odd stripes across back?

[ChubbyPony]

does anybody know what these are? you can see them now the winter coat is through?
(i have cropped the pic as im in it looking nasty, but this is the end of his back/hind quarters)
you can see the striped across his spine.

 

[ChubbyPony]

oh no the photos gone all big and blurry...its not very clear :/ how do i fix that?