In a mandarin orchard today, I saw what appeared to be sun burn damage.  The bark was missing on the top, south facing branches. It was old damage and was healing over along the edges.  I mentioned it to the grower who told me it was roof rat damage.  Several years prior when we had had a long dry winter, the rats had come out of the hills and were eating his fruit, as well as feeding on the cambium.  I've seen this damage in trees near the edges of wild country, as well as along stream and river beds.  At one orchard in Santa Paula, the feeding had actually spread Phytophthora in the canopy of lemon trees.  For control measures go to:

http://www.ipm.ucdavis.edu/PMG/r107600711.html