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Are blood marks "clusters of fleabites"?

A white-grey Quarter Horse mare with a pale blood mark on her neck, right ear and cheek

If you do a web search on bloody shoulders, you will likely come across the claim that these markings are “concentrated areas of fleabites.” Some sources will claim that they occur when clusters of fleabites “grow together,” suggesting the markings get progressively larger as the horse ages. That was always the theory that I heard. 


Is that really how horses acquire blood marks like the one on the mare featured at the top of this post? I don’t think that is the case. Visually, that’s not what the patch on her neck looks like. 

The same grey QH mare as the previous image, showing the whole right side of the horse

Up close, it looks like an earlier stage of greying before the mare turned white-grey. 


A close up of the blood mark on the QH mare's neck, showing how it is a mix of red and white hairs (but not fleabites)

That’s not to say that fleabites do not cluster together. Several horses in the previous posts had more prominent spots against a background of smaller speckles. 


the barrel and hindquarters of a fleabitten horse, with larger spots mixed with smaller ones

This is not an uncommon variation among fleabitten greys.


a Miniature Horse mare showing the same pattern of large and small fleabites as the previous image

There have even been fleabitten greys where the larger speckles were so pronounced that the horse might be mistaken for a small-spotted leopard appaloosa.

a Lipizzaner with large spots mixed with smaller fleabites

I suspect this Lipizzan pictured in a recent paper on melanomas is the mare Slavonia. (This is likely the same horse that appears in the illustration in the post “Dense Fleabitten Patterns.”) 


Here is a fleabitten grey gelding where the larger spots look more like patches of color than clusters of fleabites. Even so, their distribution across the coat looks similar to that of the larger fleabites and not much like blood marks.

Fleabitten grey gelding with several round patches of color mixed with smaller speckles

So, what are blood marks if they are not concentrated areas of fleabites? To explain what I think is happening with horses like the one at the top of this post, I need to explain something about the greying process.


Most horse people know that grey horses are born dark and get progressively lighter with age. However, when we say a grey horse is born dark, we don’t just mean darker than it will be as an adult. Grey horses are typically born darker than they should be, given their original color. There is a saying among Percheron breeders—working with a breed almost always black or grey—that the “black ones are born gray, and the grey ones are born black.” That is because foals are typically born a more muted version of the color they will be as adults; a foal destined to be black will usually be a mousey gray. Conversely, a foal destined to turn grey is born darker than average. This is known as hyperpigmentation. 


It is why bay horses turning grey often look black at the dappling stage but then develop fleabites that look very red in comparison. The coat had been hyperpigmented, making it appear much darker than the horse’s genetics would generally dictate.


close-up of the hindquarters of a horse changing from dappled grey (looking black-based) to fleabitten (with red speckles)

Normally, horses generate pigment along with each change of coat. They continue to produce pigment as needed—or at least they do so until advanced age. Grey horses gradually stop making pigment in their hairs, losing more capacity with each coat change. Hyperpigmentation can be thought of as a change in the timing of the life cycle of pigmentation. Instead of pigment spread out over the lifetime of the animal, a grey horse gets too much pigment up front and then doesn’t have enough in the years to come. 


What I suspect may be going on with blood-marked greys is that in random patches, the timing of the process is altered. This might explain why the area becomes more noticeable with age. The patch (or patches) might more closely match the rest of the coat when the horse is young, but as the horse ages, the delayed greying of the patch makes it more visible as the rest of the horse turns white. The endpoint of the greying process would be different, too, because most blood marks do not progress to white-grey.


The blood marks’ timing and endpoint vary among individual grey horses, which explains why the Quarter Horse (top) has markings with a softer contrast, while the markings on the Arabian mare (below) have more. 

QH mare with pale blood mark on her neck and face

barrel and hindquarter of an Arabian mare, showing large, dark blood marks

In this way, blood marks could be considered the opposite of chubari spotting. Chubari spotting, or Tetrarch spotting as it is sometimes called, is a pattern of paler oval spots on the coat of a greying horse. The spots look paler because those areas of the coat are greying faster than the rest of the horse. Eventually, the rest of the coat catches up, and the chubari spots fade away. Conversely, blood marks don’t fade like the rest of the coat, making them more visible with age.

hindquarters of a dark dapple grey QH showing a dozen or so chubari spots

Blood marks are also different from chubari spots in that the latter are roughly symmetrical. Normally, a horse with chubari spotting on one side will show similar spotting on the other side. 


The other side of the dapple grey QH from the previous image showing a similar pattern

The placement of blood marks, however, is often asymmetrical. This is the other side of the Quarter Horse mare at the top of this post. (Notice that this mare is not fleabitten grey.)

the QH mare from the beginning of the post, showing her left side which is plain white-grey (no fleabites)


This is the other side of the extensively patterned Arabian mare.  

the right side of the blood-marked Arabian showing a more limited mark on her hip

The asymmetrical placement of blood marks—and the fact that they do not seem heritable—suggest they are a type of somatic marking. To me, altering the speed of the greying process seems like the sort of thing that might occur in this type of scenario. 


The role of timing in grey coloring is a fascinating topic, not just as it relates to blood marks or chubari spotting. It has been known for a while that selection based on the speed of greying was effective. There were also outlier cases in both directions: horses that were near-white at birth and horses that did not show signs of greying until late in life. I wrote about one of the better-known cases of late greying in the blog’s early days. I will pull that post from the archives and follow it up with some of the cool things that have been discovered about greying since it was written. 


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