[Fiji Hill]

This is stupid and immature, not cruelty. Natural oils will break the Bond between fur and ink with no harm.

However, cats regularly cleanse themselves by licking their coats, so they could ingest the ink.

[cyclotic]

Which if you check the Sharpie website, you will find is non-toxic.

My son recently had some pins implanted in his broken elbow. The doctor wrote on his arm with a Sharpie to mark which arm and surgical locations.