What makes satin cavies shine?

23 Jun

To newcomers, one of the most confusing aspects of the cavy hobby is the inclusion of five satin breeds that, to outsiders, may seem essentially the same as similar non-satin cavies. Just ask my husband–he still struggles to determine whether a particular cavy is a silkie or a silkie satin.

But when you understand the underlying physiological changes that make cavies satin, you begin to realize that the differences are far from remote.

The Abyssinian, American, teddy, Peruvian and silkie breeds are all accompanied by a satin counterpart that under today’s standard is considered a seperate and distinct breed–the Abyssinian satins, American satins, teddy satins, Peruvian satins and silkie satins. Generally speaking, these breeds are unique in that they carry a recessive satin gene that modifies the underlying structure of the coat and gives it a distinctive sheen that makes the hair appear to shine or sparkle. Understanding exactly how that happens requires a little more in-depth scientific explanation.

One of the most common misconceptions I have heard is that this sheen is the result of a satin’s hollow hair shaft. This is false.

Fundamentals of hair shaft structure

Let’s just start at the beginning. Your basic hair shaft is composed of three main layers. The outermost, called the cuticle, is thin and scalely. Below the cuticle, the cortex is a slightly larger, spongy layer that gives hair its variable colors. At the very center of the hair is the medulla. (See below image, but note that it is not drawn to scale.)

The medulla, which is a mostly hollow tube filled with pockets of air, is like the spine of the hair shaft. It provides most of the hair’s structure and comprises the bulk of the hair’s shape and size.

What this means is that all hair is fundamentally hollow. My hair is hollow. Your hair is most likely hollow. Your cavies’ hair is hollow.

Unless, that is, your cavy is satin.

See, satin hair is actually comprised of just two layers, the cuticle and the cortex. The medulla doesn’t exist as an organized structure the way it does in regular hair. This may not sound like a big deal, but when you make such a radical change to the composition of each individual hair shaft, it changes the overall presentation of the coat in some pretty remarkable ways.

Differences in showing and judging satin cavies

The first change, and the most immediately apparent, is sheen. Though satin hair does not have a medulla, the large, air-filled cells that make up an organized medulla in most hair are still present, scattered throughout the cortex. This means that satin hair becomes semi-transparent, like thick stained glass. When light hits this hair at the right angle, the light refracts off the hair (instead of reflecting directly away from the hair, which would render the hair opaque), causing the hair to appear to glow from the inside out and producing what we know as sheen.

But this isn’t the only way in which satin hair is unique. Because there is no organized medulla, and because the medulla is generally the largest of the hair shaft’s three distinct layers, satin hair is dramatically finer. One scientific study I read found that in mice, the diameter of the hair shaft in satin-type mice was one-third the diameter of regular mouse hair.

Because the hair is so fine, satins often struggle to present density of coat the same way regular animals present it on the show table. The way we currently judge it, density of coat is basically related to two factors–the number of hairs on the animal, and the width of those hairs. Assuming the one-third ratio, a satin cavy would need three times as many individual hairs as a regular cavy in order to present the same apparent density.

Unmedullated hair is also more fragile than regular hair, because it does not have a medulla to lend structure and strength to the hair shaft. This is why many breeders report greater difficulty in groom satin cavies for show purposes.

In fact, the satin coat is so fragle that satin genotypes are of extreme concern to the textile industry, particularly where wool is concerned. Sheepherders are generally avoid satin modifiers at all costs and regard it as an extreme defect in their animals both because the wool is so fragile it cannot be spun into fabric, and because the hair cannot be dyed. It is forever white.

You read that correctly. When hair is dyed, the dye actually fills in the spaces in the medulla, thereby modifying the color of the hair (this is why you have to bleach your hair and remove some of the natural color if you want to make it lighter instead of darker). In cavies, I have observed that the satins are essentially immune to cage stain, probably for the same reason.

Varying degrees of medullation

One final thing I would like to address. I understand that among rabbit breeders, sheen is regarded as something as an on-off switch–that is, that the animal either has sheen or does not. Therefore, their logic goes, satin sheen is actually a non-issue.

Disregarding the diferences discussed above, and disregarding a wide range of factors that impact the presentation of sheen (more on that later), there is still a range of potential for sheen in satin cavies.

Multiple scientific studies I have read indicate that they observed varying degrees of medullation within satin animals of the same species. Remember how I said that, while satins lack a distinct, organized medulla, they still have cells that contain pockets of air dispersed within the cortex? Well, some satins have more of those pockets, and others have less. As these pockets become more infrequent and more diffuse, the degree of medullation is said to decrease.

I can’t explain at this point how this would work on a genetic basis. But the scientific evidence is there to support a case for, if nothing else, varying degrees of medullation. While, in theory, decreased medullation would increase the presentation of sheen, it also increases the presentation of the side-effects I just discussed–namely, the hair shaft becomes increasingly smaller and more fragile. So as breeders, we must walk a fine line between creating beautiful sheeny animals, and animals with coats so fragile they become unworkable.

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One Response to “What makes satin cavies shine?”

  1. Rosalie Beard June 23, 2015 at 8:40 pm #

    Very good article. I am glad to finally have the correct description of the satin hair shaft and the reasons for a coat that feels less dense…also for why the teddies sometimes show sheen later or it seems to come and go.

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