Tag Archives | Thayer’s Law

The Law of the Soft White Underbelly

WoodMouse

In my previous post, I said that I would be following up with a post about dun. It occurred to me, however, that it might be helpful first to give some background on some terminology. There is no question that among those who like to talk about horse color, terminology can be a minefield. Some of that comes from the limits of the English language—there are only so many words for spotted—and some of it comes from the non-linear nature of discoveries. If only the pieces of the puzzle were found in order, or if we at least got to glimpse the picture on the puzzle box before proceeding, so we knew what the end result was supposed to look like!

But since that is not the case, terms get borrowed and repurposed and stretched to fit things. And in a sense, this is going on within each community focused on a particular domestic species. There is some back-and-forth, but with the rapid pace of discovery (and the ability of non-technical audiences to talk to one another about the subject, often without direct collaboration with the academic community) there is a certain amount of divergence. This can create inconsistencies that can lead to confusion for the unaware.

The conversation surrounding the new discoveries about dun have highlighted an instance of that kind of situation for those interested in horse color. The term that may cause confusion is countershading.

If you frequent online horse color discussions with any regularity, you have probably heard this term used to describe dorsal stripes on horses that were not true duns. But countershading has a very specific meaning when speaking of animal coloration. In fact, it is the subject of a scientific law. Thayer’s Law states that animals display darker coloring along their topline and paler coloring on their undersides as a means of camouflage, because it counters the shadow cast by the sun. My husband once referred Thayer’s Law as the Law of the Soft White Underbelly. It is why so many animals, like the wood mouse pictured at the top of this post, have pale undersides.

Thayer2
Abbott Thayer used this photo of a white rooster against a white backdrop to illustrate how shadowing on the underside of solid colored animal would make it more conspicuous. Meanwhile the pale undersides of the three ibexes in the second picture help to conceal them. 

Abbott Thayer was quite enthusiastic about his theory, and he tended to overstate just how pervasive this type of coloration was in the animal kingdom. It is certainly true that horses do not display this type of coloration in the same pronounced way that many other animals do. Writers have noted that strong countershading is not really a feature of horse coloration.

Externally, horses are recognizable by the long-haired tail; the mane that is both long and thick… and the poor countershading
The Genetics of the Horse, Bowling and Ruvinsky (2000)

Even so, cave paintings suggest that wild horse coloration followed Thayer’s Law at least to some degree.

Lascaux Cave Paintings - horse

We would call the color in the above painting mealy or pangare. In her book Farben und Farbvererbung beim Pferd, this is the color Henriette Arriens associates with the term countershading, and it is the equine color that most closely fits the scenario described by Thayer. It is, however, not a particularly common coloration in modern breeds. Outside of some of the heavy draft horse and rustic pony breeds, this type of pronounced mealy pattern is rather rare. In that it is much like the other primitive equine color, dun!

MealyBelg
It is not hard to imagine that ancient cave painters were looking at horses with this type of coloration.

But the term has also been used by some authors for sootiness. In his book Equine Color Genetics, Philip Sponenberg refers to it as “black countershading”.

…the sooty effect is most commonly expressed over the top of the horse so that the back, shoulder, and croup can almost look black, whereas the horse is redder on the lower body, belly, and upper leg. This effect is called black countershading, or “sooty,” and results in an animal that is dark on top and lighter beneath.

This makes sense since the overlay of black hairs on sooty horses often reflects that same dark-on-top, light-on-bottom configuration. A countershaded bay, then, was a bay with an overlay of black hairs along the topline. This can be seen in some of the examples of sooty dappling on this Pinterest board.

This association with sootiness is probably why, when trying to distinguish the soft-edged dorsal stripes seen on some sooty horses from true dun dorsals, some began referring to markings like the one on the sooty buckskin below as “a form of countershading”. The idea being conveyed was that the marking was not related to dun, but rather was one way that sootiness (“countershading”) expressed on some horses. And since the term was so often used in online forums to argue that this or that horse was “not really dun”, for some the original connection to sootiness (never mind Thayer’s Law) was lost. Countershading was, for them at least, just another word for a non-dun dorsal stripe.

SootyDorsal

So when news came out that researchers looking for the causative mutation for Dun (D) had found an additional mutation for primitive markings “without any dilution”, quite a few people imagined that this second mutation was for “countershading”. There was confusion, however, that term did not appear in either the paper on the new discovery or the dissertation that described the research involved. That is because the scientific meaning of the term has not changed; it is still about a lighter underside countering the self-shadow of an object. Researchers may not be aware that some have given the term this different meaning, but even if they were, an audience of fellow scientists would likely be confused by the change. If the idea is to counteract the shadow cast by the mass of the body, a thin line of darker color along the back of a non-dun horse is not likely to provide much camouflage.

That is why the term is conspicuously absent in the scientific literature on dun. But I think there are other reasons why those of us who are not scientists might want to adopt some different terminology. I’ll talk more about that in the next post on the new not-quite-dun (d1) mutation.

And for those that have an interest in Abbott Thayer and his work on animal camouflage, check out “A Painter of Angels Became the Father of Camouflage.” I have a soft spot for him as another artist whose obsessive nature drew him into science (and the science of animal coloration, no less), but by any measure his work was incredibly influential. You can also access his son Gerald’s compilation of his work Concealing-Coloration in the Animal Kingdom (1909) through Google books.

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