1. How a Tokyo laboratory identified the owner of hair found in food
To find out whose hair was mixed into food, a laboratory in Tokyo first created a comparison chart. They observed hairs from rats, cows, horses, sheep, cats, and dogs under a microscope. They lined up data on appearance, surface scales, and cross-sections for each animal [1]. With these samples ready, they could compare any found hair against the chart.
The thickness of the medulla (the core inside the hair) is another clue. In the report’s table, the medulla takes up about 1/5 of the hair’s width in humans, but about 3/4 in horses [1]. Using these methods, the hairs found in miso, sliced bread, roll buns, and steamed dumplings were judged to be from humans or rats [1].
To see the scales clearly, a magnification of about 200 to 500 times is said to be needed. Sometimes a method called "Sumpu" is used to copy the surface pattern [2]. But how different are these scale shapes between animals?
2. The two main types of scales on mammal hair
Hair has three parts: the outer cuticle, the middle cortex, and the inner medulla. While this description is often used for human hair [3], a study of mammals in Sri Lanka says these three layers are a basic structure common to all mammals. Each part helps identify the animal, but experts look at several features together [4].
Scales on the outside come in two main shapes. One is "ring-like," where the scale goes completely around the hair shaft. The other is "overlapping," where scales do not go all the way around but lie over each other like roof tiles. Ring-like scales are common in small rodents and bats, but rare in human hair [4].
However, saying "ring-like means bat" is too simple.
3. Bat, cat, and otter hair scales vary more than expected
The scales of bat hair have been considered ring-like [4]. But Nason (1948) found that in 8 of 10 genera studied, overlapping scales were more common. A study of 15 genera in Sri Lanka found true overlapping scales in only 3 genera for thick hairs; the rest were ring-like. Many ring scales are deeply cracked and look like overlapping ones, making it hard even for researchers to tell them apart [4].
Another study used electron microscopes on 9 species of insect-eating bats in India. Four species had wide-spreading ring scales. Two species of the genus *Rhinolophus* had open ring scales. Two species of *Scotophilus* and others had overlapping scales [5]. Researchers said hair shape can help identify species [5].
Cat hair has irregular hexagonal net-like patterns at the root, which change to wavy and then saw-toothed shapes toward the tip. Cat hair also has a thick medulla with flat air pockets in layers [6].
Even within one animal, scale shape changes between thick and thin hairs. Leopard guard hairs are mosaic-like, while underhairs are diamond-shaped petals. Otters have spear-shaped scales for both types [4]. Scales act like name tags, but you must look at the hair type to read them.
4. Why wool sweaters shrink: the direction of the scales
If a wool sweater shrinks in the wash and does not return to its size, it is thought to be caused by scales on the wool surface. Because of these scales, friction changes depending on which way the fiber moves. This is called anisotropic friction. As a result, fibers tangle together [7].
More specifically, felting happens when fibers move slowly toward their root ends and tangle. The cause is the difference in friction based on direction (DFE). Without this difference, felting does not happen [8].
To prevent wool shrinkage, treatments like shrinking-resist processing or coating with resin are used [7]. But what if the scales are low?
5. Why alpaca fiber has lower scales but still felts easily
In experiments on alpaca fiber, scales on hairs thicker than 19 micrometers were about 0.4 micrometers high. This is half the height of wool scales, which are about 0.8 micrometers. The number of scale edges per 100 micrometers was also different: wool had 4, while alpaca had more than 9 [8].
The difference in friction by direction (DFE) was about 0.40 for wool, but only 0.16 to 0.20 for alpaca. In theory, alpaca should shrink less [8].
However, in this experiment, alpaca felted more easily than wool. The friction difference alone could not explain it; factors like fiber length may have played a role [8]. Scale height does not decide shrinkage on its own.
This result comes from one experiment. It raises the question: what would happen if other fibers were compared under the same conditions?
6. How to check your own clothing for wool and washing tips
You can look at clothing tags at home to find items labeled "wool." Checking the washing instructions might give hints about preventing shrinkage.
You can ask if a science museum or school microscope can show enlarged hair samples. Whether you can see scales depends on the magnification and the type of hair.
Manuals from Hiroshima Prefecture and annual reports from the Tokyo Metropolitan Institute of Public Health describe steps for determining which animal hair comes from [2][1]. Reports from the Ibaraki Industrial Technology Center explain wool shrinkage and treatments [7].