Beard vs. Forehead, Ten Years Later

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(Hamilton 1942; Setty 1970; Ebling and Rook 1979). Common .... We wish to thank Ms. A Fleury for kindly improving the English manuscript. This work was ...
Am. J. Human Biol. 2000, 12: 460-464.

Beard vs. Forehead, Ten Years Later Michel Cabanac1 and Heiner Brinnel2 1Department

of Physiology Faculty of Medicine Laval University Québec G1K 7P4 Canada [email protected] 2Department

of Medicine Hopital de l'Arbresle 69210 L'Arbresle France [email protected]

Number of pages: 16 Abbreviated title: Beard vs. forehead

Correspondence to: Michel Cabanac (address above), tel: 418-656-3068, fax: 418656-7898, email: [email protected]

KEY WORDS: ageing

baldness

selective brain cooling

radiator theory

2 ABSTRACT Ten years ago (Cabanac and Brinnel 1988), we observed in a sample of 100 men that the area of the glabrous skin above the eyebrows was proportional to the area of the hairy skin on the cheeks, lips and chin. Ten to eleven years later we measured again 39 of the former subjects to check longitudinally whether the relationship would still be valid. In the group of 39 subjects the correlation was again significant and the regression was practically the same as that obtained on the same sample ten years earlier. This would tend to show that ontogeny follows phylogeny. This result is understood as indirect evidence in favor of selective brain cooling in humans.

In 1988 we reported that, in a sample of 100 men, the area of the glabrous skin above the eyebrows was proportional to the area of the hairy skin on the cheeks, lips, and chin (Cabanac and Brinnel 1988). In addition, the area of the forehead of the ten least-bearded men was equal to that of a group of ten women. These results were considered from a thermoregulatory point of view.

The

hypothesis tested was that the presence of hair on the cheeks and chin hinders heat loss from the head; a large forehead was understood as a compensation for the increased thermal insulation on the lower part of the face. The measurement of sweat secretion on the various parts of the body tended to confirm this hypothesis. The skin of the scalp and the cheek sweated less when hairy than when glabrous Cabanac and Brinnel 1988). Our initial study was a cross-sectional measurement in a sample of 100 men. In the present work we report a follow up of the same variables in a group of 39 men from the original sample. Some of them were measured several times over the period of 10 yr, but the majority was measured only once ten to eleven years after the initial session.

MATERIALS AND METHODS

The techniques were carefully identical to those used ten years ago. Thirty nine men and five women who had served in the 1985 study, could be located. Their mean (±s.e.m.) age in the first study was 35.6±2.8 yr for the men and 39.4±8.4 for the women. All subjects were 10-yr older in the present work.

We measured the areas of glabrous skin on the forehead and scalp, and the hairy areas of growing beard on the cheeks, lips, chin, and neck of our cleanshaven subjects. The figures were obtained directly by sticking adhesive tape to the areas to be measured and then transferring the tape on to millimetric paper.

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Repeated measures on a same subject showed that the measurement had an error of less than 2%. The areas to be measured were defined as follows: - glabrous area: above the eyebrows and the horizontal line prolonging the eyebrows to temples up to the hair line. When a 'clearing' of glabrous skin was present in the occipital region its area was also measured; - beard area: the area where hair was seen to be growing (although cleanshaven) on the cheeks up to the eye level, the chin, the lips, and the neck down to the collar bones.

Among the male subjects, 11 were young and un-bearded at the time of their first measurement. Their bearded and glabrous skin areas were repeatedly measured approximately every other year, when possible (mean 4.8 times in 10 yr; one of the young subjects was lost after 5 yr). In addition, the head horizontal circumferences of these 11 subjects were measured at the level of the largest diameter.

The other anthropometric data measured ten years ago were not

repeated because they had been found then not correlated to the glabrous and bearded skin areas.

RESULTS

Table 1 presents the mean raw results. Figure 1 compares the initial mean areas of forehead and beard to their counterparts ten years later. It can be seen that both beard and forehead increased significantly over the period of the study. Figure 2 gives the same information for the five women. It can be seen that their foreheads increased also, although less than the men's and that they had no more beard ten years later than they had initially.

Finally, it can be seen that the

estimated total area of the head of the young men increased by ca. 2.5%, a

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magnitude far too low to explain the increases of both beard (53%) and bald skin (32%) areas.

Figures 3 and 4 give the time course of the areas of forehead and beard of the 11 young subjects who could be monitored more or less regularly over the study period. It can be seen that both forehead and beard grew monotonously over the period of observation.

Figures 5 and 6 compare the areas of forehead and beard to one another. It can be seen that the exponential regression line of forehead vs. beard for the group of 39 men was almost superimposed to the initial one after ten years. Some of the men were young at the time of the initial study and had no beard; since they would introduce a bias, they were removed from the group means in order to allow the comparison of the mean ratios presented figure 6. As a result of the parallel evolution of forehead and beard areas the ratio beard/forehead was not significantly raised over ten years (Fig. 6). In addition there was no correlation of beard area and of forehead area with head circumference in the group of young men, neither with the initial measurements (beard F=0.14, r=0.13; forehead F=0.9, r=0.11), nor ten years later (beard F=0.16, r=0.14; forehead F=0.23, r=0.17).

DISCUSSION AND CONCLUSION

Male baldness has always puzzled and upset people. The public takes it as a disease.

However, it has been repeatedly demonstrated that baldness is

genetically inherited and is not influenced by local factors such as the high scalp temperature created by the chronic wearing of a hat or living in hot climate (Hamilton 1942; Setty 1970; Ebling and Rook 1979). Common baldness is the

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result of normal involution of genetically predisposed hair follicles under the influence of androgens the secretion of which increases at puberty (Hamilton 1942; Hamilton 1951; Ebling and Rook 1979). A beard also grows when genetically predisposed hair follicles are influenced by androgens. Beard and baldness have therefore a common determinism (Setty, 1970), the influence of androgens, which explains the high correlation (but not necessarily causality) with ageing.

Beard and forehead areas were larger in this present study than ten years ago. The initial sample contained several subjects from Asia and Africa who could not be included in the present work. The data from Asian and African subjects fell in the same cluster as the data obtained from European subjects. The results obtained in this longitudinal study, although limited to subjects from European descent, confirm those obtained in the cross-sectional study of ten years ago. The highly significant but weak correlation, obtained on the 100 subjects of the 1985 group, between beard and forehead areas is especially confirmed by the results of figures 3 and 4 showing that both areas increased monotonously in the ten subjects followed over the ten years of the measurements. It should be recalled that the present results were obtained in clean-shaven men. therefore independent of whether men shaved or not.

The results are

Of course, beard and

baldness existed before shaving was invented. The positive correlation found here between the area of the bald scalp and the area of the growing beard supports the hypothesis that common baldness, far from being a disease, has developed as a compensation for the growth of beard (Cabanac and Brinnel, 1988).

The correlation found here between beard and baldness could a priori appear if both the glabrous area of the forehead and the beard area on the face were each correlated with head skin area.

However this was not the case.

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Although the volume of the head can be considered virtually constant in all adult human beings (Martin, 1958; Olivier, 1960, 1965) the estimated external area of our subject's heads slightly increased during the 10-yr span of measurements (Table I). Yet, the 1.3% increase of the head area cannot explain the 32% and 53% increases of forehead and beard taking place during the same time span.

The area of face skin covered with hair is quite variable from one man to another depending on genetic heritage (Hamilton 1942; Kherumian 1948; Martin 1958; Olivier 1960; Olivier 1965; Setty 1970). It is conceivable that a large beard, presumably a sexual signal (Guthrie 1970; Hamilton 1973) useful to display a sexual characteristic in cold climates where people wear clothing, would provide thermal insulation for the face. On the other hand, such a thermal insulation would be a handicap from the point of view of selective brain cooling.

Thus, male

common baldness might have developed from natural selection as a compensatory character for the beard.

The usefulness of baldness may be understood as

facilitating heat loss from the head and, in turn, producing selective brain cooling. Selective brain cooling tends to place the trunk in an open loop situation during muscular work. Thus, dangerous hyperthermia of the brain is avoided while heat is stored in the rest of the body for future use when the subject is at rest in the cold environment. Selective brain cooling is, therefore, mostly advantageous in a cold climate.

The enlargement of female forehead, although smaller than that of men (19% vs. 32%), was unexpected but is also compatible with the hypothesis that baldness is useful in facilitating selective brain cooling (Falk 1990; Cabanac 1995). Apes also tend to develop alopecia with ageing (Montagna and Parakai, 1974) The enlargement of glabrous skin area on the forehead of women may reflect,

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therefore, a universal trend among higher primates to enhance selective brain cooling. Like the human species, other Primates do not possess the carotid rete mirabile, believed to be the main intra-cephalic heat exchanger of many mammals. Yet, selective brain cooling during hyperthermia has been shown repeatedly to exist in several primates (Baker et al. 1972; Baker 1979; Fuller and Baker 1983).

Falk has proposed the stimulating Radiator Theory according to which the enlargement of the brain, an intense heat production tissue, might have been permitted in the early ancestors of our present species, by their increasing capacity to loose heat from their heads (Falk 1990; Falk 1992). Because this hypothesis has been criticized (see Braga and Boesch 1997; Falk and Gage 1997; Falk and Gage 1998) it is of interest to recall some recognized evidence and some recent points. Heat is a recognized limitation to brain enlargement and there is need for heat loss (Fialkowski 1978; Fialkowski 1986; Falk 1992). Since our initial finding that blood flowed inward and outward in the emissary veins in a reversible way that seemed appropriate for selective brain cooling (Caputa et al. 1978; Cabanac and Brinnel 1985), several papers have been published that would support Falk's radiator theory. Zenker and Kubik (1996) have shown that the organization of the head's vascular system seems especially devised as a heat exchanger to facilitate selective brain cooling. In addition, we showed that the surface of the head is only one pathway for the head heat loss.

Humans possess another potent and

adjustable heat loss pathway in the upper airways (Rasch et al. 1991; Cabanac and White 1995; White and Cabanac 1995; White and Cabanac 1995; White and Cabanac 1995). The discovery of this important nasal heat loss would support the concept of selective brain cooling as well as the radiator theory.

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In conclusion, in the group of 39 subjects the correlation between beard area and glabrous forehead area was again significant 10 years after the initial measurements and the regression was practically the same as that obtained on the same sample ten years earlier. This would tend to show that ontogeny follows phylogeny. This result is understood as indirect evidence in favor of selective brain cooling in humans.

ACKNOWLEDGEMENTS We wish to thank Ms. A Fleury for kindly improving the English manuscript. This work was supported by Conseil de la Recherche en Sciences Naturelles et en Génie (NSERC) Canada and by Fonds pour la Formation des Chercheurs et l'Aide à la Recherche (FCAR) Québec.

LITERATURE CITED

Baker MA 1979. A brain-cooling system in mammals. Sci Am 240(5):114-122. Baker MA, Stocking RA, Meehan JP 1972. Thermal relationship between tympanic membrane and hypothalamus in conscious cat and monkey. J appl Physiol 32:739-742. Braga J, Boesch C 1997. Further data about venous channels in South African Plio-Pleistocene hominids. J human Evol 33:423-447. Cabanac M 1995. Human Selective Brain Cooling. Austin TX, R.G. Landes Co & Springer Verlag. Cabanac M, Brinnel H 1985. Blood-flow in the emissary veins of the human head during hyperthermia. Eur J appl Physiol 54:172-176.

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Cabanac M, Brinnel H 1988. Beards, baldness and sweat secretion. Eur J appl Physiol 58:39-46. Cabanac M, White MD 1995. Core temperature thresholds for hyperpnea during passive hyperthermia in humans. Eur J appl Physiol 71:71-76. Caputa M, Perrin G, Cabanac M 1978. Écoulement sanguin réversible dans la veine ophtalmique: mécanisme de refroidissement sélectif du cerveau humain. CR hebd Séanc Acad Sci Paris 87D:1011-1014. Ebling FJ, Rook A 1979. Hair. Texbook of dermatology. A Rook, DS Wilkinson and FJG. Ebling. Oxford, Blackwell Scientific Publications: 1733-1824. Falk D 1990. Brain evolution in Homo: The "radiator theory". Behav Brain Sci 13:333-381. Falk D 1992. Brain dance. New York, Henry Holt & Co. Falk D, Gage TB 1997. Flushing the radiator?: a reply to Braga and Boesch. J. hum. Evol. 33:495-502. Falk D, Gage TB 1998. Radiators are cool: a response to Braga & Boesch's published papers and reply. J hum Evol 35:307-312. Fialkowski KR 1978. Early hominid brain evolution and heat stress: a hypothesis. Studies physic Anthropol 4:87-92. Fialkowski KR 1986. A mechanism for the origin of the human brain: a hypothesis. Curr Anthropol 27:288-290. Fuller CA, Baker MA 1983. Selective regulation of brain and body temperatures in the squirrel monkey. Am J Physiol 245:R293-R297. Guthrie RD 1970. Evolution of human threat display organs. Evolut Biol 4:257-302. Hamilton JB 1942. Male hormone stimulation is a prerequisite and an incitant in common baldness. Am J Anat 71:451-480. Hamilton JB 1951. Patterned long hair in man: types and incidence. Ann New York Acad Sci 53:708-728.

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Hamilton WJ 1973. Life's Color Code. New York, McGraw Hill Inc.

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Kherumian R 1948. Technique de la morphologie cranio-faciale. Sem Hop 24:383389. Martin R 1958. Lehrbuch der Anthropolgie in systematischer Darstellung. Stuttgart, G. Fischer. Olivier G 1960. Pratique anthropologique. Paris, Vigot Frères. Olivier G 1965. Anatomie anthropologique. Paris, Vigot Frères. Rasch W, Samson P, Cote J, Cabanac M 1991. Heat loss from the human head during exercise. J appl Physiol 71:590-595. Setty LR 1970. Hair patterns of the scalp of white and negro males. Am J phys Anthropol 33:49-56. White MD, Cabanac M 1995. Nasal mucosal vasodilatation in response to passive hyperthermia in humans. Eur J appl Physiol 70:207-212. White MD, Cabanac M 1995. Physical dilatation of the nostrils lowers the thermal strain of exercising humans. Eur J appl Physiol 70:200-206. White MD, Cabanac M 1995. Respiratory heat loss and core temperatures during submaximal exercise. J therm Biol 20:489-496. Zenker W, Kubik S 1996. Brain cooling in humans-anatomical considerations. Anat Embryol 193:1-13.

12 Table 1 Mean raw data obtained in the present work compared to those published in 1988

women's

men's

men's beard

forehead/beard

young men's

forehead

forehead

(mm2) n=39

ratio

head

(mm2) n=5

(mm2) n=39

circumference

youn

hea

(m

(mm) n=10 1985

7 837±609

15 192±1141

19 986±1557

0.97±0.11

558±4

99

565±4

10

(n=28) present work

9 336±747

19 986±1557

23 583±1322

1.36±0.11 (n=39)

Student's

5.04,

paired t

p=0.007

% increase

19

6.12, p