Chestnut with Dorsal?

I have a chestnut gelding who - in the summer - has a dark brown dorsal stripe running along his back. (He also gets dark dapples, but that's aside from the point )

It's not very dominant, but you can still see that it's there.

Now, I know for sure he's not a red dun, for he's a dark chestnut horse. I don't know whether it's something about the sun or nutritional wise that brings it out in him, or maybe he carries some sort of gene, but I'm dying to know what's going on with him. O__O

I'd appreciate any help or ideas.

Many bay/chesnut/brown horses have dorsal stripes. Maybe one of the gene people on here can explain better!

It is most likely counter shading. If the horse is truly a red dun he must have one dun parent. However there are a lot of sorrel/chestnut and even bay horses who have counter shading.

Counter shading. I'll have to look up on that. Thanks for the information.