Seal brown (horse)

The tan hair around the eyes and muzzle with a very dark body color is typical of seal brown.

The research behind the classification of seal brown as distinct from dark bay is quite new, and as a result, opinions vary on what constitutes a true seal brown. In Equine Color Genetics, Dan Phillip Sponenberg wrote "In general, all dark colors with black points that are lighter than black but darker than bay are called brown."[3] In this text, he classifies black-pointed, clear reddish coats of any shade as bay, and black-pointed coats of any shade with black countershading as brown.[4] These definitions, while precise, are no longer accurate in light of current research.

Seal brown is best described as a black or nearly-black coat with reddish or tan hairs on the "soft parts": the muzzle, eyes, inner ears, underbelly, behind the elbow, and in front of the stifle.[5][6][7][8] Like other coat colors, seal browns can range in shade. The very darkest are just about black except for their tan areas. Lighter examples are easily confused with dark bays. The mane, tail, and legs are always black.[5]

Non-horse people often refer to many horse coat colors as "brown," in particular the bay color. Among horse aficionados, a common assessment is that "...[the term] is only used by people with one horse or with two hundred."[3] The implication is that lay observers will refer to a horse's coat color to be "brown" due to a lack of vocabulary, and those discussing large populations of horses will use "brown" out of a need for a more specific vocabulary. The term "seal brown" is unlikely to be part of a novice's repertoire and is therefore preferable when discussing this specific coat color. This coat color is, illuminatingly, called "black and tan" in some languages.[6]

This horse looks "brown", but the non-black lower legs give him away as a dark liver chestnut.

The presence of other coat color genes can modify a seal brown coat. The seal brown family includes:

• Brown Buckskin, is a result of the dilution effect of a single copy of the cream gene. Sometimes called smoky brown. The black areas of the seal brown coat are unaffected or slightly lightened, while the reddish areas are more golden. These should not be confused with traditional buckskins.[14]
• Sable champagne, a result of the dilution effect of the champagne gene. Like all champagnes, sable champagnes have hazel eyes and pinkish, freckled skin. The coat is a flat, diluted grayish- or purplish-brown, somewhere between the warm pumpkin tones of the bay-based amber champagne, and the cool purplish tones of the black-based classic champagne.[15]
• Brown dun, a result of the dilution effect of the dun gene. Like all duns, brown duns have conspicuous primitive markings including at least a dorsal stripe and darker points. The primitive markings of brown duns are black, and the coat color is somewhere between the slate gray of a grulla and the tan of a bay dun.[16]

The appearance of a horse's skin, eye, and coat-color is determined by pigment chemicals called melanins. Two types of melanins are used by mammals, such as horses and humans: eumelanin, which is visually black to brown, and phaeomelanin, visually red to yellow. Specialized cells in the skin and eyes, melanocytes or pigment cells, produce melanins and deposit them into the skin and hair using complex chemical reactions. The instructions for these chemical reactions are genetically encoded as DNA, and are therefore inherited. To be seal brown, a horse must have at least one copy of the functional MC1R gene (E/E or E/E^e) and must have one of the following genotypes at the Agouti locus: A/A or A/a.

One protein with an important role in eumelanin production is Melanocortin 1 receptor (MC1R). The gene which encodes a functional MC1R protein occupies the Extension locus, or chromosomal position, and is symbolized by the capital E. A mutation, or change, in the equine MC1R gene, that resulted in a non-functioning MC1R protein, was identified in 1996.[17] This form of the gene is symbolized by the lowercase e or sometimes E^e. Since each horse has two copies of the MC1R gene, one from each parent, horses with one "broken" copy (e) can still produce eumelanin in the hair. However, if a horse has two copies of the "broken" allele and therefore no copies of the functional E allele, the horse is completely incapable of depositing black pigment in the hair. Such horses are chestnut, or at least, are not black, bay, or seal brown.

The gene with the greatest role in the seal brown coat phenotype is Agouti signalling peptide (ASIP) or simply Agouti. The functional Agouti signalling peptide (protein) acts as a switch between red-yellow phaeomelanin and black-brown eumelanin. It is ASIP that is responsible for the alternate banding of dark and light on animal hair, although it is also responsible for whole-body effects. ASIP attaches to MC1R molecules to temporarily prevent the latter from continuing the production of eumelanin, and so phaeomelanin is produced in its stead. An individual horse can only have two copies of the Agouti gene, with the following known alleles or options:[5]

• a (mutation, recessive) uniformly black coat.
• A (mutation, 2^o dominant) black restricted to black points, responsible for standard bay.

Theories of the existence of additional alleles on the Agouti locus have been proposed, but have not been verified. These are [5][18]

• A^t (mutation, 3^o dominant) poorly restricted black points, responsible for seal brown.
• A+ (wildtype, dominant) highly restricted black points.

The A and A^t alleles cannot be distinguished by DNA testing. A test had been developed, but the underlying studies were not peer-reviewed and the test was pulled from the market due to inconsistent results[1] Genetic tests exist only for two forms, dominant A and recessive a.[19] The genetic causes of seal brown and wild bay are still thought to be at Agouti.[20]

An early version of the currently-accepted equine Agouti gene theory was first presented in 1951 by Miguel Odriozola in A los colores del caballo, subsequently reviewed by William Ernest Castle in Genetics.[21] This theory prevailed until the 1990s, when discoveries of similar conditions in other species provided alternate explanations.

Black and pangaré

For a period, the seal brown phenotype - black or near-black coat with tan or red hairs on the soft areas - was described as a true black coat affected by pangaré, or mealy-factor.[22] Pangaré is a quality common to the Przewalski's horse and so-called primitive horse breeds such as the Exmoor Pony. The trait is characterized by pale hairs, typically off-white to light tan, around the eyes, muzzle, and underside of the body.[23]

These Exmoor ponies may be dark bay or seal brown, but exhibit pangaré, which is pale in comparison to the reddish areas on a seal brown.

This theory was discarded when the equine Agouti gene (ASIP) was sequenced in 2001, finding all horses fitting the seal brown phenotype did not possess the homozygous recessive a/a Agouti genotype.[24]

Extension-brown and dominant black

The allure of a pure black coat on a horse has struck horse breeders for centuries, resulting in all-black breeds like the Friesian horse. The breeding of pure black horses is attended by two problems: some black coats fade with exposure to light and sweat, and breeding two "black" horses together would sometimes produce non-black horses. In some cases, faded true black horses have lighter coats than the darkest near-black horses.[5]

Faded black horses (a/a), may be mistaken for seal brown or dark bay.

To account for this, W.E. Castle postulated that there was a third allele at the Extension locus: E^D or "dominant black". Based on the existence of such conditions in other animals, Castle suggested that the dominant black gene (E^D) would override the "points" pattern of dominant Agouti (A) and produce black or near-black horses, which could then go on to have bay offspring. The implication was that the seal brown coat color, which is often quite nearly black, could be produced by this allele.[26]

Similarly, Sponenberg once hypothesized an Extension-brown (E^B) allele, dominant over the wildtype E. He described an allele responsible for black countershading, or sootiness, which would distinguish all shades of brown from all shades of bay.

Both theories were laid to rest after the characterization of the equine MC1R or Extension, which showed no such alleles.[24] However, it remains likely that a genetic control for sootiness does exist.

Both Dark Bay horses, which have a black mane, tail, and legs with a dark reddish brown or sooty coat, and seal brown horses, which have very dark brown coats in addition to black "points", with reddish or tan hairs around their muzzle, eyes, elbows, and flanks have one of two genotypes at the Agouti locus: A/A or A/a.[19] Both coat colors exhibit a broad range of potential shades due to a variety of factors including the bleaching or fading of black hair, nutrition, and the presence of sooty or countershading factors.

Many black horses fade, sunbleach, or otherwise undergo a lightening of their coat color with exposure to sunlight and sweat. These horses are often mistaken for seal browns or dark bays. Horses which do not undergo such fading are now usually called “non-fading” black, though other terms were used in the past.[27] However, though hypothesized, there does not appear to be a separate “non-fading” allele for black, either.[19] Mineral and vitamin deficiencies can also contribute to a lighter coat, similar to sunbleaching.

Rich red tones, top-down sootiness with dappling, and relatively restricted black points identify this dark bay.

Black-pointed horses that are not uniformly black often exhibit a trait called sootiness. A sooty coat exhibits a mixing of black or darker hairs more concentrated on the dorsal aspect (top) of the animal, and less prevalent on the underparts. Sootiness is thought to be a form of countershading. Horses without any sootiness are termed "clear-coated". Sootiness can be minor or quite extensive, and often includes dappling. Dark bay horses are typically sooty. The difference between the top-down distribution of the sooty trait and the lighter soft areas of a seal brown can also be difficult to distinguish from one another. The team of French researchers who developed the DNA test for the recessive a allele also discussed the possibility that Extension might be dosage-dependent. They found a statistically significant tendency for lighter bays to be heterozygous for the dominant, wildtype Extension allele (E/e, also written E⁺/E^e) while darker bays were more often homozygous (E/E). The authors acknowledged that other factors could play a role, and that the claim needed to be studied on a greater scale.[28] This type of dosage-dependent behavior was not observed with Agouti.[29]