biology zoology blog benno meyer rochow eyebrow

Eyebrows and Eyelashes

Do they have any uses at all?

When I was a child, I often heard some adults remark what beautiful eyelashes that boy has. But as I grew older and taller the eyelashes became shorter and shorter. But what is it with eyelashes that so many people love about them? Certainly not all love them, because when my daughter Yamuna was two or three years old, she kept pulling out their exceedingly long eyelashes, because she must have felt that they were obstructing her visual field and sometimes were getting into her eyes. Trobriand Islanders, as Malinowski in his famous 1929 book The Sexual Life of Savages described (and I can support on the basis of my own observations during four stays on the Trobriand Islands), are proud not to have eyelashes as it indicates to a young girl that she’s had many lovers and therefore must be an attractive girl. Eyelashes are bitten off and eyebrows, too, are kept very short. In fact, all facial and bodily hair is considered ugly.

Some birds and many species of mammals (especially those feeding on grass and sedges or leaves of shrubs and trees) also have eyelashes and some even have noteworthy brows (although the latter are often not recognizable, because of the surrounding fur). The fact that more species which seek their nourishment in trees, bushes, grasses, etc. possess eyelashes than carnivores could suggest that one function of eyelashes is that of a warning device: if something unexpected touches the eyelashes, a nerve impulse from a pressure or stretch receptor at the base of an eyelash is sent to an appropriate eyelid muscle to close the eye to prevent damage. There is no need or time wasted to think of closing your eye: a reflex arc takes care of that. Eyelashes of camels and other mammals living in sandy surroundings serve to protect their eyes against windblown dust particles and give their eyes as well as those of ostriches, giraffes, horses and cows, etc. expressions that resemble those seen in humans. Animals like mice and kin seem not to have long eyelashes as their long whiskers alert them of danger.

Eyelashes and those of eyebrows are structurally and embryologically identical to those of the rest of the body and grow for a while, then fall out, and re-grow. There are, however, some difficulties to explain differences: while eyelashes become shorter as a person ages and the amount of hair on the scalp gets less with age (or may even disappear altogether), eyebrows tend to become fuller and bushier. Although some of the eyebrow’s hairs may function as touch receptors (similar to eyelashes) and can be quite long as in seals (where they may convey information to the seal about currents and swim speed), their main function in terrestrial animals and humans is a different one. It is actually very easy to find out how useful eyebrows are, when you go for a run in a warm, tropical country and the sweat begins to run off your forehead into your face. Getting the slightly salty sweat into your eyes irritates the eyes, but well-developed eyebrows ‘channel’ the sweat to the sides and keep your eyes functioning. A very similar observation you can make when it rains and you have no umbrella to protect yourself against the rain.

Prominent eyebrow ridges further enhance the protective function, shade the eye and reduce injury and radiation effects. A few isolated and long, bristly hairs are often found above the eyes of rodents.

Yet, eyebrows, can also express feelings, moods and intentions and that may be important for animals like apes, monkeys and humans, in which the colour of the eyebrows may be different from that of the surrounding skin and the hair on the scalp. Perhaps that is the reason why women often darken their eyebrows and, removing excess hairs, turn them into thin lines to show without words what they feel. (Besides, it may reduce the chance of Demodex mites colonizing the eyebrows and eyelashes).

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content. 

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