Lash science · The natural lash
Evolution and function
Most mammals have eyelashes, and in the ones that have been measured the lashes come to the same proportion of the eye. That is a clue to what they are for. This chapter separates what has been measured from what is still a story.
Who has them
Hair lashes belong to mammals
Hair is the covering of mammals and feathers the covering of birds, so a true eyelash is a mammal's. A survey of animal image collections found lashes or fur around the eye in 87 of 126 living mammal families.221
Some birds grow lash-like feathers instead. In two species of hornbill the upper part of the bird's view is seen through them. Camels carry very thick, long lashes on both lids.523
When engineers at Georgia Tech set out to measure lashes, they photographed preserved specimens of 22 mammals from 12 different orders.1
The 22 mammals measured
Amur hedgehognorthern tamanduanine-banded armadilloelephant shrewporcupineVirginia opossumblack-tailed jackrabbitred-fronted lemurred pandaAmerican badgergiant pandajaguarundichimpanzeeGobi argalicamelsnow leopardcougarBaird's tapirAfrican wild dogwild boarred kangaroogiraffe
Photographed from museum specimens at the American Museum of Natural History.
The one-third rule
One proportion across the mammals
Across the 22 species, lash length rose in step with the width of the eye, at a slope of 0.34. The relationship held after correcting for how closely the species are related.1
A model eye in a wind tunnel, a dish of water 20 mm wide ringed with mesh or human-hair lashes, showed why. At about a third of the eye's width the lashes halved both evaporation and the dust landing on the surface. Tilted 20 degrees to the wind, the best length fell to about a fifth and the benefit halved.1
A separate computer model with realistic human lash shapes put the best length a little shorter, at 15 to 30 percent of the eye's width, and found that longer lashes raised evaporation again.2
The suggestion that lashes evolved so the eye needs to blink less is the authors' idea, not a measurement.1
At the atelier
False lashes were tested in the same wind tunnel. Curved and dense, they let through as much evaporation as a much sparser mesh. What that means for a living eye was not measured, and we claim no more than that. We do keep length close to the eye's own proportion.
Length, in the libraryLines redrawn from the published slopes, not individual animals. The birds measured do not follow the mammal rule.
Twenty-two mammals, twelve orders, one proportion.
From the hedgehog to the giraffe, the lashes come to about a third of the width of the eye.
Water and light
Shedding water, and a little shade
A human lash line throws off water quickly. A curved, flexible, water-repellent row of lashes cut the time liquid stays in contact by about a fifth compared with straight, rigid slopes.3
Lashes may also shade the eye. A computer model of the head estimated that they cut the ultraviolet light reaching the cornea by 12 to 14 percent on average, and by up to 24 percent. No one has measured it on people.4
In hornbills, whose lash-like feathers sit at the top of the visual field, the researchers read them as a sunshade. That is their interpretation of where the feathers sit.5
What lashes take off, in percent
The water figure is measured in the laboratory. The two paler bars come from a computer model.
Touch
Built like a whisker
In the primate eyelid, every lash follicle carries a large stretch receptor, and between neighbouring lashes sit small pegs of touch cells. The design resembles the receptors on monkeys' whiskers and facial guard hairs.6
The anatomists who described it presumed it protects the eye. In human upper lids a separate web of nerves at the rim of the lid serves the lashes.67
The reflex it is thought to trigger is fast. Measured in 50 healthy people with a small electrical pulse, the lid muscle answered at about 10 thousandths of a second, and again at 29, with no difference between men and women.8
At the atelier
A lash feels what it carries. That is one more reason we keep extensions light and never let a fibre touch the skin of the lid.
Attachment, in the libraryThe lash as an instrument
- At every lash
- a large stretch receptor
- Between lashes
- small pegs of touch cells
- The same design as
- whiskers and facial guard hairs in monkeys
- First answer of the lid
- about 10 thousandths of a second
- Second answer
- about 29 thousandths of a second
The timings are for an electrical pulse to the nerve above the eye. A blink from touching a lash has not been timed.
What sets the length
Two genes and a glaucoma drop
Lash length is controlled. When the gene FGF5 is switched off by a mutation, the growing phase runs on and lashes grow extremely long, a condition called trichomegaly. In the laboratory the same protein sends a human hair follicle into retreat.10
When the gene FOXC2 is switched off, a second row of lashes grows behind the first, a condition called distichiasis.11
The clearest outside switch was found by accident. Patients treated in one eye with a prostaglandin glaucoma drop grew more, longer, thicker, more curved and darker lashes on that eye. In mice, the drug lengthens the growing phase without making new follicles.1314
At the atelier
Growth serums are covered in the health chapter, with what the trials found and what the regulators say.
Health: keeping and improving natural lashesDrawn to show the idea, not to scale.
A readout
Lashes can report on health
Unusually long lashes, over 12 mm by one definition, can be inborn, caused by medicines, or acquired. The listed causes include allergic disease, eczema, some inflammatory eye conditions and certain cancer drugs.289
In children, longer lashes went with allergy. In one study allergic children had lashes about a millimetre longer than their peers, and in another the length rose with the severity of the eye allergy. In adults, lashes shorten with age.15161720
Lash loss has its own list. Chronic inflammation of the lid is the condition most often linked to it. Thyroid disease, alopecia, hair pulling, radiotherapy and some chemotherapy can all cause it, and it can be the first sign of something serious.1218
At the atelier
If your natural lashes change suddenly, growing much longer or falling out, tell your doctor first. We will pause a set rather than cover a change that needs a diagnosis.
Average upper lash length, in millimetres
From three different studies, so compare within each pair, not across them.
Signal
What is still a story
Some of what is said about lashes is measured, and some is interpretation. The two are easy to blur.
The human eye has the whitest and most exposed sclera of any primate, which is measured. That it evolved to make our gaze easy to read is the authors' interpretation, and later studies have both questioned it and partly supported it.212425
People rate faces with lashes near a third of the eye's width as the healthiest and most attractive. That we prefer that length because it marks health is an explanation nobody has tested.26
Often said
What the evidence shows
Men have longer lashes than women.
No measurement of adult lash length by sex was found. One count from photographs found no difference in the number of lashes.19
People from sunny, dusty or cold places have longer lashes.
No study links lash length to climate. The largest comparison found no difference in length between groups, only in thickness, lift and number.27
Camels have a double row of lashes.
Their lashes are very thick and long on both lids. The row count could not be confirmed.23
At the atelier
What evolution suggests for a set
A free 30-minute consultation comes first. We look at your natural lashes under magnification and draw the map before a single fibre is placed.
- Length
- close to the eye's own proportion
- Weight
- light enough for a sensor to carry
- Change
- a sudden one goes to a doctor first
Sources
Every numbered line above comes from one of these. Where a claim is still a hypothesis, the text says so.
- 1Eyelashes divert airflow to protect the eyeAmador, Mao, DeMercurio, Montero, Clewis, Alexeev and Hu. Journal of the Royal Society Interface, 2015.Measured: 22 species and a wind tunnel
- 2How eyelashes can protect the eye through inhibiting ocular water evaporation: a chemical engineering perspectiveZou, Zha, Xiao and Chen. Journal of the Royal Society Interface, 2019.Computer model
- 3Rapid water drainage on human eyelashes of a hydrophobic Brachistochrone fiber arrayZhou, Chen, Cheng, Gao, He, Wang, Jiang, Dai and Dong. Science Advances, 2024.Measured in the laboratory
- 4Modeling the protective role of human eyelashes against ultraviolet light exposureMarro, Moccozet and Vernez. Computers in Biology and Medicine, 2022.Computer model
- 5Visual fields in hornbills: precision-grasping and sunshadesMartin and Coetzee. Ibis, 2004.Measured in two species; the sunshade is an interpretation
- 6The sensory innervation of primate eyelidHalata and Munger. The Anatomical Record, 1980.Measured: microscopy
- 7Sensory innervation of the upper eyelid and its implication for upper eyelid surgeryMayr-Riedler, Broer, Hirtler and Grill. Ophthalmic Plastic and Reconstructive Surgery, 2023.Measured: 10 dissections
- 8Blink reflex: normal values and its findings on peripheral facial paralysisLu and Tang. Chinese Medical Journal, 1996.Measured: 50 healthy people
- 9The eyelash follicle features and anomalies: a reviewAumond and Bitton. Journal of Optometry, 2018.Review of the published research
- 10FGF5 is a crucial regulator of hair length in humansHiggins, Petukhova, Harel, Ho, Drill, Shapiro, Wajid and Christiano. Proceedings of the National Academy of Sciences, 2014.Measured: affected families and cultured follicles
- 11Mutations in FOXC2 are responsible for the hereditary lymphedema-distichiasis syndromeFang, Dagenais, Erickson and colleagues. American Journal of Human Genetics, 2000.Measured: affected families
- 12Madarosis: a marker of many maladiesKumar and Karthikeyan. International Journal of Trichology, 2012.Review of the published research
- 13Hypertrichosis and increased pigmentation of eyelashes in patients treated with unilateral topical latanoprostJohnstone. American Journal of Ophthalmology, 1997.Measured: 43 patients, the other eye as control
- 14Characterization of an in vivo model for the study of eyelash biology and trichomegalyTauchi, Fuchs, Kellenberger, Woodward, Paus and Lütjen-Drecoll. British Journal of Dermatology, 2010.Animal study: mice
- 15Eyelash length in children and adolescents with allergic diseasesLevy, Segal, Ben-Amitai and Danon. Pediatric Dermatology, 2004.Measured: 60 patients, 80 controls
- 16Long eyelashes in a case series of 93 children with vernal keratoconjunctivitisPucci, Novembre, Lombardi and colleagues. Pediatrics, 2005.Measured: 93 patients with matched controls
- 17Eyelash length for the diagnosis of atopic dermatitis and ichthyosis vulgaris in childrenSingh and Pawar. European Journal of Pediatrics, 2018.Measured: one centre
- 18MadarosisKhong, Casson, Huilgol and Selva. Survey of Ophthalmology, 2006.Review of the published research
- 19Retrospective review of eyelash number in patients who have undergone full-thickness eyelid resectionFreitag, Lee, Lee, Johnstone and Sires. Ophthalmic Plastic and Reconstructive Surgery, 2014.Measured: 38 patients, small
- 20Epidemiologic analysis of change in eyelash characteristics with increasing age in a population of healthy womenGlaser, Jones, Carruthers and colleagues. Dermatologic Surgery, 2014.Measured: 179 women
- 21Unique morphology of the human eye and its adaptive meaningKobayashi and Kohshima. Journal of Human Evolution, 2001.Measured across primates; the gaze function is an interpretation
- 22A new scenario for the evolutionary origin of hair, feather, and avian scalesDhouailly. Journal of Anatomy, 2009.Review of the published research
- 23Anatomical and histological characteristics of the dromedary eyeRahmoun, Lieshchova, Gherissi and Hadjeris. Theoretical and Applied Veterinary Medicine, 2020.Descriptive anatomy, 10 camels
- 24Ocular pigmentation in humans, great apes, and gibbons is not suggestive of communicative functionsCaspar, Biggemann, Geissmann and Begall. Scientific Reports, 2021.Measured: 15 ape species
- 25Experimental evidence that uniformly white sclera enhances the visibility of eye-gaze direction in humans and chimpanzeesKano, Kawaguchi and Hanling. eLife, 2022.Experiment
- 26Long lashes, mixed signals: investigating the effect of eyelash length on perceived health, attractiveness, and sexual receptivityPazhoohi. Archives of Sexual Behavior, 2025.Ratings study; the evolutionary reason is a hypothesis
- 27Ethnic characteristics of eyelashes: a comparative analysis in Asian and Caucasian femalesNa, Kwon, Kim, Park, Oh, Kim, Cho and Eun. British Journal of Dermatology, 2006.Measured: 4,661 lashes on 30 women
- 28Eyelash trichomegaly: review of congenital, acquired, and drug-associated etiologiesPaul, Cohen and Kurzrock. International Journal of Dermatology, 2012.Review of the published research
Lash science
The natural lash