Why do some black dog coats get sun burned and get a rusty color why other black coats don't?

my dogs are the same. they are all black and white but the smallest poochi has his black fur changing to red from the sun. he seems to like sunning himself more than the husky and my border collie. it must be the hair folliclules that make the hair go lighter just like some people who get high lights from the sun and it seems like they had it streaked. just watch the skin so it doesn't get sunburnt and u can put sun screen on the one who is more prone to the chnging color or limit his exposure. apply it on his nose,ears and any fur less spots.
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I don't think it's sunburn as much as genetic variation in the dog… brown hair genes that allow lightening in the sun…
much like dark haired humans whose hair lightens or turns red in the summer….
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its not sunburn! it is a mineral deficency.( and old age will do it too) black angus cattle and some sheep show it too. i cant remember off hand, what mineral, sorry. type in PAT COLBY to google and read her pages, she has info there- its magnesium i think- but do go check! she does a lot of good stuff on all animals but goats are her main love. a lot of it applies to many creatures. including us!
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wide reading habits and age

This is an interesting question.

In Siberians specifically, a lot of it depends on whether or not the black dog is red-factored. By that, I mean whether or not it has genes for red coat in its background. Black dogs with "red" genes seem to have a little bit of that red in their coat, which means that, when the black coat dies or gets burnt, it will turn brownish. Dogs who are not red-factored (do not carry the gene for red coloring) do not have this problem.

It is a well-known fact in the Siberian world that red coats sunburn easily. So it would make sense that this "red tint" in a black coat would burn and make the coat look brown.

I have a black/white Champion male who is out of two grey/white parents. However, his coat does burn brown in the sun, and he does produce red in his litters. Red is a recessive color, and so both parents must carry it in order for a puppy to be red.

I also had a black/white female out of a grey/white parent and a dark red/white parent who burned brown in the sun.

However, I have black/agouti dogs with no red-factoring who never burn brown.

This may be more info than you wanted. But I wouldn't chalk the burnt coat up to any mineral deficiencies in your dogs, especially if you know for a fact that the two dogs in question come from a red parent.

Hope this helps!
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Siberian husky owner/exhibitor, musher, rescue, breeder for 20 years.