ANNALS OF HUMAN BIOLOGY, 1976, VOL. 3, NO. l,
11-22
Environmental correlations of skin colour D. F. R O B E R T S and D. P. S. K A H L O N Department of Human Genetics, University of Newcastle upon Tyne
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[Received 2 October 1974; revised 14 January 1975] Summary. Skin colour data obtained by reflectance spectrophotometry on indigenous populations are compared with environmental variables--latitude, temperature, humidity and altitude. Association with latitude predominates at all wavelengths. Temperatures show a small but appreciable association at the shorter wavelengths, humidity at wavelengths above 595 nm. Over 80 per cent of the total interpopulation variance at each wavelength is accounted for by these variables, more at the shorter wavelengths. It is suggested that skin colour should be regarded as a complex of entities of differing selective values rather than a single entity.
1. Introduction The general regularity of geographical distribution of pigmentation in the world's populations has been repeatedly pointed out. This observation, however, has tended primarily to stimulate speculation rather than to provide any solid analysis of the selective role of pigmentation. Partly this has been due to the absence of objective and quantitative records, and today such studies as that of Davenport (1926) or of Biasutti (1953) are of little more than historical interest. Fleure's (1945) discussion drew attention to obvious exceptions to the pigmentation gradient, but his attempts to explain them too, in the absence of agreement on the selective function of pigmentation, essentially remain mere speculation. It is over 20 years since Weiner (1951) described a portable reflectance spectrophotometer (EEL), suitable for field work. This made it possible to obtain, outside the confines of a laboratory, objective and accurate measures of skin colour on human populations in their native environments in the form of reflectance measures at standard wavelengths, and there are already sufficient data on different populations for some attempt to be made at their analysis in relation to geographical variables. To examine the relative strengths of association of mean reflectance readings at different wavelengths with different environmental variables is the object of the present study. 2. Data Although some 120 samples, approximately 60 of each sex, are available to represent the world's indigenous populations, results are not all technically comparable. Most were obtained with the E E L spectrophotometer, and some others with the photovolt model where the wavelengths used are not identical. For only a proportion of the samples are the data given at all nine wavelengths available
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12
D. F. Roberts and D. P. S. Kahlon
on the E E L instrument (425, 465, 485, 515, 545, 575, 595, 655, 685 nm). The skin sites examined are not always the same. The samples vary in size, but for present purposes none was included that contained fewer than 15 subjects. The samples included here all relate to what may be regarded as indigenous populations, and no sample of recent migrants to their present habitat was included; for example, the South American samples relate only to South American Indians and not to whites. All the samples relate to adults, though the inclusion of sub-adult individuals in so-called adult samples may affect the variability. A further possible source of error comes in the climatic variables assigned to each sample. Each sample was located sufficiently precisely for values of the following variables to be assigned to it: latitude, mean annual temperature, mean maximum temperature (average of the highest each year), mean minimum temperature (average of the lowest each year), maximum midday humidity (highest monthly mean), minimum midday humidity (lowest monthly mean), and altitude. T o assign the climatic variables, the figures given in the Meteorological Office Tables (1958-1967) were used, with interpolation between adjacent stations where necessary, taking into account local topographic variation.
Latitude Mean temperature Maximum temperature Minimum humidity Maximum humidity Altitude
Mean temp. (*F) --0.720
Max. temp. ( oF) --0-183 0.700
Min. humidity (per cen0 0.073 --0"212 --0"569
Max. humidity (per cen0 0.089 0"012 --0-235 0-720
Altitude (fee0 --0-363 "0"274 --0-480 --0" 184 --0-401
Min. temp. ( *F) --0-659 0"569 0-082 0-186 0-149 0-002
Table 1. Zero-order correlation coefficients between environmental variables (61 localities). The geographic variables are, of course, not independent of each other. Table 1 shows the zero order correlations of the environmental variables as assigned to all the populations now studied. From these there appear as expected the obvious influence of latitude on temperature; the close correlation of mean temperature with maximum temperature and minimum temperature; similar close correlation with each other of minimum and maximum humidities, while altitude shows no high correlation with any of the others. On account of these intercorrelations, the following analysis takes the form of an examination of zero-order correlation coefficients of reflectance readings with each environmental variable to show the general associations, and subsequent examination by stepwise regression in an endeavour to identify the order in which these environmental variables contribute to the variation in skin pigmentation. 3. Results Upper inner arm The first examination of the reflectance at the upper inner arm incorporated all adequate indigenous samples both male and female; most samples (77) were available at wavelength 685 nm, fewest (32) at wavelengths 485 and 655 nm. For
N = 21
0'920 --0.869 --0-576 --0.574 0-540 0'426 --0-328
485 nm
N=32
0"938 --0.913 --0"616 --0"687 0-623 0'506 --0.372
485 nm
N = 29
0"886 --0.757 --0.538 --0.444 0.312 0.245 --0.231
515 nm
Male samples only
N=45
0-907 --0.797 --0"575 --0'511 0"317 0.284 --0'255
515 nm
N = 34
0.873 --0.809 --0.563 --0'550 0.372 0'250 --0.237
545 nm
N--=55
0.880 --0"836 --0.596 --0"616 0-400 0"318 --0.265
545 nm
N=21
0.906 --0.843 --0.625 --0.559 0.536 0.408 --0-332
575 nm
N=32
0.928 --0.898 --0.657 --0.680 0.622 0.478 --0.371
575 nm
N = 23
0"904 --0"838 --0"629 --0-533 0.507 0'367 --0'323
595 nm
N=36
0"928 --0"898 --0"658 --0'654 0'590 0'446 --0.347
595 nm
N = 34
0.843 --0.504 --0.564 --0'017 0.087 0.204 --0"383
655 nm
N=51
0-889 --0.598 --0"599 --0-101 0.103 0.210 --0-405
655 nm
Zero order correlation coefficients of the environmental variables with mean reflectance at upper inner arm.
N=29
N = 37
Table 2.
0-910 --0"756 --0.533 --0.414 0.233 0"221 --0-261
0.885 --0.898 --0'503 --0"507 0.315 4'311 --0"209
Latitude Mean temperature Minimum temperature Maximum temperature Minimum humidity Maximum humidity Altitude
465 nm
425 nm
N=45
N=61
Wavelength:
0-932 --0'795 --0'581 --0'481 0.239 0.264 --0.285
0.895 --0'824 --0"533 --0-556 0.347 0.403 --0"238
Latitude Mean temperature Minimum temperature Maximum temperature Minimum humidity Maximum humidity Altitude
465 nm
425 nm
Wavelength :
Male and female samples
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N = 46
0'825 --0"652 --0"585 --0"346 0'231 0"257 --0"220
685 nm
N=77
0"835 --0"681 --0.590 --0"400 0.281 0.331 --0"236
685 nm
U,
~' "~
~-
e~
15.9
3"3 0"1
Max. temp.
Altitude
Table 3.
11"1
Mean temp.
Max. humidity
Latitude
N=36
13.3
86.2
N=32
11"7
88.0
0"6 0.1
Mean temp. Min. temp.
N=51
1.7 0"9
Max. temp.
79.0
Max. humidity
Latitude
Wavelength 655 nm
Mean temp.
Latitude
Wavelength 485 nm
0"I
Mean temp.
Min. humidity
0.2 N=77
1.2 0.7
Mean temp.
1-6
4"0
6"6
69"7
Altitude
Min. temp.
Max. temp.
Max. humidity
Latitude
Wavelength 685 nm
N=45
0.6 0.5
Max. humidity Min. temp.
2"6 0"9
12.3
82.3
Altitude Min. humidity
Max. temp.
Latitude
Wavelength 515 nm
Min. humidity
Max. temp.
Latitude
N=55
0"8
21.4
77"4
Wavelength 545 nm
Contribution to regression of various environmental variables on nine variables at upper inner arm (male and female samples).
N=32
86.0
Latitude
Wavelength 595 nm
Wavelength 575 nm
2"7 0"3
9.9
86.9
N=45
Min. temp.
Altitude
Max. temp.
Latitude
Wavelength 465 nm
N=61
Min. temp.
80-1
Latitude
Wavelength 425 nm
Contributions to variance (per cent)
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4~
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Environmental correlations o[ skin colour
15
all wavelengths, the zero order correlations suggest that association with latitude predominates (table 2), accounting for between 88 per cent (485 nm) and 70 per cent (685 nm) of the total variance in mean reflectance. The second important variable appears to be mean temperature, which, considered alone, would account for between 83 per cent (485 nm) and 36 per cent (655 nm) of the total variance. The order of the remaining variables differs from wavelength to wavelength, but in general the extreme temperature readings come next, then the humidity, while altitude shows the lowest correlations. Associations of reflectance with latitude and the humidities are consistently positive, with temperatures and altitude negative. These zero order correlation coefficients considered alone demonstrate the strength of the association with geographical variables, reflectance increasing with increasing latitude, decreasing temperature and increasing humidity. In part, these overall associations are, of course, attributable to the intercorrelation of the geographical variables. When these are taken into account in order of importance (i.e. proportion (R 2) of the total variance accounted for) by stepwise regression, the order of the variables is therefore different (table 3). Latitude alone (since it predominates in the zero order analysis) accounts for between 77 and 88 per cent of the total variance for all wavelengths except 685 nm where it accounts for 70 per cent. The addition as an independent second variable of maximum temperature at wavelengths 425, 465, 515 and 545 nm accounts for an additional 10-21 per cent, or of mean temperature at wavelengths 485 and 575 nm for 11 per cent. At the three longer wavelengths maximum humidity moves up into second place, accounting for some 2-13 per cent. The only other variable to produce an increase of R: as great as 1 per cent at any of the first eight wavelengths is altitude (2-3 per cent at 425, 465 and 515 nm) while at wavelength 685 nm all three temperature variables make contributions of 1-4 per cent. Selection of latitude as the predominant variable and the subordinate roles of the others is clearly demonstrated by examination of the first-order partial correlations at the four wavelengths for which there are most samples (table 4). The correlation with latitude diminishes very slightly, if at all, after exclusion of the effects of maximum temperature or maximum humidity, diminishes rather more but still remains appreciable after the exclusion of mean temperature. Clearly the latitudinal correlation owes little to these three variables. Excluding the effects of latitude, the correlations with mean temperature diminish very markedly, those with maximum temperature or maximum humidity rather less. The stepwise analysis was repeated employing only male samples. Fewer samples were available at each wavelength (table 2) so there is some slight
Correlation with latitude excluding max. temp. Correlation with latitude excluding mean temp. Correlation with latitude excluding max. humidity Correlation with max. temp. excluding latitude Correlation with mean temp. excluding latitude Correlation with max. humidity excluding latitude
425 nm
Wavelength 545 nm 655 nm
685 nm
0-821 0.628 0.867 --0'406 --0-372 0-326
0.794 0.578 0-858 --0.470 --0.420 0"242
0"788 0.716 0-812 --0.256 --0-166 0"259
0-901 0.950 0"877 0-063 0-086 0.132
Table 4. Partial correlations (first order) of pzrcentage reflectance at upper inner arm (both sexes).
D. F. Roberts and D. P. S. Kahlon
16
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difference in the subordinate variables and the multiple correlation coefficients tend to be slightly lower, but the substantive results are similar to those for the combined sexes. It appears, therefore, that the overall associations owe little to differential sex representation. The regression equations of reflectance on the relevant environmental variables at each wavelength are set out in table 5 for male and female samples. The proportion of the total variance for which these account is indeed remarkable. Only for the two longest wavelengths does this drop below 97 per cent, though exact comparison is precluded by the different number of samples available at each wavelength. Male and female samples Wavelength 425 nm
Y=
59-69+0-46 Latitude--0-51 Max. temp.
--0.0009 Altitude
(R~=0.994)
465 nm 485 nm
Y= Y=
59"74+0"64Latitude--0-53 Max. temp. 53.31+0"55 Latitude--0.62 Mean temp.
--0"001 Altitude
(Ra=0.995) (R2=0-998)
515 nm
Y=
67.93+0.63 Latitude--0.59 Max. temp.
--0"001 Altitude
(R2=0.972)
545 nm
Y=
50.22+0-57 Latitude--0.42 Max. temp.
+0-06
54.83+0-52 Latitude--0.57 Mean temp.
575 nm
Y=
595 nm
Y= --16.55+0"88 Latitude+0-42 Max. humidity
655 nm 685 nm
Y = - - 7.78+0"90 Latitude+0"18 Max. humidity+0"14 Y= 39.26+0-67 Latitude+0.21 Max. humidity--0-26
Table 5. Regression equations:
Min. humidity (R~=0-996) (R~=0.971) (RZ=0.995) Max. temp. Max. temp.
(RZ=0.816) (R~=0.803)
upper inner arm percentage reflectance on the main environmental variables.
There is no doubt about the dominating influence of the latitudinal associations, nor is there any doubt of the appreciable independent contribution of maxim u m or mean temperature at wavelengths 425 nm to 575 nm. Humidity makes a negligible contribution to the variance at wavelengths below 595 nm, the effect being restricted to m a x i m u m humidity at the three longest wavelengths. The remaining environmental variables make negligible contributions. A particularly interesting feature is the contrast between shorter and longer wavelengths in the importance of temperature and humidity.
Forehead Data are less numerous for forehead readings, there being only 39 samples (both sexes) at wavelength 685 nm, fewer at all other wavelengths. The zero-order correlation coefficients (table 6) are generally similar to but lower than those on the upper inner arm. Latitude is again dominant, accounting for 66-96 per cent of the total variance, followed by mean temperature. Again the associations with latitude and humidity are positive, and negative with temperature and altitude. Taking into account the intercorrelations of the geographical variables (table 7), latitude exerts the m o s t important effect at all wavelengths (65-94 per cent of the v a r i a n c e ) f o l l o w e d by m a x i m u m humidity at 425 and 685 nrn or a temperature variable at the other wavelengths. The contribution of this second variable is in general less than it w a s at the upper inner arm site.
Altitude
Table 6.
N=27
N = 37
N = 16
--0.385
0.775
0-587
--0'553
--0.482
--0.901
0.969
485 nm
N = 27
--0'300
0"209
0'102
--0'290
--0'519
--0"723
0"939
515 nm
N=31
--0.261
0.160
0'179
--0.321
--0"503
--0.724
0.884
545 nm
N = 16
--0.432
0.763
0'566
--0-511
--0.531
--0.870
0"967
575 nm
N = 16
--0.438
0"746
0"564
-0.507
--0"539
--0.879
0.980
595 nm
N = 36
--0'461
0-245
--0'041
0-125
--0'563
--0.460
0.897
655 nm
Zero order correlation coetficients of the environmental variables with mean reflectance at forehead.
--0.340
0.214
0'062
--0.251
--0.500
-0.697
0.947
465 nm
--0'177
0.332
Maximum humidity
--0-378
Maximum temperature
0-243
--0'460
Minimum temperature
Minimum humidity
--0.742
0'832
425 nm
Mean temperature
Latitude
Wavelength
Male and female samples
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N=39
--0.158
0.273
0-172
--0'219
--0"630
--0.628
0.8 I0
685 nm
e5
-W
Mean temp.
Latitude
Table 7.
N=I6
6.3
93 "5
N = 16
0"1
4.3
93.8
0.1
Mean temp. Min. temp.
N = 36
2.3 0.5
Min. humidity
8-7 5.7
Max, temp.
80.4
Max. humidity
Latitude
Wavelength 655 nm
Altitude
Min. temp.
Latitude
Wavelength 485 nm
N=27
3.7
0.8
1-3
1.8
88"1
N = 39
1.6 0.5
Mean temp.
1.8
6"3
6"1
4.1
65.6
Min. temp.
Min. humidity
Max. temp.
Altitude
Max. humidity
Latitude
Wavelength 685 nm
Max. humidity
Min. humidity
Max. temp,
Min. temp.
Latitude
Wavelength 515 nm
Min. humidity
Max. humidity
Mean temp.
Min. temp.
Max. temp.
Latitude
N=31
0-1
0"2
0.1
1"0
2"6
78"2
Wavelength 545 nm
Contribution to regression of various environmental variables on nine variables at forehead (both sexes).
N=16
2.0 0.2
Min. temp.
96.0
Altitude
Latitude
Wavelength 595 nm
Wavelength 575 nm
0.01
3.9
2.9
N=27
0.4
Max. temp.
Mean temp.
Max. temp.
0"8
2"7
89"7
N=37
0.5
Mean temp.
Min. humidity
1.4
0.7
Min. temp.
Min. humidity
Max. humidity
6"7
Altitude
Min. temp.
6.7
Max. humidity
Latitude
69.0
Wavelength 465 nm
Latitude
Wavelength 425 nm
Contribution to variance (per cent)
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t~
Oo
Environmental correlations o[ skin colour
19
The regression equations incorporating the effects of the first three variables are set out in table 8. In view of the sampling error attaching to such a small number of samples, the extraction of variates after the first becomes decreasingly reliable, and all that can safely be concluded is that all variables except latitude appear to exert a negligible effect. The multiple regression coefficients appear rather lower than for the upper inner arm site, but the smaller number of samples precludes direct comparison. Male and female samples Ann Hum Biol 1976.3:11-22. Downloaded from informahealthcare.com by McMaster University on 11/28/14. For personal use only.
Wavelength 425 nm
Y= --10.82+0-41 Latitude+0.20 Max. humidity +0.0008 Altitude
465 nm
Y = - - 7.72+0-70 Latitude+0-10 Min. temp.
+0.07
485 nm
Y=-
+0-0002 Altitude
8.13+0-91 Latitude+0-19 Min. temp.
(R2=0.826)
Max.humidity (R2=0.932) (R2=0.982)
515 nm
Y=
10-90+0.67Latitude+0.09 M;n. temp.
--0.107 Max. temp.
(R2=0'911)
5-',5 nm
Y=
18.!0+0.47 Latitude--0.11 Max. temp.
+0-05
(R2=0,817)
575 nm
Y=
34.71+0'50 Latitude--0.33 Mean temp.
595 nm
Y=
3' 16-t- 1-03 Latitude+0.13 Min. temp.
Min. temp.
(R-~=0,988) --0.0003 Altitude
655 nm
Y= --55-04+0.91 Latitude+0-50 Max. temp.
685 nm
Y = - - 3-24+0-74 Latitude+0-30 Max. humidity+0-001 Altitude
+0-28
(R~=0.982)
Max.humidity (R2=0.947) (R2=0.758)
Table 8. Regression equations: forehead percentage reflectance on the main environmental variables.
Site difference The difference between the shorter and longer wavelengths is brought out well by the examination of the difference between forehead and upper inner arm site though this unfortunately can be computed on only a small number of samples. For wavelengths 425-575nm, the zero-order correlation with latitude varies between 0.52 and 0-67. For wavelengths 595-685 nm, this diminishes to +0"03 to --0.25. For mean temperature, the values are respectively --0-43 to --0-60, and --0.13 to -t-0-16. For the four filters at which more than 30 samples are available (table 9) the contributions of various environmental factors to the regression are appreciably lower, and at the two longest wavelengths account for a very small proportion of the total variance. But too much weight should not be attached to this, for though this measure may perhaps be related to the amount of tanning to which the individual is subject, it may equally well be of little biological validity. 4. D i s c u s s i o n
Overall, for both sites and all wavelengths, these results indicate a remarkably close relationship of mean pigmentation with environmental variables. In particular, there is a dominating association of pigmentation with latitude, in that latitude accounts for a very great proportion of the total variance in the reflectance means observed in human populations. It is reasonable to argue therefore that some factor associated with latitude has a strong biological influence. Of the environmental variables associated with latitude, the amount of ultra-violet radiation received at the earth's surface varies inversely with latitude (Kendrew, 1938), while temperature is also strongly associated with latitude; the effect of B2
20
D. F. Roberts and D. P. S. Kahlon
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Wavelength 425 nm Latitude 37.7 Mean temp. 3.7 Min. humidity 3"1 Max. humidity 0"7 Max. temp. 0"7 Altitude 0-3 Min. temp. 0.1 N=37
Contribution to regression (per cent) Wavelength Wavelength 545 nm 655 nm Latitude Mean temp. Max. humidity Max. temp.
27.4 0.5 0"3 0"8
N=31
Latitude 6-0 Min. temp. 3-7 Min. humidity 1"5 Max. humidity 1.l Altitude 1.5 Max. temp. 3-3 Mean temp. 1-3 N=33
Wavelength 685 nm Mean temp. Max. temp. Min. humidity Min. temp. Latitude Max. humidity
2-4 1"3 2"7 2.2 1"2 0.4
N=39
Table 9. Contribution to regression of various environmental variables on site difference (upper inner arm-forehead), males and females. both at a given locality is, of course, modified by other variables such as cloudiness or altitude. Temperature is already taken into account in the analysis, and the latitude association of reflectance values is independent of it, as the partial correlation coefficients show (table 4). It seems most likely, therefore, that ultra-violet radiation is the factor responsible for the latitudinal association of pigmentation. Thermoregulation, also invoked as a selective mechanism for skin pigmentation, appears to occupy a subordinate role, though one that is measurable. For the upper inner arm, association with temperature, particularly maximum temperature, is at the shorter wavelengths; the next most important environmental variable independent of latitude, maximum humidity, though exerting an effect at the three longest wavelengths, is of relatively minor importance. These two variables are of prime relevance in thermoregulatory stress, and the findings may be taken perhaps to indicate some thermoregulatory importance in pigmentary variation. For the forehead similarly, the effect of the variables relevant to thermoregulation is relatively slight, particularly at the shorter wavelengths, by comparison with the association with latitude, again suggesting that thermoregulation is rather less important. The apparent contrast in the association pattern of upper inner arm reflectance readings at the three longest wavelengths to those at the shorter is striking. The difference between them is that at the lower wavelengths the effects of other factors besides melanin concentration are incorporated in the readings, whereas at the longer, particularly at 685nm, melanin concentration dominates. This finding suggests that the varying trends of association of the blue-green and red spectral regions with the environmental variables reflect the pattern of underlying pigmentary components, so that 'skin colour' should perhaps be regarded as a complex of entities of differing selective values rather than a single entity. The distinct contribution of maximum temperature apparent from tables 4 and 5 at the blue and green filters (425, 465, 515, 545 nm) may well reveal the role that temperature plays in variation of the content and flow of blood in regions of warmer climate. But why temperature should be replaced by humidity at the longer wavelengths, and why there should occur the distinctly decreased effect of latitude on the red filter (685 nm) compared to the others is not at all dear. Problems of sampling,
E n v i r o n m e n t a l correlations of skin colour
21
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particularly w h e n continental groups m a y be differentially r e p r e s e n t e d , is one possibility. H o w e v e r , t h e latitude c o n t r i b u t i o n at this w a v e l e n g t h still r e m a i n s very high, so that the e m p i r i c a l analysis of the reflectance data so far a v a i l a b l e supports the contention that there exists a causal relationship b e t w e e n melanin, the chief p i g m e n t a r y c o m p o n e n t in the red region, and latitude t h r o u g h its m a i n effect, ultra-violet radiation. B u t to validate these interpretations, collection of i n f o r m a t i o n on m a n y m o r e s a m p l e s is required so that the analysis can be r e p e a t e d within c o n t i n e n t a l groups of p o p u l a t i o n s as well as on a w o r l d w i d e species basis.
References (I) General Biasutti, R. (1953). Le razze e i popoli della terra. Torino: Unione Tipografice-editrice Torinese. Davenport, C. B. (1926). The skin colours of the races of mankind. Journal of the American Museum of Natural History, 26, 44-49. Fleure, H. J. (1945). The distribution of the types of skin colour. Geographical Review, 35, 580-95. Kendrew, W. G. (1938). Climate, 2nd edition. Oxford University Press. Weiner, L S. (1951). A spectrophotometer for measurement of skin colour. Man, 51, 152-3. (2) Sources o[ data Barnicot, N. A. (1958). Reflectometry of the skin in S. Nigerians and in some Mulattoes. Human Biology, 30, 150-160. Buchi, E. C. (1957-58). Eine spectrophotometrische Untersuchung der Hauffarbe yon Angehorien verschiedener Kasten in Bengalen. Bulletin der Schweizerische Gesellschafl fiir Anthropologie und Ethnoloeie, 34, 7-8. Conway, D. L., and Baker P. T. (1972). Skin reflectance of Quechua Indians, the effects of genetic admixture, sex and age. American Journal of Physical Anthropology, 36, 267-282. Das, S. R., and Mukherjee, D. P. (1963). A spectrophotometric skin colour survey among four Indian castes and tribes. Zeitschrifl fur Morphologie und Anthropologie, 54, 190-200. Diaz Ungria, A. G. de (1965). La pigmentation de la piel en las indigenas Gushibos. Homenaje a Juan Comas, Vol. 2, 63-82. Editorial libres de Mexico, S.A. Harrison, G. A., and Owen, J. J. T. (1964). Studies on the inheritance of human skin colour. Annals o[ Human Genetics, 28, 27-37. Harrison, G. A., and Salzano, F. M. (1966). The skin colour of the Caingang and Guarani Indians of Brazil. Human Biology, 38, 104-111. Huizinga, J. (1965). Reflectometry of the skin in Dogons. Proceedings, I(oninklijke Nederlandse Akademie van Wetenschappen, Series C, 68, 289-296. Hulse, F. S. (1967). Selection for skin colour among the Japanese. American Journal of Physical Anthropology, 27, 143-I 55. Hulse, F. S. (1970). Skin colour among the Yemenite Jews of the isolate from Habban. Proceedings of the Vlllth Congress of Anthropological and Ethnological Science, p. 226. Hulse, F. S. (1972). Skin colour in Northumberland. In Genetic Variation in Britain, eds. Roberts, D. F., and Sunderland, E. London: Taylor & Francis. Kalla, A. K. (1969). Affinities in skin pigmentation of some Indian populations. Human Heredity, 19, 499-505. Kalla, A. K. (1971). A study of age differences in skin pigmentation in males. Zinruigaku Zassi, 77, 246-253. Leguebe, A. (1961). Contribution ~t l'6tude de la pigmentation chez l'homme. Bulletin de l'Institut royale des sciences naturelles de Belgique, 37, 1-29. Ojikutu, R. O. (1965). Die Rolle yon Hautpigment und Schweibdrusen in der Klimaansparsung des Menschen, Homo, 16, 77. Omoto, K. (1968). Studies of skin colour among hybrid subjects in Japan with special reference to measurements of suntanning. Journal of the Anthropological Society of Nippon. 76, 122 34. Pollitzer, W. S., Namboodiri, K. K., EIston, R. C., Brown, W. H., and Leyshon, W. C. (1970). The Seminole Indians of Oklahoma: morphology and serology. American Journal of Physical Anthropology, 33, 15-30. Smith, J., and Mitchell, R. J. 0972). Skin colour studies in South Wales, the Isle of Man and Cumbria. In Genetic Variation in Britain, eds. Roberts, D. F., and Sunderland, E. London: Taylor & Francis.
Ann Hum Biol 1976.3:11-22. Downloaded from informahealthcare.com by McMaster University on 11/28/14. For personal use only.
22
D. F. R o b e r t s and D. P. S. K a h l o n
Sunderland, E. (1967). Skin colour of the people of Azraq, Eastern Jordan. Human Biology, 39, 65-70. Tiwari, S. C. (1963). Studies of crossing between Indians and Europeans. Annals of Human Genetics, 26~ 219-227. Tiwari, S. C., and Kalla, A. K. (1968). Dimorphism in the effect of adolescence on skin pigmentation. The Anthropologist, Sp. vol. 169. Tobias, P. V. (1961). Studies on skin reflectance in Bushman-European hybrids. Proceedings of the Second International Congress o/ Human Genetics, 14~ 401. Tournel, J. V. (1965). Pigmentation de la peau de Belges et d'Africains. Bulletin de la Socidtd royale Beige d'Anthropologie et de Prdhistoire, 76~ 79-96. Walsh, R. J. (1963). Variations of melani.n pigmentation of the skin in some Asian and Pacific peoples. Journal of the Royal Anthropological Institute, 93, 126-133. Wasserman, H. P., and Heyl, T. (1968). Quantitative data on skin pigmentation in South African races. South African Medical Journal, 42, 98-101. Weiner, J. S., Sebag-Montefiore, N. C., and Peterson, J. N. (1963). A note on the skin colour of Aguarana Indians of Peru. Human Biology, 35, 470-473. Weiner, J. S., Harrison, G. A., Singer, R., Harris, R., and Jopp, W. (1964). Skin colour in southern Africans. Human Biology, 36, 294-307. Weninger, M. (1969). Spektrophotometrische Untersuchungen der Haut an einen Bantu-stamm (Chope) aus Mozambique. An'hropologie, 7, 53-8. Address correspondence to: Dr. D. F. Roberts, Department of Human Genetics, University of Newcastle upon Tyne, 19 Claremont Place, Newcastle upon Tyne NE2 4AA. Zusammenfassung. Hautfarbendaten, gemessen mit dem Reflex-Photospektrometer an Eigenborenen-Bevtilkerungen, werden mit den Umweltvariablen Breitengrad, Temperatur, Luftfeuchte und H6he ti. M. verglichen. Die Verbindung mit dem Breitengrad dominiert in allen Wellenliingen. Die Temperaturen zeigen eine kleine, aber klare Beziehung bei den kiirzeren Wellen, die Feuchtedaten bei tiber 595 nm. Ober 80 prozent der Varianz zwischen den Bevtilkerungen gehen auf diese Variablen zurtick, bei den ktirzeren Wellenl~ingen sogar mehr. Es wird vorgeschlage_n, dass die Hautfarbe als ein Komplex verschiedener Selektivwerte aufgefasst werden sollte und nicht als Einheit. R6sum~. Les donnEes sur la couleur de la peau obtenues par spectrophotom6trie de la reflectance sont mises au regard de variables de l'environnement: latitude, temp6rature, humidit6 et altitude. L'association ~t la latitude pr6domine ~t toutes les longueurs d'onde. La temp6rature montre une association faible mais appr6ciable aux longueurs d'ondes les plus courtes, rhumidit6 aux longueurs d'ondes sup6rieures ~ 595 nm. Plus de 80 per cent de la variance totale interpopulationnelle h chaque longueur d'onde est assur6e par ces variables, davantage encore aux longueurs d'ondes les plus courtes. 11 est sugg6r6 que la couleur de la peau devrait 8tre consid4rEe comme un complexe d'entit6s h valeurs s61ectives diff6rentes plut6t que comme une entit6 unique.
11-22
Environmental correlations of skin colour D. F. R O B E R T S and D. P. S. K A H L O N Department of Human Genetics, University of Newcastle upon Tyne
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[Received 2 October 1974; revised 14 January 1975] Summary. Skin colour data obtained by reflectance spectrophotometry on indigenous populations are compared with environmental variables--latitude, temperature, humidity and altitude. Association with latitude predominates at all wavelengths. Temperatures show a small but appreciable association at the shorter wavelengths, humidity at wavelengths above 595 nm. Over 80 per cent of the total interpopulation variance at each wavelength is accounted for by these variables, more at the shorter wavelengths. It is suggested that skin colour should be regarded as a complex of entities of differing selective values rather than a single entity.
1. Introduction The general regularity of geographical distribution of pigmentation in the world's populations has been repeatedly pointed out. This observation, however, has tended primarily to stimulate speculation rather than to provide any solid analysis of the selective role of pigmentation. Partly this has been due to the absence of objective and quantitative records, and today such studies as that of Davenport (1926) or of Biasutti (1953) are of little more than historical interest. Fleure's (1945) discussion drew attention to obvious exceptions to the pigmentation gradient, but his attempts to explain them too, in the absence of agreement on the selective function of pigmentation, essentially remain mere speculation. It is over 20 years since Weiner (1951) described a portable reflectance spectrophotometer (EEL), suitable for field work. This made it possible to obtain, outside the confines of a laboratory, objective and accurate measures of skin colour on human populations in their native environments in the form of reflectance measures at standard wavelengths, and there are already sufficient data on different populations for some attempt to be made at their analysis in relation to geographical variables. To examine the relative strengths of association of mean reflectance readings at different wavelengths with different environmental variables is the object of the present study. 2. Data Although some 120 samples, approximately 60 of each sex, are available to represent the world's indigenous populations, results are not all technically comparable. Most were obtained with the E E L spectrophotometer, and some others with the photovolt model where the wavelengths used are not identical. For only a proportion of the samples are the data given at all nine wavelengths available
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12
D. F. Roberts and D. P. S. Kahlon
on the E E L instrument (425, 465, 485, 515, 545, 575, 595, 655, 685 nm). The skin sites examined are not always the same. The samples vary in size, but for present purposes none was included that contained fewer than 15 subjects. The samples included here all relate to what may be regarded as indigenous populations, and no sample of recent migrants to their present habitat was included; for example, the South American samples relate only to South American Indians and not to whites. All the samples relate to adults, though the inclusion of sub-adult individuals in so-called adult samples may affect the variability. A further possible source of error comes in the climatic variables assigned to each sample. Each sample was located sufficiently precisely for values of the following variables to be assigned to it: latitude, mean annual temperature, mean maximum temperature (average of the highest each year), mean minimum temperature (average of the lowest each year), maximum midday humidity (highest monthly mean), minimum midday humidity (lowest monthly mean), and altitude. T o assign the climatic variables, the figures given in the Meteorological Office Tables (1958-1967) were used, with interpolation between adjacent stations where necessary, taking into account local topographic variation.
Latitude Mean temperature Maximum temperature Minimum humidity Maximum humidity Altitude
Mean temp. (*F) --0.720
Max. temp. ( oF) --0-183 0.700
Min. humidity (per cen0 0.073 --0"212 --0"569
Max. humidity (per cen0 0.089 0"012 --0-235 0-720
Altitude (fee0 --0-363 "0"274 --0-480 --0" 184 --0-401
Min. temp. ( *F) --0-659 0"569 0-082 0-186 0-149 0-002
Table 1. Zero-order correlation coefficients between environmental variables (61 localities). The geographic variables are, of course, not independent of each other. Table 1 shows the zero order correlations of the environmental variables as assigned to all the populations now studied. From these there appear as expected the obvious influence of latitude on temperature; the close correlation of mean temperature with maximum temperature and minimum temperature; similar close correlation with each other of minimum and maximum humidities, while altitude shows no high correlation with any of the others. On account of these intercorrelations, the following analysis takes the form of an examination of zero-order correlation coefficients of reflectance readings with each environmental variable to show the general associations, and subsequent examination by stepwise regression in an endeavour to identify the order in which these environmental variables contribute to the variation in skin pigmentation. 3. Results Upper inner arm The first examination of the reflectance at the upper inner arm incorporated all adequate indigenous samples both male and female; most samples (77) were available at wavelength 685 nm, fewest (32) at wavelengths 485 and 655 nm. For
N = 21
0'920 --0.869 --0-576 --0.574 0-540 0'426 --0-328
485 nm
N=32
0"938 --0.913 --0"616 --0"687 0-623 0'506 --0.372
485 nm
N = 29
0"886 --0.757 --0.538 --0.444 0.312 0.245 --0.231
515 nm
Male samples only
N=45
0-907 --0.797 --0"575 --0'511 0"317 0.284 --0'255
515 nm
N = 34
0.873 --0.809 --0.563 --0'550 0.372 0'250 --0.237
545 nm
N--=55
0.880 --0"836 --0.596 --0"616 0-400 0"318 --0.265
545 nm
N=21
0.906 --0.843 --0.625 --0.559 0.536 0.408 --0-332
575 nm
N=32
0.928 --0.898 --0.657 --0.680 0.622 0.478 --0.371
575 nm
N = 23
0"904 --0"838 --0"629 --0-533 0.507 0'367 --0'323
595 nm
N=36
0"928 --0"898 --0"658 --0'654 0'590 0'446 --0.347
595 nm
N = 34
0.843 --0.504 --0.564 --0'017 0.087 0.204 --0"383
655 nm
N=51
0-889 --0.598 --0"599 --0-101 0.103 0.210 --0-405
655 nm
Zero order correlation coefficients of the environmental variables with mean reflectance at upper inner arm.
N=29
N = 37
Table 2.
0-910 --0"756 --0.533 --0.414 0.233 0"221 --0-261
0.885 --0.898 --0'503 --0"507 0.315 4'311 --0"209
Latitude Mean temperature Minimum temperature Maximum temperature Minimum humidity Maximum humidity Altitude
465 nm
425 nm
N=45
N=61
Wavelength:
0-932 --0'795 --0'581 --0'481 0.239 0.264 --0.285
0.895 --0'824 --0"533 --0-556 0.347 0.403 --0"238
Latitude Mean temperature Minimum temperature Maximum temperature Minimum humidity Maximum humidity Altitude
465 nm
425 nm
Wavelength :
Male and female samples
Ann Hum Biol 1976.3:11-22. Downloaded from informahealthcare.com by McMaster University on 11/28/14. For personal use only.
N = 46
0'825 --0"652 --0"585 --0"346 0'231 0"257 --0"220
685 nm
N=77
0"835 --0"681 --0.590 --0"400 0.281 0.331 --0"236
685 nm
U,
~' "~
~-
e~
15.9
3"3 0"1
Max. temp.
Altitude
Table 3.
11"1
Mean temp.
Max. humidity
Latitude
N=36
13.3
86.2
N=32
11"7
88.0
0"6 0.1
Mean temp. Min. temp.
N=51
1.7 0"9
Max. temp.
79.0
Max. humidity
Latitude
Wavelength 655 nm
Mean temp.
Latitude
Wavelength 485 nm
0"I
Mean temp.
Min. humidity
0.2 N=77
1.2 0.7
Mean temp.
1-6
4"0
6"6
69"7
Altitude
Min. temp.
Max. temp.
Max. humidity
Latitude
Wavelength 685 nm
N=45
0.6 0.5
Max. humidity Min. temp.
2"6 0"9
12.3
82.3
Altitude Min. humidity
Max. temp.
Latitude
Wavelength 515 nm
Min. humidity
Max. temp.
Latitude
N=55
0"8
21.4
77"4
Wavelength 545 nm
Contribution to regression of various environmental variables on nine variables at upper inner arm (male and female samples).
N=32
86.0
Latitude
Wavelength 595 nm
Wavelength 575 nm
2"7 0"3
9.9
86.9
N=45
Min. temp.
Altitude
Max. temp.
Latitude
Wavelength 465 nm
N=61
Min. temp.
80-1
Latitude
Wavelength 425 nm
Contributions to variance (per cent)
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"~
.~
4~
Ann Hum Biol 1976.3:11-22. Downloaded from informahealthcare.com by McMaster University on 11/28/14. For personal use only.
Environmental correlations o[ skin colour
15
all wavelengths, the zero order correlations suggest that association with latitude predominates (table 2), accounting for between 88 per cent (485 nm) and 70 per cent (685 nm) of the total variance in mean reflectance. The second important variable appears to be mean temperature, which, considered alone, would account for between 83 per cent (485 nm) and 36 per cent (655 nm) of the total variance. The order of the remaining variables differs from wavelength to wavelength, but in general the extreme temperature readings come next, then the humidity, while altitude shows the lowest correlations. Associations of reflectance with latitude and the humidities are consistently positive, with temperatures and altitude negative. These zero order correlation coefficients considered alone demonstrate the strength of the association with geographical variables, reflectance increasing with increasing latitude, decreasing temperature and increasing humidity. In part, these overall associations are, of course, attributable to the intercorrelation of the geographical variables. When these are taken into account in order of importance (i.e. proportion (R 2) of the total variance accounted for) by stepwise regression, the order of the variables is therefore different (table 3). Latitude alone (since it predominates in the zero order analysis) accounts for between 77 and 88 per cent of the total variance for all wavelengths except 685 nm where it accounts for 70 per cent. The addition as an independent second variable of maximum temperature at wavelengths 425, 465, 515 and 545 nm accounts for an additional 10-21 per cent, or of mean temperature at wavelengths 485 and 575 nm for 11 per cent. At the three longer wavelengths maximum humidity moves up into second place, accounting for some 2-13 per cent. The only other variable to produce an increase of R: as great as 1 per cent at any of the first eight wavelengths is altitude (2-3 per cent at 425, 465 and 515 nm) while at wavelength 685 nm all three temperature variables make contributions of 1-4 per cent. Selection of latitude as the predominant variable and the subordinate roles of the others is clearly demonstrated by examination of the first-order partial correlations at the four wavelengths for which there are most samples (table 4). The correlation with latitude diminishes very slightly, if at all, after exclusion of the effects of maximum temperature or maximum humidity, diminishes rather more but still remains appreciable after the exclusion of mean temperature. Clearly the latitudinal correlation owes little to these three variables. Excluding the effects of latitude, the correlations with mean temperature diminish very markedly, those with maximum temperature or maximum humidity rather less. The stepwise analysis was repeated employing only male samples. Fewer samples were available at each wavelength (table 2) so there is some slight
Correlation with latitude excluding max. temp. Correlation with latitude excluding mean temp. Correlation with latitude excluding max. humidity Correlation with max. temp. excluding latitude Correlation with mean temp. excluding latitude Correlation with max. humidity excluding latitude
425 nm
Wavelength 545 nm 655 nm
685 nm
0-821 0.628 0.867 --0'406 --0-372 0-326
0.794 0.578 0-858 --0.470 --0.420 0"242
0"788 0.716 0-812 --0.256 --0-166 0"259
0-901 0.950 0"877 0-063 0-086 0.132
Table 4. Partial correlations (first order) of pzrcentage reflectance at upper inner arm (both sexes).
D. F. Roberts and D. P. S. Kahlon
16
Ann Hum Biol 1976.3:11-22. Downloaded from informahealthcare.com by McMaster University on 11/28/14. For personal use only.
difference in the subordinate variables and the multiple correlation coefficients tend to be slightly lower, but the substantive results are similar to those for the combined sexes. It appears, therefore, that the overall associations owe little to differential sex representation. The regression equations of reflectance on the relevant environmental variables at each wavelength are set out in table 5 for male and female samples. The proportion of the total variance for which these account is indeed remarkable. Only for the two longest wavelengths does this drop below 97 per cent, though exact comparison is precluded by the different number of samples available at each wavelength. Male and female samples Wavelength 425 nm
Y=
59-69+0-46 Latitude--0-51 Max. temp.
--0.0009 Altitude
(R~=0.994)
465 nm 485 nm
Y= Y=
59"74+0"64Latitude--0-53 Max. temp. 53.31+0"55 Latitude--0.62 Mean temp.
--0"001 Altitude
(Ra=0.995) (R2=0-998)
515 nm
Y=
67.93+0.63 Latitude--0.59 Max. temp.
--0"001 Altitude
(R2=0.972)
545 nm
Y=
50.22+0-57 Latitude--0.42 Max. temp.
+0-06
54.83+0-52 Latitude--0.57 Mean temp.
575 nm
Y=
595 nm
Y= --16.55+0"88 Latitude+0-42 Max. humidity
655 nm 685 nm
Y = - - 7.78+0"90 Latitude+0"18 Max. humidity+0"14 Y= 39.26+0-67 Latitude+0.21 Max. humidity--0-26
Table 5. Regression equations:
Min. humidity (R~=0-996) (R~=0.971) (RZ=0.995) Max. temp. Max. temp.
(RZ=0.816) (R~=0.803)
upper inner arm percentage reflectance on the main environmental variables.
There is no doubt about the dominating influence of the latitudinal associations, nor is there any doubt of the appreciable independent contribution of maxim u m or mean temperature at wavelengths 425 nm to 575 nm. Humidity makes a negligible contribution to the variance at wavelengths below 595 nm, the effect being restricted to m a x i m u m humidity at the three longest wavelengths. The remaining environmental variables make negligible contributions. A particularly interesting feature is the contrast between shorter and longer wavelengths in the importance of temperature and humidity.
Forehead Data are less numerous for forehead readings, there being only 39 samples (both sexes) at wavelength 685 nm, fewer at all other wavelengths. The zero-order correlation coefficients (table 6) are generally similar to but lower than those on the upper inner arm. Latitude is again dominant, accounting for 66-96 per cent of the total variance, followed by mean temperature. Again the associations with latitude and humidity are positive, and negative with temperature and altitude. Taking into account the intercorrelations of the geographical variables (table 7), latitude exerts the m o s t important effect at all wavelengths (65-94 per cent of the v a r i a n c e ) f o l l o w e d by m a x i m u m humidity at 425 and 685 nrn or a temperature variable at the other wavelengths. The contribution of this second variable is in general less than it w a s at the upper inner arm site.
Altitude
Table 6.
N=27
N = 37
N = 16
--0.385
0.775
0-587
--0'553
--0.482
--0.901
0.969
485 nm
N = 27
--0'300
0"209
0'102
--0'290
--0'519
--0"723
0"939
515 nm
N=31
--0.261
0.160
0'179
--0.321
--0"503
--0.724
0.884
545 nm
N = 16
--0.432
0.763
0'566
--0-511
--0.531
--0.870
0"967
575 nm
N = 16
--0.438
0"746
0"564
-0.507
--0"539
--0.879
0.980
595 nm
N = 36
--0'461
0-245
--0'041
0-125
--0'563
--0.460
0.897
655 nm
Zero order correlation coetficients of the environmental variables with mean reflectance at forehead.
--0.340
0.214
0'062
--0.251
--0.500
-0.697
0.947
465 nm
--0'177
0.332
Maximum humidity
--0-378
Maximum temperature
0-243
--0'460
Minimum temperature
Minimum humidity
--0.742
0'832
425 nm
Mean temperature
Latitude
Wavelength
Male and female samples
Ann Hum Biol 1976.3:11-22. Downloaded from informahealthcare.com by McMaster University on 11/28/14. For personal use only.
N=39
--0.158
0.273
0-172
--0'219
--0"630
--0.628
0.8 I0
685 nm
e5
-W
Mean temp.
Latitude
Table 7.
N=I6
6.3
93 "5
N = 16
0"1
4.3
93.8
0.1
Mean temp. Min. temp.
N = 36
2.3 0.5
Min. humidity
8-7 5.7
Max, temp.
80.4
Max. humidity
Latitude
Wavelength 655 nm
Altitude
Min. temp.
Latitude
Wavelength 485 nm
N=27
3.7
0.8
1-3
1.8
88"1
N = 39
1.6 0.5
Mean temp.
1.8
6"3
6"1
4.1
65.6
Min. temp.
Min. humidity
Max. temp.
Altitude
Max. humidity
Latitude
Wavelength 685 nm
Max. humidity
Min. humidity
Max. temp,
Min. temp.
Latitude
Wavelength 515 nm
Min. humidity
Max. humidity
Mean temp.
Min. temp.
Max. temp.
Latitude
N=31
0-1
0"2
0.1
1"0
2"6
78"2
Wavelength 545 nm
Contribution to regression of various environmental variables on nine variables at forehead (both sexes).
N=16
2.0 0.2
Min. temp.
96.0
Altitude
Latitude
Wavelength 595 nm
Wavelength 575 nm
0.01
3.9
2.9
N=27
0.4
Max. temp.
Mean temp.
Max. temp.
0"8
2"7
89"7
N=37
0.5
Mean temp.
Min. humidity
1.4
0.7
Min. temp.
Min. humidity
Max. humidity
6"7
Altitude
Min. temp.
6.7
Max. humidity
Latitude
69.0
Wavelength 465 nm
Latitude
Wavelength 425 nm
Contribution to variance (per cent)
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t~
Oo
Environmental correlations o[ skin colour
19
The regression equations incorporating the effects of the first three variables are set out in table 8. In view of the sampling error attaching to such a small number of samples, the extraction of variates after the first becomes decreasingly reliable, and all that can safely be concluded is that all variables except latitude appear to exert a negligible effect. The multiple regression coefficients appear rather lower than for the upper inner arm site, but the smaller number of samples precludes direct comparison. Male and female samples Ann Hum Biol 1976.3:11-22. Downloaded from informahealthcare.com by McMaster University on 11/28/14. For personal use only.
Wavelength 425 nm
Y= --10.82+0-41 Latitude+0.20 Max. humidity +0.0008 Altitude
465 nm
Y = - - 7.72+0-70 Latitude+0-10 Min. temp.
+0.07
485 nm
Y=-
+0-0002 Altitude
8.13+0-91 Latitude+0-19 Min. temp.
(R2=0.826)
Max.humidity (R2=0.932) (R2=0.982)
515 nm
Y=
10-90+0.67Latitude+0.09 M;n. temp.
--0.107 Max. temp.
(R2=0'911)
5-',5 nm
Y=
18.!0+0.47 Latitude--0.11 Max. temp.
+0-05
(R2=0,817)
575 nm
Y=
34.71+0'50 Latitude--0.33 Mean temp.
595 nm
Y=
3' 16-t- 1-03 Latitude+0.13 Min. temp.
Min. temp.
(R-~=0,988) --0.0003 Altitude
655 nm
Y= --55-04+0.91 Latitude+0-50 Max. temp.
685 nm
Y = - - 3-24+0-74 Latitude+0-30 Max. humidity+0-001 Altitude
+0-28
(R~=0.982)
Max.humidity (R2=0.947) (R2=0.758)
Table 8. Regression equations: forehead percentage reflectance on the main environmental variables.
Site difference The difference between the shorter and longer wavelengths is brought out well by the examination of the difference between forehead and upper inner arm site though this unfortunately can be computed on only a small number of samples. For wavelengths 425-575nm, the zero-order correlation with latitude varies between 0.52 and 0-67. For wavelengths 595-685 nm, this diminishes to +0"03 to --0.25. For mean temperature, the values are respectively --0-43 to --0-60, and --0.13 to -t-0-16. For the four filters at which more than 30 samples are available (table 9) the contributions of various environmental factors to the regression are appreciably lower, and at the two longest wavelengths account for a very small proportion of the total variance. But too much weight should not be attached to this, for though this measure may perhaps be related to the amount of tanning to which the individual is subject, it may equally well be of little biological validity. 4. D i s c u s s i o n
Overall, for both sites and all wavelengths, these results indicate a remarkably close relationship of mean pigmentation with environmental variables. In particular, there is a dominating association of pigmentation with latitude, in that latitude accounts for a very great proportion of the total variance in the reflectance means observed in human populations. It is reasonable to argue therefore that some factor associated with latitude has a strong biological influence. Of the environmental variables associated with latitude, the amount of ultra-violet radiation received at the earth's surface varies inversely with latitude (Kendrew, 1938), while temperature is also strongly associated with latitude; the effect of B2
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D. F. Roberts and D. P. S. Kahlon
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Wavelength 425 nm Latitude 37.7 Mean temp. 3.7 Min. humidity 3"1 Max. humidity 0"7 Max. temp. 0"7 Altitude 0-3 Min. temp. 0.1 N=37
Contribution to regression (per cent) Wavelength Wavelength 545 nm 655 nm Latitude Mean temp. Max. humidity Max. temp.
27.4 0.5 0"3 0"8
N=31
Latitude 6-0 Min. temp. 3-7 Min. humidity 1"5 Max. humidity 1.l Altitude 1.5 Max. temp. 3-3 Mean temp. 1-3 N=33
Wavelength 685 nm Mean temp. Max. temp. Min. humidity Min. temp. Latitude Max. humidity
2-4 1"3 2"7 2.2 1"2 0.4
N=39
Table 9. Contribution to regression of various environmental variables on site difference (upper inner arm-forehead), males and females. both at a given locality is, of course, modified by other variables such as cloudiness or altitude. Temperature is already taken into account in the analysis, and the latitude association of reflectance values is independent of it, as the partial correlation coefficients show (table 4). It seems most likely, therefore, that ultra-violet radiation is the factor responsible for the latitudinal association of pigmentation. Thermoregulation, also invoked as a selective mechanism for skin pigmentation, appears to occupy a subordinate role, though one that is measurable. For the upper inner arm, association with temperature, particularly maximum temperature, is at the shorter wavelengths; the next most important environmental variable independent of latitude, maximum humidity, though exerting an effect at the three longest wavelengths, is of relatively minor importance. These two variables are of prime relevance in thermoregulatory stress, and the findings may be taken perhaps to indicate some thermoregulatory importance in pigmentary variation. For the forehead similarly, the effect of the variables relevant to thermoregulation is relatively slight, particularly at the shorter wavelengths, by comparison with the association with latitude, again suggesting that thermoregulation is rather less important. The apparent contrast in the association pattern of upper inner arm reflectance readings at the three longest wavelengths to those at the shorter is striking. The difference between them is that at the lower wavelengths the effects of other factors besides melanin concentration are incorporated in the readings, whereas at the longer, particularly at 685nm, melanin concentration dominates. This finding suggests that the varying trends of association of the blue-green and red spectral regions with the environmental variables reflect the pattern of underlying pigmentary components, so that 'skin colour' should perhaps be regarded as a complex of entities of differing selective values rather than a single entity. The distinct contribution of maximum temperature apparent from tables 4 and 5 at the blue and green filters (425, 465, 515, 545 nm) may well reveal the role that temperature plays in variation of the content and flow of blood in regions of warmer climate. But why temperature should be replaced by humidity at the longer wavelengths, and why there should occur the distinctly decreased effect of latitude on the red filter (685 nm) compared to the others is not at all dear. Problems of sampling,
E n v i r o n m e n t a l correlations of skin colour
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particularly w h e n continental groups m a y be differentially r e p r e s e n t e d , is one possibility. H o w e v e r , t h e latitude c o n t r i b u t i o n at this w a v e l e n g t h still r e m a i n s very high, so that the e m p i r i c a l analysis of the reflectance data so far a v a i l a b l e supports the contention that there exists a causal relationship b e t w e e n melanin, the chief p i g m e n t a r y c o m p o n e n t in the red region, and latitude t h r o u g h its m a i n effect, ultra-violet radiation. B u t to validate these interpretations, collection of i n f o r m a t i o n on m a n y m o r e s a m p l e s is required so that the analysis can be r e p e a t e d within c o n t i n e n t a l groups of p o p u l a t i o n s as well as on a w o r l d w i d e species basis.
References (I) General Biasutti, R. (1953). Le razze e i popoli della terra. Torino: Unione Tipografice-editrice Torinese. Davenport, C. B. (1926). The skin colours of the races of mankind. Journal of the American Museum of Natural History, 26, 44-49. Fleure, H. J. (1945). The distribution of the types of skin colour. Geographical Review, 35, 580-95. Kendrew, W. G. (1938). Climate, 2nd edition. Oxford University Press. Weiner, L S. (1951). A spectrophotometer for measurement of skin colour. Man, 51, 152-3. (2) Sources o[ data Barnicot, N. A. (1958). Reflectometry of the skin in S. Nigerians and in some Mulattoes. Human Biology, 30, 150-160. Buchi, E. C. (1957-58). Eine spectrophotometrische Untersuchung der Hauffarbe yon Angehorien verschiedener Kasten in Bengalen. Bulletin der Schweizerische Gesellschafl fiir Anthropologie und Ethnoloeie, 34, 7-8. Conway, D. L., and Baker P. T. (1972). Skin reflectance of Quechua Indians, the effects of genetic admixture, sex and age. American Journal of Physical Anthropology, 36, 267-282. Das, S. R., and Mukherjee, D. P. (1963). A spectrophotometric skin colour survey among four Indian castes and tribes. Zeitschrifl fur Morphologie und Anthropologie, 54, 190-200. Diaz Ungria, A. G. de (1965). La pigmentation de la piel en las indigenas Gushibos. Homenaje a Juan Comas, Vol. 2, 63-82. Editorial libres de Mexico, S.A. Harrison, G. A., and Owen, J. J. T. (1964). Studies on the inheritance of human skin colour. Annals o[ Human Genetics, 28, 27-37. Harrison, G. A., and Salzano, F. M. (1966). The skin colour of the Caingang and Guarani Indians of Brazil. Human Biology, 38, 104-111. Huizinga, J. (1965). Reflectometry of the skin in Dogons. Proceedings, I(oninklijke Nederlandse Akademie van Wetenschappen, Series C, 68, 289-296. Hulse, F. S. (1967). Selection for skin colour among the Japanese. American Journal of Physical Anthropology, 27, 143-I 55. Hulse, F. S. (1970). Skin colour among the Yemenite Jews of the isolate from Habban. Proceedings of the Vlllth Congress of Anthropological and Ethnological Science, p. 226. Hulse, F. S. (1972). Skin colour in Northumberland. In Genetic Variation in Britain, eds. Roberts, D. F., and Sunderland, E. London: Taylor & Francis. Kalla, A. K. (1969). Affinities in skin pigmentation of some Indian populations. Human Heredity, 19, 499-505. Kalla, A. K. (1971). A study of age differences in skin pigmentation in males. Zinruigaku Zassi, 77, 246-253. Leguebe, A. (1961). Contribution ~t l'6tude de la pigmentation chez l'homme. Bulletin de l'Institut royale des sciences naturelles de Belgique, 37, 1-29. Ojikutu, R. O. (1965). Die Rolle yon Hautpigment und Schweibdrusen in der Klimaansparsung des Menschen, Homo, 16, 77. Omoto, K. (1968). Studies of skin colour among hybrid subjects in Japan with special reference to measurements of suntanning. Journal of the Anthropological Society of Nippon. 76, 122 34. Pollitzer, W. S., Namboodiri, K. K., EIston, R. C., Brown, W. H., and Leyshon, W. C. (1970). The Seminole Indians of Oklahoma: morphology and serology. American Journal of Physical Anthropology, 33, 15-30. Smith, J., and Mitchell, R. J. 0972). Skin colour studies in South Wales, the Isle of Man and Cumbria. In Genetic Variation in Britain, eds. Roberts, D. F., and Sunderland, E. London: Taylor & Francis.
Ann Hum Biol 1976.3:11-22. Downloaded from informahealthcare.com by McMaster University on 11/28/14. For personal use only.
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D. F. R o b e r t s and D. P. S. K a h l o n
Sunderland, E. (1967). Skin colour of the people of Azraq, Eastern Jordan. Human Biology, 39, 65-70. Tiwari, S. C. (1963). Studies of crossing between Indians and Europeans. Annals of Human Genetics, 26~ 219-227. Tiwari, S. C., and Kalla, A. K. (1968). Dimorphism in the effect of adolescence on skin pigmentation. The Anthropologist, Sp. vol. 169. Tobias, P. V. (1961). Studies on skin reflectance in Bushman-European hybrids. Proceedings of the Second International Congress o/ Human Genetics, 14~ 401. Tournel, J. V. (1965). Pigmentation de la peau de Belges et d'Africains. Bulletin de la Socidtd royale Beige d'Anthropologie et de Prdhistoire, 76~ 79-96. Walsh, R. J. (1963). Variations of melani.n pigmentation of the skin in some Asian and Pacific peoples. Journal of the Royal Anthropological Institute, 93, 126-133. Wasserman, H. P., and Heyl, T. (1968). Quantitative data on skin pigmentation in South African races. South African Medical Journal, 42, 98-101. Weiner, J. S., Sebag-Montefiore, N. C., and Peterson, J. N. (1963). A note on the skin colour of Aguarana Indians of Peru. Human Biology, 35, 470-473. Weiner, J. S., Harrison, G. A., Singer, R., Harris, R., and Jopp, W. (1964). Skin colour in southern Africans. Human Biology, 36, 294-307. Weninger, M. (1969). Spektrophotometrische Untersuchungen der Haut an einen Bantu-stamm (Chope) aus Mozambique. An'hropologie, 7, 53-8. Address correspondence to: Dr. D. F. Roberts, Department of Human Genetics, University of Newcastle upon Tyne, 19 Claremont Place, Newcastle upon Tyne NE2 4AA. Zusammenfassung. Hautfarbendaten, gemessen mit dem Reflex-Photospektrometer an Eigenborenen-Bevtilkerungen, werden mit den Umweltvariablen Breitengrad, Temperatur, Luftfeuchte und H6he ti. M. verglichen. Die Verbindung mit dem Breitengrad dominiert in allen Wellenliingen. Die Temperaturen zeigen eine kleine, aber klare Beziehung bei den kiirzeren Wellen, die Feuchtedaten bei tiber 595 nm. Ober 80 prozent der Varianz zwischen den Bevtilkerungen gehen auf diese Variablen zurtick, bei den ktirzeren Wellenl~ingen sogar mehr. Es wird vorgeschlage_n, dass die Hautfarbe als ein Komplex verschiedener Selektivwerte aufgefasst werden sollte und nicht als Einheit. R6sum~. Les donnEes sur la couleur de la peau obtenues par spectrophotom6trie de la reflectance sont mises au regard de variables de l'environnement: latitude, temp6rature, humidit6 et altitude. L'association ~t la latitude pr6domine ~t toutes les longueurs d'onde. La temp6rature montre une association faible mais appr6ciable aux longueurs d'ondes les plus courtes, rhumidit6 aux longueurs d'ondes sup6rieures ~ 595 nm. Plus de 80 per cent de la variance totale interpopulationnelle h chaque longueur d'onde est assur6e par ces variables, davantage encore aux longueurs d'ondes les plus courtes. 11 est sugg6r6 que la couleur de la peau devrait 8tre consid4rEe comme un complexe d'entit6s h valeurs s61ectives diff6rentes plut6t que comme une entit6 unique.
