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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New Pinterest boards

SootyBoard

I have uploaded a few new boards to the Equine Tapestry Pinterest site. These all deal with different types of sootiness, which I’ll be talking about a little bit in my next blog post. These boards are not complete, because there are a few types that I have not included, and some of the boards are quite sparsely populated with just a few pins. I will likely be adding to these in the next few weeks, but I wanted to get the set up so I could reference them easily in future posts.

This will also—I hope— eventually get tied back in with the set of posts on pigment-type switching, because this is related to that, too.

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Telling the story a different way

NewModSlide

As I stated on my biography page, my background was originally in art—not genetics. When I started writing about horse color, I was primarily speaking to other artists, so it made sense to use a visual framework to explain it. I wanted to organize the concepts about horse coloration in a way that would resonate with my audience, and that would make a rather technical subject seem less intimidating. That visual approach can be seen in the slides that I put together for a presentation I made fifteen years ago.

OldPresent

A Visual Framework

I still use this basic framework (but not cartoons drawn with magic markers!), as the current slide at the top of this post shows. If asked to give a 30-second rundown, this is how I would lay out this “base color + modifiers” approach:

  • You start out with a basic color: bay/brown, black or chestnut
  • You can add various kinds of modifiers to that basic color: shade/intensity, dilutions, progressive depigmentation, white patterns, reversions
  • Every horse’s color, no matter how complex, can be reduced to a base color + modifiers.
  • Every modifier has a version for each base color (although not all combinations result in a visible alteration).
  • Modifiers have relationships with one another, both hierarchical (this trait trumps that trait) and interactive (one changes the other, sometimes in surprising ways)

The visual framework has proven to be a very effective way to present the story of horse color, not just to artists, but to non-scientists in general. It certainly is the most common approach taken by online horse color resources. It is how I laid out my own book. It is not, however, the only way to tell this story. In some cases, it isn’t necessarily the simplest way to tell the story.

The visual framework can invite certain misunderstandings, like the idea that “bay is a modified form of black“. It also leaves out a concept that is pretty central to the science of genetics, which is that of a wild-type. I talked a bit about that in a previous post. Recent discoveries about the color dun (D) have given that idea new relevance, which will be the subject of my next post. But for now, what I want readers to consider is that the visual framework is just one way to tell the story of horse coloration. And really, that is all the whole concept of “base + modifier” is: a way to tell this story. It is not the science of coat color genetics itself, but a way to talk about it. There are others.

An Evolutionary Framework

Tahki1

We can also tell the story from an evolutionary perspective. That story might go something like this.

At one time, wild horses were uniformly bay dun, or perhaps more likely—given the appearance of other wild equids like the Przewalski’s Horse above—mealy wild bay dun. Their coloration was the result of a pigmentary process, controlled by a number of genes. While still in the wild, mutations to those genes changed part of that process, which in turn changed the color of the horses. Later, as horses were domesticated, additional mutations were selected for by human beings because they found them appealing or felt that the new color better served a particular purpose.

This is quite a different way of looking at horse color. Someone coming from a visual framework, looking at a yellow dun horse, would think of dun as a “modifier of bay”—as something added to the “normal color” of a horse. But from an evolutionary standpoint, bay dun is the wild-type, and our visual explanation is actually backwards. Dun was not added to bay, but rather dun horses became bay because something (a mutation) happened to remove the dun. The wide range of equine color came about in the same way; the wild-type coloration was altered by a variety of mutations, each of which changed some aspect of the original pigmentation of the species. Telling the story of when and how those changes came about is another way to explaining horse color.

Here is the 30-second rundown for this approach:

  • The wild-color for horses is bay dun. This is no longer speculation, but knowledge gained from multiple studies that involved the testing ancient remains.
  • Fairly early, the ancestors of domestic horses acquired a mutation to partially—but not fully—remove the dun coloration. They also acquired a mutation that resulted in progressive loss of color in the coat (Leopard Complex).
  • Mutations to the sites controlling black and red pigment resulted in all-black and all-red phenotypes.
  • Another mutation in domestic horses resulted in individuals with no sign of the dun coloration. Additional mutations occurred giving diluted phenotypes as well as those with white (depigmented) areas. These are believed to have been retained and selected for because human beings liked them.
  • Among modern domestic horses, individuals still appear carrying new mutations. This is particularly true for white markings or patterning.

Not a revolution, but an experiment

What I have described are two very different ways of explaining horse color. I want to be clear that I am not saying that teaching horse color from a visual standpoint is incorrect, or that anyone should stop using it. I will be guest lecturing this upcoming Tuesday, using the slide at the top of the post, so obviously I haven’t overhauled my own approach! But I think it is worthwhile to point out that there are many ways to talk about horse color. They all have their strong points and their pitfalls. Certainly the visual approach is easier for many non-scientists because horse color is such a visual subject. But for those that want to explore the subject further, it can actually be a bit disorienting to read published papers or textbooks on animal coloration, because the evolutionary point of view is the scientific framework for that setting. It is also a more logical framework when the subject veers into the history of domestication and differentiation in horses, because it takes into account chronology. That means that at times an evolutionary framework might make a lot of sense, even for people who were originally taught a visual approach.

It happens that discussing the recent discovering about dun is one of those times. So, we’ll be using a slightly different framework of understanding on the blog in the coming weeks. I am going to ask that readers more familiar with “base and modifiers” set that aside for just a bit while we tell the story of horses and their colors a slightly different way.

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