Y representing velocity in m/set, and u representing the kinematic viscosity of air, 15.3 x 10-6m2/sec. Using these relationships, the resulting heat transfer coefficients are 1.605 (7’, - T,)’ for natural convection and 5.85 YO.*D -*.* for forced convection (Gates, 1980). These equations were subsequently used to determine heat loss in each specific instance. Surface area of the ears was calculated using measurements provided by zoo personnel. Although the ears are not a regular shape, a rectangle of the same proportions was found to approximate the ear surface in each case. The diameter of a circle the same area as the ear was determined for use as the critical dimension in the heat flow calculations. Turbulent, rather than laminar flow was considered to exist under all conditions for several reasons. The sizes of the elephants’ ears are large enough to assure turbulent flow at virtually all times. Gates (1980) suggests that under natural convection a Grashof number of more than 2 x IO’ is usually sufficient to indicate turbulent conditions. He also indicates that large Reynolds numbers, in excess of 104,are associated with turbulent flow in forced convection. With very large Reynolds numbers, the size of the turbulent layer is much greater than the laminar layer. Large leaves (D > 0.5 m) experience turbulence at velocities of 3 m/set or more. In addition, the heat transfer coefficient (II,) is somewhat uncertain for large vertical surfaces over 2 feet, due to high turbulence (Brown and Marco, 1958). The size of an elephant ear means the elephant experiences turbulent flow across the pinna of the ear. In man, a wind velocity range of 0.1-0.2 m/see results in natural and forced convection assuming equal importance (Mitchell, 1973). Thus, careful determination of wind velocity was necessary when examining the elephants’ heat losses. Although it is common for elephants to flap their ears continuously, in some ambient conditions this does not occur, for instance at lower ambient temperatures. The elephant house is large and drafty, so a wind speed of 0.2m/sec was used as a common indoor wind speed (Mitchell, 1973) for those occasions when the elephants were not moving their pinnae. When the elephants’ pinnae were in motion, wind velocity was estimated from the average speed of the pinna. The pinnae traveled in an arc of approximately 100” per flap. Since the width of the pinna was known, the distance it travelled could be determined by the following equation:
Two-thirds of the width (Wi) was used as the radius of the arc through which the ear travels rather than tbe entire width for two reasons: (1) the ear varies in width from top to bottom and (2) the outer portion travels a greater distance than the inner sections. The distance was doubled since each flap has both forward and backward movement along the arc. Speed of pinna movement was determined by observing the number of flapping motions that each elephant made in I min. Average speed was calculated by: V=L
-g 0
in which L is arc length and N is number of flaps made in 1 min. This value became the wind velocity, which varies from elephant to elephant and day to day since ear sizes and flapping rates varied even when ambient temperature remained the same. Benedict (1936) calculated basal metabolic rate (BMR) for an elephant lying down in a post absorptive state as 13 kc&/kg/day. Since eIephants rarely lie down and spend a majority of their day eating, it is more practical to calculate a standard metabolic rate (SMR). Benedict determined this
696
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K.
PHILLIPS and JAMFS EDWARD HEATH
value for a 3672kg female Asiatic elephant. He then deduced 10% for the influence of standing and 15% for digestion to arrive at the listed basal rate. Since the relationship between size and heat production remains fairly con-
stant after 500 kg, it was assumed that the metabolic rates of the elephants-in this study would be similar to that of Benedict’s elephant, Jap. Using this method, the SMRs were calculated using 17 kcal/kg/day or 0.82 15 W/kg. Since the areas of the pinnae were previously calculated, per cent of the total surface area included in the pinnae was a simple calculation. Table 2 lists metabolic rates and surface areas of the elephants as calculated from the data provided by the zoo.
Summaryof measuredpinnaesurfacetemperatures and subsequentinstantaneousheat losses
Table 3.
T,
T,
Id I.
17nx _16.79
18.0
20.95 22.03 21.55
19.6
Animal Affie
20.13
0.79 0.65
25.42 34.68 21.85
0.82 1.12 I .33
Affie
19.99
10.67
0.65
23.0
27.54 25.60 29.64
26.98 15.49 33.80
1.64 0.94 0.94
25.6
31.72 31.58 31.25 30.60
54.48 53.17 35.82 31.48
I .76 I .72
Affie
2.18 I .92
Mame
27.2
31.62 33.09 30.74’ 33.97
48.87 66.70 38.48 76.21
2.97 4.06 2.34 4.64
Mame
32. I
35.05 35.22 34.81’ 33.85 __^.
55.03 32.49 28.86 19.91 ._--
,‘+.“.a
L I .*
35.33
34.81
1.78 I .98 1.76 1.21 1.20 2.94 3.73
RESULTS Measured surface temperatures and corresponding instantaneous heat losses at given ambient temperatures are listed in Table 3 for each animal. Heat losses are given both in Watts and as percentages of standard metabolic rates. Not all animals were available at each ambient temperature. In general, surface temperature increased with ambient temperature, although the gradient between the two did not change noticeably as ambient temperature increased. Surface temperatures ranged from 16.79”C at an air temperature of 14.4 to 3522°C at 32.1”C ambient temperature. The smallest gradient was 0.39” at 19.6”C ambient temperature. The largest gradient was 6.77” at an ambient temperature of 27.2”C. Two measurements were made for the back of Mame’s pinnae. In each instance, the temperature was less than 1” lower than that recorded for the front with gradients of 0.85” at T, = 27.2 and 0.35 at T, = 32.1. Wind velocities were calculated based on the flapping rates of the pinnae at the two warmest ambient conditions. Those values at T, = 32.1 were 1.24 m/set for me, 0.96 m/set for Mame, 1.03 m/set for M’toto and 1.28 m/set for Christy. At T, = 27.2, velocity was 0.64 m/set for Mame and 0.80 m/set for M’toto. Instantaneous heat losses ranged from 10.67 to 76.21 W. Those values correspond to 0.65-4.64% of the subjects’ standard metabolic rates. The highest heat losses were calculated when ambient temperature was 27.2”C which corresponds to the largest temperature gradient between surface and air. In 18°C ambient conditions, Mame exhibited an unusual surface temperature pattern (Fig. 2). In this case, she had selectively dilated vessels so that 26% of her left pinna had a measured surface temperature of 24.13”C, while the remaining surface measured 20.8O”C. Heat losses from the warm and cool areas separately were 9.49 and 12.96 W, respectively, which
%SMR
$, 2AAX_ _
I
Mame M’toto Mame Christy
M’toto Affie Mame M’toto Christy
Values concern Ihe front side of one pinna only unless otherwise indicated. *Back side of pinna.
corresponds to 0.58% and 0.79% of her SMR. These values combined yield a slightly higher heat loss than that listed in Table 3 which was calculated using mean surface temperature of 21.55”C. The flat plate model qf the pinna was calculated at several ambient temperatures. The example shown (Fig. 3) for a 2000 kg elephant at 20°C ambient conditions indicates a heat loss of 375 W if the pinna surface is 36°C and there is a 5 m/set wind. At lesser wind velocities and lower surface temperatures the calculated heat loss decreases linearly.
DISCUSSION
The natural habitat of the African elephant is savana which contains both wooded areas and open grasslands (Carrington, 1959). Air temperatures can range from near 50°C to below freezing on the mountain plains up to 4600 m. The air can be humid or dry and desert-like (Sikes, 1971). Elephants range throughout these environmental zones and are able to handle the conditions they encounter. Zoo elephants do not see these wide ranges in their environment, however parallels can be drawn between them and their wild relatives. Large vessels in the pinnae, particularly when seen through thermoTable 2. Metabolic rates and surface areas based on Benedict’s graphy, indicate that elephants can control heat loss estimates of 17 and 13 kcal/kg/day for BMR and SMR,respectively, through vasomotion in the pinna. Others have looked and 0.1Wt”’ for SA at elephants through thermography. Cena (1973) BMR SMR shows a thermograph of an elephant on a cool (15”C), Elephant (kcal/day) (kcal/day) (W) (:, ,&, windy day. He records the ear surface temperature as AffiC JVoo 64,ooo 3093 24.2 20.2 lower than the rest of the body which happens when Mame 26,ooO 34,000 1643 15.9 21.9 blood flow is reduced. M’toto 26JIOO 34,000 1643 15.9 20.2 Christy 14,800 19,300 933 10.9 24.9 The pinna of the African elephant serves three Total ear area was calculated using data from Table 1. functions in the control of body temperature. Heat
Elephant heat exchange
Fig. 2. Thermogram of Mame at 18°C. She has selectively dilated vessels in a section of her pinna. That section, 26% of the pinna surface, has a mean temperature of 24.13”C. The surface temperature of the remaining 74% is 20.8O”C. Overall mean surface temperature is 21.55”C accounting for 1.33% of her SMR.
can be dissipated, conserved or deliberately lost prevent freezing. The thermogram of Mame (Plate suggests the dilation of specific vessels in sections the ear can be controlled to facilitate heat loss needed. In this same manner, the vessels can
0
50
100
150
200
250
300
350
to 2) of as be
400
Watts
Fig. 3. Flat plate model of heat loss by the pinna of a 2000 kg African elephant at 20°C ambient temperature. Heat losses from one side of one ear only over five wind velocities are indicated; This model estimates that up to 91% of the heat the elephant produces can be dissipated via the four sides of its pinnae.
constricted to reduce heat loss, thereby conserving heat. Freezing of the pinnae is also a risk for the elephants living on the high altitude savanna. Benedict’s elephant was a circus animal which showed evidence of frostbite on her ears from exposure. The elephant may increase ear surface temperature by dilation of the vessels there to avoid injury. Because of their value, great care is taken with the zoo elephants. They are allowed outside only when the weather is warm. Because of their size, elephants produce considerable amounts of metabolic heat which must be lost. In addition, they gain heat throughout the day from their environment. An absence of deep body temperatures has made it difficult to state for certain that elephants are capable of heat storage (Wright and Luck, 1984). Obtaining an elephant’s deep body temperature is not a simple task, particularly with adults. Until recently there were few figures available (Brody, 1945; Benedict et al., 1921). Vaughan Langman (personal communication) has shown that elephants do store heat by allowing their body temperatures to rise during the day. They dissipate that corresponding amount of heat at night. This provides them with another means of dealing with heat. Elephants do not have sweat glands (Wright and Luck, 1984). The animal may meet its heat loss need through trans-epidermal evaporation. Elephants prefer humid shelter, and when water is readily
POLLYK. PHILLIPS and Jm
698
available they have no difficulty with heat loss (Wright and Luck, 1984). Although there is no physiological provision for keeping the skin moist and flexible, it has developed wrinkles for water retention. Folds which are too big for that purpose serve to enhance adherence of mud which also helps in evaporation (Lillywhite and Stein, 1987). In periods of drought or intense solar radiation, the animals must depend on other avenues for heat loss. The use of pinna for heat loss by convection and radiation is not unique to elephants. New Zealand white rabbits use vasomotion in their pinnae for the same purpose. The pulsing of blood into the rabbit pinna is easily noticeable through thermography (Mohler, 1987). Wright (1984) measured the blood flow in the right ear of a 2400 kg elephant and found values ranging from 6 to 12 I/min. Those changes may indicate that elephants, like many mammals, have pulsatile flow to the skin. The movement of the pinna facilitated both radiative heat loss by exposing both of its surfaces and convective heat loss by increasing air movement (Wright, 1984). Any injury which interferes with the elephant’s ear flapping ability may seriously affect its health (Sikes, 1971). Benedict (1936) considered that the change in blood flow to the pinna was due to nervousness. While it is obvious that an elephant uses its pinna for other purposes, such as aggressive display (Sikes, 1971), two observations indicate that the main purpose of movement may be for heat loss. In the model, heat losses increase linearly with increases in wind velocity. The movement of the elephant’s ear creates significant relative air movement. The elephants move their ears faster in warmer ambient conditions, and more often when outside in the sun than when inside. Observations made at the coolest of ambient temperatures found little ear movement. Elephants dissipate excess heat by raising skin temperature. In Plates 1 and 2, warm patches are visible on the elephants’ bodies. A similar pattern is visible in humans when viewed by thermography
a.
Fig. 4. Heat distribution The change
EDWARLIHEATH
@ersonal observation). The Asiatic elephant (Elephus muximw) looks similar to the African elephant in thermograms. Their pinna are approximately onethird the size of the African elephant (Carrington, 1959), therefore their calculated theoretical heat loss from the pinnae is only one-third as much of their metabolic rate as compared to the African elephant. At an ambient temperature of 20°C with a 5 m/set wind and a 26” gradient, a 2000 kg African elephant can lose 1500 W from both sides of two ears (Fig. 1) which is 91.3% of her SMR of 1643 W. An Asiatic elephant of the same size and metabolic rate, but with ears only one-third the size loses only 544 W or 33.1% of its SMR. The New Zealand white rabbit loses 33.1% of the heat it produces through its pinnae in windless ambient conditions at 20°C and 43.3% at 25°C (Mohler, 1987). This is through the front of the ear only and while these seem to be much larger figures than those obtained for the African elephant, it is important to remember that the rabbit has few other places from which to lose heat due to the insulating value of its fur. In addition, the ears of the rabbit may account for a greater percentage of body surface area than do those of the African elephant. The calculated heat loss for the elephants in this study may not seem large when considering the size of the pinnae, the lack of sweat glands, and the great amounts of heat they generate which must be lost. However, there was always water available to the animals. They were bathed daily and much care was taken to prevent them from becoming overheated. These elephants perhaps did not need to use their ears for heat loss to the extent to which the model suggests is possible. Had larger heat losses been necessary, the elephants needed only to increase the temperature gradient, increase wind velocity by speeding up ear movements, or store the heat for dissipation later. Presumably, the first choice would be to move to a cooler spot. The pattern of heat dissipation in the bill of the white Pekin duck varies with changes in ambient
b.
across the surface of Me’s right pinna at T* = 18°C (a) and at T, = 32.1”C (b). in pattern indicates a change in blood flow occurs at higher temperatures.
Elephant heaIt exchange temperature (Hagen and Heath, 1980). The same appears to be true for the pinnae of the African elephant. Heat distribution patterns differ when comparing a cool ear to a warm one (Fig. 4). The cool ear shows very warm temperatures at the point where the ear attaches to the head and an increasing gradient from base to top. In the warm ear, the pattern shifts to concentric circles with the gradient increasing toward the center of the ear. This seems to indicate that a change in blood flow occurs, causing the varying patterns of surface temperature. The surface temperatures observed for the back of the ear appear to conflict with previous studies. Benedict et al. (1921) found the back of the ear to be warmer than the front, while the opposite was true in this study. The ambient temperature was 19.W when Benedict made his recordings. The elephant probably held the pinna very close to her head and did not move it much, thus accounting for a warmer temperature being recorded on the back surface. The observations made in this study were at much warmer ambient conditions of 32.1 and 27.2% Only when the animal obliged by flapping her ear while at the proper angle to the camera could the surface temperature be obtained for the back of the ear. Movement keeps the ear away from the body, eliminating transfer of heat from that area as well as increasing wind velocity. One must also consider that thermography gives an overall average temperature while other methods, such as thermocouples, measure only points, such as directly over vessels, which may not be representative. The African elephant has a great capacity for heat loss through the pinnae of its ears. Evaporation aside, all of the elephant’s heat loss needs can be met by vasomotion in the pinnae over a wide range of conditions. Infrared thermography indicates the elephant is able to control blood flow to the pinnae. Heat can be dissipated through dilation of the ear vessels or conserved by constriction. The pinnae
699
vessels may dilate to prevent freezing. Heat loss is amplified by movement of the pinnae. Acknowledgement-The
animals used in this study were made available by Brookfield 200, Brookfield, IL, U.S.A. REFERENCES
Benedict F. G. (1936) The Physiology of the Elephant. Carnegie Institute, Washington, DC. Benedict F. G., Fox E. L. and Baker M. L. (1921) The surface temperature of the elephant, rhinoceros and hippopotamus. Am. J. Physiol. 56, 464474. Brody S. (1945) Bioenergetics and Growth. Reinhold Publishing, New York. Brown A. I. and Marco S. M. (1958) Introduction to Heat Trunsfer. McGraw-Hill, New York. Carrington R. (1959) Elephants. Basic Books, New York. Cena K. (1973) Radiative heat loss from animals and man. In Heat Loss from Animals and Man (Edited by Monteith J. L. and Mount L. E.), Vol. 1, pp. 33-58. Butterworths, London. Gates D. M. (1962) Energy Exchange in the Biosphere. Harper and Row, New York. Gates b. M. (1980) Biophysical Ecology. Springer, New York. Hanan A. A. and Heath 3. E. (1980) Reaulation of heat loss ii the duck by vasomotion in the bill. J. ther. Eiol. 5, 95-101. Lillywhite H. B. and Stein B. R. (1987) Surface sculpturing and water retention of elephant skin. J. Zool., Land. 211, 727-734. Mitchell D. (1973) Convective heat transfer from man and other animals. In Heat Loss from Animals and Man (Edited by Monteith J. L. and Mount L. IT.), Vol. 1, pp. 59-76. Butterworths, London. Mohler F. S. (1987) Oscillating heat flow from the pinna of the ear of the rabbit (Oryctolagus cuniculus). PhD thesis, University of Illinois. Sikes S. K. (1971) The Natural History of the African Elephant. Elsevier, New York. Wright P. G. (1984) Why do elephants flap their ears? S. Afr. J.
Zool. 19, 266-269.
Wright P. G. and Luck C. P. (1984) Do elephants need to sweat? S. Afr. J. Zool. 19, 270-274.
Biochem.
Physiol.
Vol.
IOiA, No.4,
pp. 693-699,
1992
Printed inGreatBritain
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03W9629/92 ES.00 + 0.00 1992 Pergamon Press pk
HEAT EXCHANGE BY THE PINNA OF THE AFRICAN ELEPHANT (LOXODONTA AFRICANA) POLLYK.PHILLIPS* and JA~EDWARDH~~ Department of Physiology and Biophysics, University of Illinois, Urbana, IL 61801, U.S.A. Telephone: (217) 333-1735; Fax: (217) 333-1133 (Received 2 August 1991)
Abstract-1. Surface temperatures of the pinnae of four female African elephants were measured at ambient temperatures between 14 and 32°C using infrared thermography. Instantaneous heat losses calculated using those values ranged from 10.67 to 76.2 W under the observed conditions. 2. Using a value of 17 k&/kg/day, those heat losses account for 0.65-4.64% of the animals’ standard metabolic rates, considering one side of one ear only. 3. A model of heat flow across a flat vertical plate was constructed and compared to the actual values. Up to 100% of an African elephant’s heat Ioss needs can be met by movement of its pinnae and by vasodilation. 4. Thermography indicates that the temperature distribution pattern across the pinna changes with amnbient temperature and that areas of specialized motor control exist.
~RODU~ION
Homeotherms appear to have adapted to new or changing environments by varying the size and shape of their bodies and extremities. These physical changes facilitated heat conservation or dissipation as dictated by ambient conditions. Larger animals need to develop means of dealing with the great amounts of heat that they produce. The African elephant (Loxodonra africana), the largest land mammal, has accordingly developed the largest the~o~~lato~ organ known in any animal, the pinna or external ear, which it uses as a radiator-convector. The extent to which the elephant uses its ears in thermoregulation has been disputed. In his classic study of elephant physiology, Benedict (1936) claimed that a large, hairless animal such as the elephant would have no need for a special heat regulating mechanism in its ears and if it did possess it, it would be a singuiar provision in nature. Others, however, have indicated that the pinna of an elephant’s ear is the main external organ connected with body temperature regulation (Sikes, 1971). The combined surface area of both sides of both ears of an African elephant is about 20% of its total surface area (Wright, 1984). The ears are sensitive, thin, and easily frozen (Benedict, 1936). The extensive vascular network of subcutaneous vessels lying to the medial side of the ear cartilage (Sikes, 1971) combined with the high surface to volume ratio and large surface area of the ear indicate this area has a role in temperature regulation. Further, the almost constant motion of the pinnae exposes the medial side and corresponding vessels to air currents and could increase heat loss from the ear as well as from the fanned body surface (Wright, 1984). *To whom all correspondence should be addressed. cm4
IOI,4-E
Sikes (1971) states that evidence has been found which indicates that the degree of dilation of the ear vessels can be controlled, although she does not explain what that evidence is. Benedict et al. (1921) note wide and rapid changes in ear temperatures which suggests large fluctuation in blood flow even at the ear tips. They also found the protected, medial side of the ear to be warmer than the front, with marked differences in small areas. A 4000 kg elephant needs to maintain a heat loss of 4.65 kW or more while moving and feeding (Wright, 1984), which indicates that the elephant not only creates a great deal of heat, but must have an effective means for controlling heat flow. The pinna appears to suit that purpose very well. Infrared thermography provides a non-invasive measure of pinna temperature to estimate the African elephant’s use of its pinnae for regulation of body temperature and heat loss. Converting the surface temperatures to a model of heat flow and comparing this to metabolic rates may clarify the role of the pinnae in the thermoregulation of the largest land animal. MATERlAISANDMETHODS The four female elephants used in this study were made available through the cooperation and generosity of Brookfield Zoo, Brookfield, IL, U.S.A. and Mr Bruce Brewer, curator of mammals. The vital statistics of the elephants, listed in Tabk 1, were obtained through the assistance of Mr Brewer and zoo personnel. Infrared scans of the elephants, either under overcast conditions or indoors, were taken on 5 days of varying ambient temperature using an Inframetrics model 525 infrared (i.r.) scanner and electronics/control unit. When using this system thermal radiation is detected by the scanner across a band of wavelengths from 8 to 12 pm at which energy absorption by the atmosphere is at a minimum. Radiation from points in the field of view is compared to a
693
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POLLYK. ~H~LLIPSand JAMES EDWARDHEATH
Table 1. Vital statistics of the Brookfield Zoo elephants Ear Ear Weight width Height length Elephant (kg) (m) (en0 (em) 3764 88.9 Affie 2.56 137.0 68.6 Mame 2000 2.31 127.0 13.1 2000 2.29 M’toto 109.0 1136 63.5 Christy 1.80 106.7
liquid nitrogen internal standard temperature of - 196°C. The imaging system then creates a black and white picture at the rate of 10 frames per set by electronically converting the radiation at each point to light intensity. The images produced were recorded for later analysis using standard videotape equipment. The standard thermogram produced by the Inframetrics 525 scanner includes a calibrated gray scale across the bottom of the picture (Fig. 1). Constant temperature contours am intensified over the normal image when the scanner is in the isotherm mode. in this mode, temperatures of two or more areas can be determined relative to one another. A IO-point scale is displayed to the left of the screen with a cursor which indicates the difference as a fraction of the temperature range. The range can be varied but, in most instances, a range of either 10 or 20” was used. The range size is indicated by the size of the vertical bar within the displayed IO-point scale. When an object of known temperature is included within the field of view, it becomes a constant temperature reference body for any other objects in that field of view. Ambient and reference temperatures were obtained by use of a copper/constantan thermocouple and a Bailey Instruments Company BAT-12 model thermometer.
Surface temperatures were determined by analysis of single frames on a DEC PDP-11 computer with a video frame analysis package. When using this system, single frames are frozen and stored on computer diskette. The intensity value of each pixel within that frame is digitized by the computer. Plotting of pixel intensity vs position along the calibrated gray scale enables minimum and maximum temperature to be read. From this information and the temperature of the known reference body, the computer calculates a calibration curve of temperature versus pixel intensity. The examined temperature range corresponds to a linear 2/3 section of this curve, falling midway between the two extremes. The reference body also falls within the linear portion of the curve. The computer is able to set the limits of each isotherm within a specified area of the picture, then sums the total number of pixels included within each isotherm. The average temperature of each isotherm, along with the pixel tally is calculated then printed out. Mean surface temperature of each pinna was determined by use of this information in a separate program. Alternatively, the following equation can be used: T,=cT,
; , 0 t
where T, is average temperature of an isotherm, I’, is pixel tally of that isotherm, and P, is total number of pixels in the analysed image (Mohler, 1987). Heat exchange across the elephants’ pinnae was calculated using the mean surface temperatures acquired through pixel talley and the above equation. In one case, specific sections were found to vary greatly in temperature from the remaining portion of the ear. In that instance, surface temperature and thus heat loss was determined
Fig. 1. Standard thermogram produced by the Inframetrics system showing M’toto at T. = 23°C. The calibrated gray scale is visible across the bottom of the picture. A 10” range is indicated by the bar in the IO-point scale at the left. This scale and cursor identify computer enhanced isothermais and can be used to determine temperature of two or more points relative to one another. Mean pinna surface temperature is 27.54”C.
Elephant heat exchange separately for each section and then totaled. The surface temperature calculated in this method was not significantly different than that calculated by using mean surface temperature. The elephants’ pinnae did not contact any solid objects during filming. Radiative heat flux in essentially bareskinned animals such as the elephant is governed by skin temperature (Cena, 1973). Both natural and forced convection must be considered since the continuous flapping of the elephants’ ears creates wind of significant velocity, yet the presence of this force does not eliminate natural convection (Mitchell, 1973). Finally, although African elephants can theoretically meet up to 75% of their heat loss requirements by trans-epidermal evaporation (Wright and Luck, 1984), it was not the intent nor within the realm of this study to determine heat loss through that avenue. To estimate the use the African elephant makes of its ears for heat loss, a model of heat flow across a flat plate was constructed. Since Mame was one of the medium sized elephants, and was observed at most tem~ratures, the model was constructed using her ear size. The elephants’ theoretical heat losses were calculated at various wind velocities and ambient temperatures. Heat exchange was calculated for radiation, natural convection and forced convection, then summed for a total. Radiative heat loss was calculated as:
where T, and T, represent surface and ambient temperatures in Kelvin (converted by K = “C + 273.14), R represents the area in m* (Gates, 1962), (r represents the Stefan-Boltzman Constant, 5.673 x lo-’ Wjm*/K, and p represents emissivity for which the standard value for biological tissue is 0.96 (Mohler, 1987). Both natural and forced convection can be calculated from the same equation:
(Gates, 1980). The difference in calcutating the heat flux due to the two types of convection occurs in the determination of the heat transfer coefficient (h,) which is defined as:
h,=y, in which k is the thermal conductivity of air, 25.7 x 10-3W/m,YC, D represents the critical dimension of the object in meters, and Nu is the Nusselt number for that object (Gates, 1980). The Nusselt number used for the elephant ear in still air was that of a vertical plate in still air under turbulent conditions: Nu = 0.13 (Gr x Pr)*‘r where Pr is Prandtl’s number, considered constant at 0.72 over the range of environmental conditions encountered by animals (Gates, 1980). Gr represents Grashof’s number in still air as determined by the following equation: Gr = (15.4 x I07)(T,-
T,) D’.
The Nussdt number for the elephant ear in forced convertion corresponds to the value for a vertical plate under turbulent flow: Nu = 0.032 Re”,s where the Reynold’s number (Re) is further defined as follows:
695
with Y representing velocity in m/set, and u representing the kinematic viscosity of air, 15.3 x 10-6m2/sec. Using these relationships, the resulting heat transfer coefficients are 1.605 (7’, - T,)’ for natural convection and 5.85 YO.*D -*.* for forced convection (Gates, 1980). These equations were subsequently used to determine heat loss in each specific instance. Surface area of the ears was calculated using measurements provided by zoo personnel. Although the ears are not a regular shape, a rectangle of the same proportions was found to approximate the ear surface in each case. The diameter of a circle the same area as the ear was determined for use as the critical dimension in the heat flow calculations. Turbulent, rather than laminar flow was considered to exist under all conditions for several reasons. The sizes of the elephants’ ears are large enough to assure turbulent flow at virtually all times. Gates (1980) suggests that under natural convection a Grashof number of more than 2 x IO’ is usually sufficient to indicate turbulent conditions. He also indicates that large Reynolds numbers, in excess of 104,are associated with turbulent flow in forced convection. With very large Reynolds numbers, the size of the turbulent layer is much greater than the laminar layer. Large leaves (D > 0.5 m) experience turbulence at velocities of 3 m/set or more. In addition, the heat transfer coefficient (II,) is somewhat uncertain for large vertical surfaces over 2 feet, due to high turbulence (Brown and Marco, 1958). The size of an elephant ear means the elephant experiences turbulent flow across the pinna of the ear. In man, a wind velocity range of 0.1-0.2 m/see results in natural and forced convection assuming equal importance (Mitchell, 1973). Thus, careful determination of wind velocity was necessary when examining the elephants’ heat losses. Although it is common for elephants to flap their ears continuously, in some ambient conditions this does not occur, for instance at lower ambient temperatures. The elephant house is large and drafty, so a wind speed of 0.2m/sec was used as a common indoor wind speed (Mitchell, 1973) for those occasions when the elephants were not moving their pinnae. When the elephants’ pinnae were in motion, wind velocity was estimated from the average speed of the pinna. The pinnae traveled in an arc of approximately 100” per flap. Since the width of the pinna was known, the distance it travelled could be determined by the following equation:
Two-thirds of the width (Wi) was used as the radius of the arc through which the ear travels rather than tbe entire width for two reasons: (1) the ear varies in width from top to bottom and (2) the outer portion travels a greater distance than the inner sections. The distance was doubled since each flap has both forward and backward movement along the arc. Speed of pinna movement was determined by observing the number of flapping motions that each elephant made in I min. Average speed was calculated by: V=L
-g 0
in which L is arc length and N is number of flaps made in 1 min. This value became the wind velocity, which varies from elephant to elephant and day to day since ear sizes and flapping rates varied even when ambient temperature remained the same. Benedict (1936) calculated basal metabolic rate (BMR) for an elephant lying down in a post absorptive state as 13 kc&/kg/day. Since eIephants rarely lie down and spend a majority of their day eating, it is more practical to calculate a standard metabolic rate (SMR). Benedict determined this
696
POLLY
K.
PHILLIPS and JAMFS EDWARD HEATH
value for a 3672kg female Asiatic elephant. He then deduced 10% for the influence of standing and 15% for digestion to arrive at the listed basal rate. Since the relationship between size and heat production remains fairly con-
stant after 500 kg, it was assumed that the metabolic rates of the elephants-in this study would be similar to that of Benedict’s elephant, Jap. Using this method, the SMRs were calculated using 17 kcal/kg/day or 0.82 15 W/kg. Since the areas of the pinnae were previously calculated, per cent of the total surface area included in the pinnae was a simple calculation. Table 2 lists metabolic rates and surface areas of the elephants as calculated from the data provided by the zoo.
Summaryof measuredpinnaesurfacetemperatures and subsequentinstantaneousheat losses
Table 3.
T,
T,
Id I.
17nx _16.79
18.0
20.95 22.03 21.55
19.6
Animal Affie
20.13
0.79 0.65
25.42 34.68 21.85
0.82 1.12 I .33
Affie
19.99
10.67
0.65
23.0
27.54 25.60 29.64
26.98 15.49 33.80
1.64 0.94 0.94
25.6
31.72 31.58 31.25 30.60
54.48 53.17 35.82 31.48
I .76 I .72
Affie
2.18 I .92
Mame
27.2
31.62 33.09 30.74’ 33.97
48.87 66.70 38.48 76.21
2.97 4.06 2.34 4.64
Mame
32. I
35.05 35.22 34.81’ 33.85 __^.
55.03 32.49 28.86 19.91 ._--
,‘+.“.a
L I .*
35.33
34.81
1.78 I .98 1.76 1.21 1.20 2.94 3.73
RESULTS Measured surface temperatures and corresponding instantaneous heat losses at given ambient temperatures are listed in Table 3 for each animal. Heat losses are given both in Watts and as percentages of standard metabolic rates. Not all animals were available at each ambient temperature. In general, surface temperature increased with ambient temperature, although the gradient between the two did not change noticeably as ambient temperature increased. Surface temperatures ranged from 16.79”C at an air temperature of 14.4 to 3522°C at 32.1”C ambient temperature. The smallest gradient was 0.39” at 19.6”C ambient temperature. The largest gradient was 6.77” at an ambient temperature of 27.2”C. Two measurements were made for the back of Mame’s pinnae. In each instance, the temperature was less than 1” lower than that recorded for the front with gradients of 0.85” at T, = 27.2 and 0.35 at T, = 32.1. Wind velocities were calculated based on the flapping rates of the pinnae at the two warmest ambient conditions. Those values at T, = 32.1 were 1.24 m/set for me, 0.96 m/set for Mame, 1.03 m/set for M’toto and 1.28 m/set for Christy. At T, = 27.2, velocity was 0.64 m/set for Mame and 0.80 m/set for M’toto. Instantaneous heat losses ranged from 10.67 to 76.21 W. Those values correspond to 0.65-4.64% of the subjects’ standard metabolic rates. The highest heat losses were calculated when ambient temperature was 27.2”C which corresponds to the largest temperature gradient between surface and air. In 18°C ambient conditions, Mame exhibited an unusual surface temperature pattern (Fig. 2). In this case, she had selectively dilated vessels so that 26% of her left pinna had a measured surface temperature of 24.13”C, while the remaining surface measured 20.8O”C. Heat losses from the warm and cool areas separately were 9.49 and 12.96 W, respectively, which
%SMR
$, 2AAX_ _
I
Mame M’toto Mame Christy
M’toto Affie Mame M’toto Christy
Values concern Ihe front side of one pinna only unless otherwise indicated. *Back side of pinna.
corresponds to 0.58% and 0.79% of her SMR. These values combined yield a slightly higher heat loss than that listed in Table 3 which was calculated using mean surface temperature of 21.55”C. The flat plate model qf the pinna was calculated at several ambient temperatures. The example shown (Fig. 3) for a 2000 kg elephant at 20°C ambient conditions indicates a heat loss of 375 W if the pinna surface is 36°C and there is a 5 m/set wind. At lesser wind velocities and lower surface temperatures the calculated heat loss decreases linearly.
DISCUSSION
The natural habitat of the African elephant is savana which contains both wooded areas and open grasslands (Carrington, 1959). Air temperatures can range from near 50°C to below freezing on the mountain plains up to 4600 m. The air can be humid or dry and desert-like (Sikes, 1971). Elephants range throughout these environmental zones and are able to handle the conditions they encounter. Zoo elephants do not see these wide ranges in their environment, however parallels can be drawn between them and their wild relatives. Large vessels in the pinnae, particularly when seen through thermoTable 2. Metabolic rates and surface areas based on Benedict’s graphy, indicate that elephants can control heat loss estimates of 17 and 13 kcal/kg/day for BMR and SMR,respectively, through vasomotion in the pinna. Others have looked and 0.1Wt”’ for SA at elephants through thermography. Cena (1973) BMR SMR shows a thermograph of an elephant on a cool (15”C), Elephant (kcal/day) (kcal/day) (W) (:, ,&, windy day. He records the ear surface temperature as AffiC JVoo 64,ooo 3093 24.2 20.2 lower than the rest of the body which happens when Mame 26,ooO 34,000 1643 15.9 21.9 blood flow is reduced. M’toto 26JIOO 34,000 1643 15.9 20.2 Christy 14,800 19,300 933 10.9 24.9 The pinna of the African elephant serves three Total ear area was calculated using data from Table 1. functions in the control of body temperature. Heat
Elephant heat exchange
Fig. 2. Thermogram of Mame at 18°C. She has selectively dilated vessels in a section of her pinna. That section, 26% of the pinna surface, has a mean temperature of 24.13”C. The surface temperature of the remaining 74% is 20.8O”C. Overall mean surface temperature is 21.55”C accounting for 1.33% of her SMR.
can be dissipated, conserved or deliberately lost prevent freezing. The thermogram of Mame (Plate suggests the dilation of specific vessels in sections the ear can be controlled to facilitate heat loss needed. In this same manner, the vessels can
0
50
100
150
200
250
300
350
to 2) of as be
400
Watts
Fig. 3. Flat plate model of heat loss by the pinna of a 2000 kg African elephant at 20°C ambient temperature. Heat losses from one side of one ear only over five wind velocities are indicated; This model estimates that up to 91% of the heat the elephant produces can be dissipated via the four sides of its pinnae.
constricted to reduce heat loss, thereby conserving heat. Freezing of the pinnae is also a risk for the elephants living on the high altitude savanna. Benedict’s elephant was a circus animal which showed evidence of frostbite on her ears from exposure. The elephant may increase ear surface temperature by dilation of the vessels there to avoid injury. Because of their value, great care is taken with the zoo elephants. They are allowed outside only when the weather is warm. Because of their size, elephants produce considerable amounts of metabolic heat which must be lost. In addition, they gain heat throughout the day from their environment. An absence of deep body temperatures has made it difficult to state for certain that elephants are capable of heat storage (Wright and Luck, 1984). Obtaining an elephant’s deep body temperature is not a simple task, particularly with adults. Until recently there were few figures available (Brody, 1945; Benedict et al., 1921). Vaughan Langman (personal communication) has shown that elephants do store heat by allowing their body temperatures to rise during the day. They dissipate that corresponding amount of heat at night. This provides them with another means of dealing with heat. Elephants do not have sweat glands (Wright and Luck, 1984). The animal may meet its heat loss need through trans-epidermal evaporation. Elephants prefer humid shelter, and when water is readily
POLLYK. PHILLIPS and Jm
698
available they have no difficulty with heat loss (Wright and Luck, 1984). Although there is no physiological provision for keeping the skin moist and flexible, it has developed wrinkles for water retention. Folds which are too big for that purpose serve to enhance adherence of mud which also helps in evaporation (Lillywhite and Stein, 1987). In periods of drought or intense solar radiation, the animals must depend on other avenues for heat loss. The use of pinna for heat loss by convection and radiation is not unique to elephants. New Zealand white rabbits use vasomotion in their pinnae for the same purpose. The pulsing of blood into the rabbit pinna is easily noticeable through thermography (Mohler, 1987). Wright (1984) measured the blood flow in the right ear of a 2400 kg elephant and found values ranging from 6 to 12 I/min. Those changes may indicate that elephants, like many mammals, have pulsatile flow to the skin. The movement of the pinna facilitated both radiative heat loss by exposing both of its surfaces and convective heat loss by increasing air movement (Wright, 1984). Any injury which interferes with the elephant’s ear flapping ability may seriously affect its health (Sikes, 1971). Benedict (1936) considered that the change in blood flow to the pinna was due to nervousness. While it is obvious that an elephant uses its pinna for other purposes, such as aggressive display (Sikes, 1971), two observations indicate that the main purpose of movement may be for heat loss. In the model, heat losses increase linearly with increases in wind velocity. The movement of the elephant’s ear creates significant relative air movement. The elephants move their ears faster in warmer ambient conditions, and more often when outside in the sun than when inside. Observations made at the coolest of ambient temperatures found little ear movement. Elephants dissipate excess heat by raising skin temperature. In Plates 1 and 2, warm patches are visible on the elephants’ bodies. A similar pattern is visible in humans when viewed by thermography
a.
Fig. 4. Heat distribution The change
EDWARLIHEATH
@ersonal observation). The Asiatic elephant (Elephus muximw) looks similar to the African elephant in thermograms. Their pinna are approximately onethird the size of the African elephant (Carrington, 1959), therefore their calculated theoretical heat loss from the pinnae is only one-third as much of their metabolic rate as compared to the African elephant. At an ambient temperature of 20°C with a 5 m/set wind and a 26” gradient, a 2000 kg African elephant can lose 1500 W from both sides of two ears (Fig. 1) which is 91.3% of her SMR of 1643 W. An Asiatic elephant of the same size and metabolic rate, but with ears only one-third the size loses only 544 W or 33.1% of its SMR. The New Zealand white rabbit loses 33.1% of the heat it produces through its pinnae in windless ambient conditions at 20°C and 43.3% at 25°C (Mohler, 1987). This is through the front of the ear only and while these seem to be much larger figures than those obtained for the African elephant, it is important to remember that the rabbit has few other places from which to lose heat due to the insulating value of its fur. In addition, the ears of the rabbit may account for a greater percentage of body surface area than do those of the African elephant. The calculated heat loss for the elephants in this study may not seem large when considering the size of the pinnae, the lack of sweat glands, and the great amounts of heat they generate which must be lost. However, there was always water available to the animals. They were bathed daily and much care was taken to prevent them from becoming overheated. These elephants perhaps did not need to use their ears for heat loss to the extent to which the model suggests is possible. Had larger heat losses been necessary, the elephants needed only to increase the temperature gradient, increase wind velocity by speeding up ear movements, or store the heat for dissipation later. Presumably, the first choice would be to move to a cooler spot. The pattern of heat dissipation in the bill of the white Pekin duck varies with changes in ambient
b.
across the surface of Me’s right pinna at T* = 18°C (a) and at T, = 32.1”C (b). in pattern indicates a change in blood flow occurs at higher temperatures.
Elephant heaIt exchange temperature (Hagen and Heath, 1980). The same appears to be true for the pinnae of the African elephant. Heat distribution patterns differ when comparing a cool ear to a warm one (Fig. 4). The cool ear shows very warm temperatures at the point where the ear attaches to the head and an increasing gradient from base to top. In the warm ear, the pattern shifts to concentric circles with the gradient increasing toward the center of the ear. This seems to indicate that a change in blood flow occurs, causing the varying patterns of surface temperature. The surface temperatures observed for the back of the ear appear to conflict with previous studies. Benedict et al. (1921) found the back of the ear to be warmer than the front, while the opposite was true in this study. The ambient temperature was 19.W when Benedict made his recordings. The elephant probably held the pinna very close to her head and did not move it much, thus accounting for a warmer temperature being recorded on the back surface. The observations made in this study were at much warmer ambient conditions of 32.1 and 27.2% Only when the animal obliged by flapping her ear while at the proper angle to the camera could the surface temperature be obtained for the back of the ear. Movement keeps the ear away from the body, eliminating transfer of heat from that area as well as increasing wind velocity. One must also consider that thermography gives an overall average temperature while other methods, such as thermocouples, measure only points, such as directly over vessels, which may not be representative. The African elephant has a great capacity for heat loss through the pinnae of its ears. Evaporation aside, all of the elephant’s heat loss needs can be met by vasomotion in the pinnae over a wide range of conditions. Infrared thermography indicates the elephant is able to control blood flow to the pinnae. Heat can be dissipated through dilation of the ear vessels or conserved by constriction. The pinnae
699
vessels may dilate to prevent freezing. Heat loss is amplified by movement of the pinnae. Acknowledgement-The
animals used in this study were made available by Brookfield 200, Brookfield, IL, U.S.A. REFERENCES
Benedict F. G. (1936) The Physiology of the Elephant. Carnegie Institute, Washington, DC. Benedict F. G., Fox E. L. and Baker M. L. (1921) The surface temperature of the elephant, rhinoceros and hippopotamus. Am. J. Physiol. 56, 464474. Brody S. (1945) Bioenergetics and Growth. Reinhold Publishing, New York. Brown A. I. and Marco S. M. (1958) Introduction to Heat Trunsfer. McGraw-Hill, New York. Carrington R. (1959) Elephants. Basic Books, New York. Cena K. (1973) Radiative heat loss from animals and man. In Heat Loss from Animals and Man (Edited by Monteith J. L. and Mount L. E.), Vol. 1, pp. 33-58. Butterworths, London. Gates D. M. (1962) Energy Exchange in the Biosphere. Harper and Row, New York. Gates b. M. (1980) Biophysical Ecology. Springer, New York. Hanan A. A. and Heath 3. E. (1980) Reaulation of heat loss ii the duck by vasomotion in the bill. J. ther. Eiol. 5, 95-101. Lillywhite H. B. and Stein B. R. (1987) Surface sculpturing and water retention of elephant skin. J. Zool., Land. 211, 727-734. Mitchell D. (1973) Convective heat transfer from man and other animals. In Heat Loss from Animals and Man (Edited by Monteith J. L. and Mount L. IT.), Vol. 1, pp. 59-76. Butterworths, London. Mohler F. S. (1987) Oscillating heat flow from the pinna of the ear of the rabbit (Oryctolagus cuniculus). PhD thesis, University of Illinois. Sikes S. K. (1971) The Natural History of the African Elephant. Elsevier, New York. Wright P. G. (1984) Why do elephants flap their ears? S. Afr. J.
Zool. 19, 266-269.
Wright P. G. and Luck C. P. (1984) Do elephants need to sweat? S. Afr. J. Zool. 19, 270-274.
