<$BlogRSDURL$>

Thursday, April 29, 2004

African Lips As Health Signals


J.D. O’Dea, Visiting Fellow, School of Botany and Zoology, Australian National University, Canberra, Australia.


Human lips vary in the extent to which they are rolled up to expose the pink membranous portion, a phenomenon known as lip eversion.

Lip eversion has been described as particularly a trait of Africans, involving outrolling and exposure of the mucus membrane of the mouth and thickening of the lips making them stand forward from the teeth (1). The preponderance of the trait in Africa has not been explained, although it has been noted that lip eversion is a warm and/or moist climate trait (1). In line with this observation, it has been suggested (2) that everted lips may have some cooling capacity because capillaries run very close to their surface and the slight moistness of the lips could help in cooling by evaporation. However any cooling effect seems likely to be minimal.

The question of why many Africans have everted lips with visible pink mucus membranes could also be approached from a sexual signalling perspective. It has been suggested (3) that the pink lips of the human female mimic the pink labia of her vulva, and noted that “…during intense sexual arousal, both the lips of the mouth and the genital labia become swollen and deeper in colour, so that they not only look alike, but also change in the same way in sexual excitement.” It might therefore be argued that the eversion of African lips to reveal some pink surface would be necessary to achieve this resemblance. On the other hand, such an hypothesis would not explain why supposedly labia-mimicking lips occur in men and children (4).

Some anatomical features are thought to have evolved as fitness indicators that signal freedom from parasites (5). For example, the uakari monkey’s bright red face may have evolved to indicate that it is not infected by blood parasites, such as malaria, that would cause anemic pallor (6).

In lighter-skinned humans, cheeks and lips vary in color depending on the amount of blood they receive, turning pale or bluish when the blood vessels narrow in reaction to cold, ill health, or other stresses, thereby serving as potential signals of health and fitness. This situation has been compared (4) to what are believed to be similar indicators of healthy blood flow in other species such as the rooster’s comb, the turkey’s featherless head and the small patch of bare skin on the forehead of the chick of the great crested grebe.

In the darker-skinned Africans, the outer areas of the lips are dark but eversion of the lips very often reveals the inner pink mucus membrane. The vertical dimensions of the lips (the “vermilion”) vary in different ethnicities; for example, African-American males and females have 13.3 and 13.6 mm upper lip and 13.2 and 13.8 mm lower lip, respectively. North American Caucasian vermilion height norms of upper and lower lip for males and females are 8.0 and 8.7 mm and 9.3 and 9.4 mm, respectively (7). The greater height of African lips is associated with lip eversion and visibility of pink mucus membrane despite the otherwise dark skin color.

The characteristic eversion of the lips in African humans may function to make visible the pink mucus membrane so as to signal health. In the African environment, the most probable form of health and fitness being indicated would be the absence of anemia or jaundice due to diseases, including parasitic diseases such as malaria. In such disease states the exposed mucus membrane would not have its normal pink color. The capillaries running close to the surface of the lips, alluded to above, would therefore generate a clear indication of hematological health.

In summary, the characteristic eversion of African lips may have evolved to permit the display of a pink surface on the face, providing a conspicuous signal of hematological health. Non-Africans would not require everted lips, either because they have relatively fair skin on the entire face (especially on the cheeks in many Europeans) that would clearly signal hematological health or because of the lesser incidence outside Africa of diseases likely to cause jaundice and anemia.

It has been noted (1) that the lip eversion trait is not found in every part of Africa. If the proposal in this note is correct, it might be expected that the trait would be most common in areas where diseases likely to lead to anemia, such as malaria, have been most prevalent.

References

(1) Krantz, G. S. 1980. Climatic Races and Descent Groups. The Christopher Publishing House, Massachusetts.

(2) Fischer, M. (n.d.) Foundations of human culture – human morphological variation.
http://lucy.ukc.ac.uk/Courses/SE302/humanmorpohvar.html

(3) Morris, D. 1967. The naked ape. Corgi Books, London.

(4) Zahavi, A. and A. Zahavi. 1997. The handicap principle: a missing piece of Darwin’s puzzle. Oxford University Press, Oxford, New York.

(5) Hamilton, W. D. and M. Zuk. 1982. Heritable true fitness and bright birds: a role for parasites? Science 218: 384-387.

(6) Miller, G. 2001. The mating mind: how sexual choice shaped the evolution of human nature. Vintage, Random House, London.

(7) Farkas, L.G. 1981. Anthopometry of the head and face in medicine. Elsevier Science, New York.


|

This page is powered by Blogger. Isn't yours?