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What is a marten sable?
A normal sable mouse is a red tan (Ay/at C/-), and has a dark brown back which fades down to a bright tan belly. In simple genetic terms a marten sable is a normal sable diluted by the chinchilla gene (Ay/at c[ch]/c[ch]). This dilutes the tan of the sable to white, whilst leaving the top of the mouse dark otter brown. In truth, it's rather more difficult than this. The modifiers needed to give a dark sepia back and a snow white belly take a lot of condensing over generations. Even then the resulting martens are very variable in appearance and when decent martens are appearing regularly in litters, the breeder will still find a lot appearing with a light top and white belly, or a dark top and yellow-cream belly. In a lot of respects it's similar to breeding a marked variety, in that only a small percentage of mice bred are suitable for showing.

Tips for breeding marten sables:
As a double dose of the dominant red gene is lethal, all marten sables have one red gene and one tan gene; which means every litter from marten sable parents will produce silver foxes. These aren't usually good enough to show, but do need to be kept for breeding to keep the strength and contrast of colour in the marten sable. It is helpful, but not necessary, to keep a show line of black silver foxes to provide the occasional outcross.

If one can't find marten sables to start with, the best way to start breeding them is by crossing dominant red or sable with black silver fox, which results in a litter containing sables and black tans. The black tans won't be useful for breeding or showing but the sables can often be shown. The sables from this litter should be bred back to black silver fox, which results in a litter containing sable, marten sable, black tan, and black silver fox. From there it is a case of refining the colour and shading by breeding marten sable to marten sable and marten sable to black silver fox. If the shading is becoming a delineated line, marten sable x marten sable is needed. If the contrast is disappearing from the top and belly, marten sable x silver fox is needed.

Marten sables are prone to obesity, although this seems to be less severe than in regular sables, and if they get too fat they may become lazy breeders and/or infertile. I find it helpful to make sure that a buck always has a doe in with him, to keep him fit and active. Attempting to 'diet' mice by feeding less or feeding a nutrient-poor diet only results in the mice being in poor condition, it will not keep the weight off because the weight is a direct side effect of the dominant red gene. Mice should always be allowed to eat their fill.

Above: young marten sable doe showing a good dark face, but the shading is not gradual enough
and is in danger of becoming a demarcation line, and the belly is not white enough.

What colours can they come in?
Marten sables can be bred in various different colours by using different dilute genes to change the colour of the back, but only the dark otter-brown form is currently standardised in the UK.

Outcrossing marten sables
The only good outcross for marten sable is black silver fox, and it is necessary to breed silver fox into a marten sable line on a regular basis. Black foxes from lines that throw chocolate or blue foxes should be avoided as these will result in poor coloured or unstandardised marten sables down the line. PEW or cream might be added for improving type, but it will take a long time to get the colour contrast back. Red and sable can be used if no foxes, whites, or creams are available, but this outcross will set back the whiteness of the belly fur a long way.

Chinchilla might be a viable outcross for marten sable, and I don't see why it wouldn't be, but I have no experience with this cross so can't recommend it.

Above: black silver fox. Good examples of these are extremely useful in a marten sable line.

Common faults in marten sables:
The most common fault in marten sables is a pale face. According the the NMC standard for the variety, the dark brown should continue from nose to tail. This is difficult to achieve without losing the shading down the sides of the mouse, but it is not impossible.

Another common fault is a cream tinted belly. Marten sable bellies should be snow white, the fur thick and smooth; which is why a line of good silver foxes is useful to keep on hand for an outcross. Some marten sables can have quite a yellowy tinge to the side shadings and face in particular. This should be avoided and cleaned up by crossing to decent black silver foxes.

Above: mature marten sable doe with a much too pale face, but a lovely white belly.
(She has also had her whiskers barbered by a cagemate)

Showing marten sables:
Like all shaded mice, marten sables are a bit of a headache to show. They are prone to the horrible patchiness caused by casting, and you may enter a spectacular example on a Tuesday evening, only for that mouse to have cast out into patches by the time of the show the following Saturday - and once a mouse has cast into patches, it will always be patchy. To show regularly, you need to keep quite a few and make sure they are all of different ages, in the hope that at least a couple of them won't be casting on show day. Does suffer from this more than bucks, as hormones regulate casting and a doe's hormones have a such a short cycle. This means that to show marten sables competitively, you're likely to need to keep a lot of bucks. This means that you need to have a lot of cages, as each buck will need his own cage. Bucks also tend to have thicker belly fur, which helps the snow white effect as the skin underneath is less likely to show through. The dominant red gene means that they will run to fat eventually; this and the casting mean even a good marten sable will have a short show life.

Above: an example of the patches caused by casting. Although these move around the body, once it's happened the mouse is forever patchy and can't be shown.