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, 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 with appropriate and specific direction to the original content. 


Five-legged frogs and other abnormalities

I once lived for four months in a Zoological Museum in a small room just under the roof of the building. The great advantage of that abode was that I did not have to pay any rent for staying there; the disadvantage was that the toilet was in the cellar. Therefore, if I needed to go there it required careful planning in advance. First down a narrow flight of stairs, then through the storage area, past the huge stuffed bison, along the displays of various skeletons, down to the taxidermically prepared monkeys and birds, silent and motionlessly in their glass cages, to finally descend into the dark vault of the cellar’s WC. It was an afternoon when a man entered this museum, wanting to see the Director. Since he wasn’t there, I offered to help. The man had with him a five-legged frog he wanted to donate to the museum.

It was a fully grown male frog with a third front leg between the two regular legs. Interestingly the extra leg also possessed the typical nuptial thumb pad that male frogs develop during the mating season to help them cling to a slippery female. Despite its extra leg the frog seemed normal enough and obviously had been able to grow into a mature individual. Together with Professor Juergen Koebke I carried out an anatomical and ultrastructural study on the extra leg’s muscles, blood supply and nerves. We found that the extra leg’s muscles were normal and could contract, but that joints between the bones showed little wear, because the leg did not actively participate when the frog jumped. The question was: what could have caused the development of the extra leg. That there was “a second individual” somehow involved (as with conjoined twins or a partially absorbed twin) was outrightly dismissed. An injury during the tadpole stage to a leg bud resulting in a growth of an additional leg seemed a possibility and that the extra leg might have been the consequence of an exposure to ionizing radiation or the response to a parasitic  trematode attack (because such cases had been described before) were other possibilities.

Abnormalities such as these have always alarmed as well as fascinated humans and when present in humans were (in the past) usually linked to superstitions blaming either the mother, the devil, weather phenomena, or God. More recently we see connections with various kinds of pollutants and for this reason had started to monitor abnormal growths like skin cancers and reduced or increased numbers of limbs and toes in Japanese newts in one specific region of Japan over a period of 10 years. This kind of research that involves field and laboratory work is labour-intensive and despite its importance was unfortunately discontinued. However, what we did find was that the incidences of abnormalities in the newt population had not changed in the 10 years we monitored them. That this should not lead us to be complacent is self-understood, especially with regard to the rise in plastics and their residues in the environment, but it could also suggest that a certain small percentage of abnormalities is a “normal” feature of a natural population. However, to distinguish an abnormality from a variety (e.g., regarding respective wing or feather colours in insects and birds) is not always clear.

Although humans with cyclopia, or cows with two heads or chickens with four-legs etc. are sometimes referred to as “monstrosities”, for humans at least such terminology is unacceptable and efforts are made to identify (and then eliminate) the causes of malformations. This requires collaborations between geneticists, toxicologists, biochemists and scientists of various other disciplines. What I personally find sad is that goldfish monstrosities and Folded Ear cats (even if the genetic defects are known to have serious consequences for their health) are ‘celebrated’ and given prizes in breeder competitions. Isn’t it a bit abnormal to love such abnormalities when one knows that the animals are suffering? 

How Small Can Small Be ?

A study of the eyes of miniature insects

In the year 2010, I published with my PhD-students S. Fischer and C. Müller in the journal “Visual Neuroscience” an article with the provocative title “How small can small be”. Although the article dealt with the compound eye of the parasitoid wasp Trichogramma evanescens, an insect of 0.4-mm total body length, the question of how small an animal or a part of it can be before it stops to function is of course an intriguing one. The smallest mammals are the Etruscan shrew and the bumblebee bat Craseonycteris thonglongyai; the smallest fish of just 1 cm total body size is Paedocypris progenetica, and the smallest frog and reptiles are Paedophryne sp. and dwarf geckos of 1 and 2 cm respective sizes.

Back to insect compound eyes. In 1975 I had examined the tiny eye of a beetle known as Corylophodes, whose total body size was below 1 mm. Compound eyes consist of a multitude of facets, each equipped with a small lens and a retinula, which contains a rod-like light-perceiving structure known as the rhabdom. It is obvious that for an eye of a given size sensitivity (related to the amount of light that can enter the facet) and spatial resolution (the amount of detail the eye can resolve) are in conflict with each other. Photography and camera-buffs know the relationship between aperture setting and exposure time: for high resolution you want a small aperture, but need more light (e.g. longer exposure).

If we take a look at the hemispherical eye of an insect and assume a constant facet diameter, increase of the eye would lead to an increase in sensitivity and resolution, because more facets would allow more light to be perceived and the angle between neighbouring facets would decrease, thereby improving resolution). But what about the situation in very small insects? They would have a problem, because a small head offers less space for big compound eyes. So, the facet size should be reduced, but there is a limit and a researcher named Barlow as long ago as 1952 calculated that smaller facets would be increasingly limited by diffraction. He concluded facet sizes below 10 µm would probably not occur. If facet sizes weren’t allowed to become reduced as the eye itself got smaller and saw its curvature increased, then the angles between neighbouring facets would become wider and resolving power would suffer. How tiny insects find a ‘compromise’ to solve this impasse is what interests us.

Another problem is that the rod-like rhabdom mentioned above acts like a light-guide in the bigger insect eyes, but as Alan Snyder 1979 worked out, effectiveness of a light-guide based on total inner reflection drops off as the diameter of the light guide gets smaller. With diameters below 1.5 µm light in the form of modes travels partly on the outside of the rhabdom, making it unavailable for the photo pigment in the rhabdom’s constituent membranes. Doekele Stavenga in the Netherlands showed that the fraction of the light conducted inside the rhabdom decreases with increasing wavelength for all modes. Thus, shorter wavelengths like blue, violet and UV would be the most useful for the tiny eyes of small insects. And yet, surprises still await the researcher: on the island of Hachijojima I caught a fly of 0.65 mm body length and examined its eyes with Dr Yumi Yamahama. Not only did this fly have three compound eyes (two lateral ones with 35 facets each and one dorsal eye with 90 facets on the top of its head), it also had an extremely shallow retinula and surprisingly thick lenses. We published the results in 2019 in the journal Entomologie Heute, but are still at a loss to explain how this miniature insect could be such a swift flyer, repeatedly being attracted to the light of my computer screen at midnight.

Finally, here’s a challenge: there are mini-snails with body lengths of 1mm or even less. I noticed they have black eyespots. How would their eyes be constructed and what could they possibly see with them?

© Dr V.B. Meyer-Rochow and, 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 with appropriate and specific direction to the original content.