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Curly Hair

March 20, 2023

My maternal grandmother was Polish, and I would often visit her in her cozy apartment on the second floor of a two-family house. As she was making tea for us, I would browse through the Polish language weekly newspaper she received in the mail, trying to make sense of that language. The 32 letters of the Polish alphabet resemble the Latin alphabet; so, how hard could that be? In fact, it was impossible, since Polish is not a Latin-Germanic language like English. One interesting feature of that newspaper was the short Little Orphan Annie cartoon, presented in Polish. Annie's most distinctive feature is her large ball of curly hair. Curly hair in humans may have evolved as a means to insulate our overheated brain from the environment.[1-2]

Portion of the first Little Orphan Annie comic strip

A portion of the first Little Orphan Annie comic strip from the New York Daily News, August 5, 1924, page 26. This shows Annie washing dishes in an orphanage.

This cartoon was created by Harold Gray (1894–1968), and it ceased publication in 2020.

(Edited portion of a Wikimedia Commons image.)

From a physics perspective, the spherical cow model for this property can be found in glass wool thermal insulation known to most homeowners.[3] Dry air at room temperature has the very low thermal conductivity of 2.6 x 10-2 watt/meter-kelvin (W/m-K). The glass fibers of the glass wool have a thermal conductivity of about 0.8 W/m-K, but the glass wool entraps air, and this results in a low overall thermal conductivity. The physical process is explained by the watts per meter-kelvin units of thermal conductivity. A watt, which is a joule per second, expresses the rate of energy loss, and this depends on the temperature difference from one side of the thermal insulator to the other and its thickness.

Of course, thicker is better, and among the first things I did when my family moved into our present house was to add an additional four inches of glass wool insulation above the existing three inches between the ceiling joists. It was hard work for someone who spent most of his time seated in front of a computer display; but, the hourly labor rate derived from subsequent savings on home heating cost was far beyond what I earned as a scientist. Not so much of a concern then as it is now was my reduced contribution to global warming.

A roll of glass wool insulation

A common sight to home builders - A roll of glass wool attic insulation.

Since people associate better quality with larger numbers, home insulation is tagged with an R-value. This number is essentially the inverse of the thermal conductivity for the specific thickness of the material.

(Image, a Knauf Insulation Rafter Loft Roll, licensed under a Creative Commons License. Click for larger image.)

Humans are unusual among primates for not having bodies covered with fur. Our loss of fur happened about 1.6 million years ago as an evolutionary adaptation to our change of behavior to that of animals traveling long distances in search of food and water.[4] We have, however, retained hair atop our heads, and a new study of the possible reasons for retention of such hair has been published in bioRxiv.[1] Research team members were from Pennsylvania State University (State College, Pennsylvania), the University of Southern California (Los Angeles, California), Loughborough University (Loughborough, Leicestershire, England), and the University of Sydney (Sydney, Australia).[1]

As we all have observed, scalp hair type is variable between people, but the consequence of such variation has never been been studied.[1] Scalp hair, especially curly hair, may have evolved as a means of thermoregulation.[2] The idea of hair insulating our heads from the cold seems obvious, and our ancestors did live through an ice age.[1] People with short hair, of the order of 5 millimeters lose heat more quickly than those with longer hair, of the order of 100-130 millimeters.[1] However, hair will also act to insulate from heat.[1] Hominids with their hairless bodies and bipedal posture may have needed to evolve by keeping scalp hair to protect our large brains from solar radiation.[1-2] A 2010 study found that bald heads absorb more heat.[2]

Figure caption

Who better to illustrate an article with mentions of evolution and baldness than Charles Darwin (1809-1882). Another bald scientist is Muppet character, Dr. Bunsen Honeydew.

This is a photograph of Charles Darwin taken around 1874 by his son, Leonard Darwin (1850–1943), also noted as a mentor of the preeminent statistician, Ronald Fisher (1890-1962).

(Wikimedia Commons image. Click for larger image.)

The research team designed an experiment to examine the potential evolutionary advantage of human scalp hair and the variation of its morphology. The experiment used a mannequin wearing human hair wigs in a temperature and humidity-controlled environment at different wind speeds.[1-2] Solar radiation was simulated with a lamp, and data were collected for convective, radiative, and evaporative heat fluxes to and from the scalp.[1] The authors state that these experiments are the first of their type.[2]

Experimental setup and results for the influence of solar radiation on dry skin for various hair types.

Experimental setup and results for the influence of solar radiation on dry skin for various hair types. The types are no hair, straight, moderately curled, and tightly curled. The conditions were 30 °C, and 60% relative humidity. (Left, a portion of fig. 1 from ref. 1; and right, a portion of fig. 3 from ref. 1.[1] Licensed under a Creative Commons License. Click for larger image.)

It was found the bald mannequin absorbed a considerably greater amount of heat, and curly hair resulted in significantly less absorbed heat.[2] More tightly curled hair offered increased protection against heat gain from solar radiation.[1] The results indicate that any type of head covering reduces heat gain from the Sun.[2] It appears that scalp hair evolved in response as a defense against over heating after our species' evolution of upright posture and large brains.[2] Decreased sweating would extend how long an individual could engage in strenuous physical activity before needing water.[1]

As the researchers write,
"The emergence (or retention) of scalp hair may have struck an optimal balance between maximizing heat loss across the large surface area of the body and minimizing solar heat gain on the small surface area of the scalp, directly over the brain."[1]


  1. Tina Lasisi, James W Smallcombe, W. Larry Kenney, Mark D. Shriver, Benjamin Zydney, Nina G. Jablonski, and George Havenith, "Human scalp hair as a thermoregulatory adaptation," bioRxiv, January 25, 2023, doi: https://doi.org/10.1101/2023.01.21.524663. Supplementary material here.
  2. Carly Cassella, "Cool New Experiment Explains Why We Evolved Curly Hair," sciencealert, February 17, 2023.
  3. Games Slayter, "Fibrous products and method and apparatus for producing same," US Patent No. 3,012,923, December 12, 1961 (Via Google Patents).
  4. Nina G. Jablonski, "The Naked Truth: Why Humans Have No Fur," Scientific American, February 1, 2010.

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