How to create wrinkles?

 Hello fellow users!

I'm working on a greyscale vector illustration of a rhino. Everything is going well, except I can't for the life of me figure out how to get a 'wrinkle' effect. There needs to be varying degrees of wrinkles (some thicker than others). I would upload a pic of what I'm trying to describe, but I can't figure out how to upload an attachment on the new layout (I haven't been here in a while). Because it has to be designed in vector, it's proving quite a challenge for me. I've tried using the pen tool, as well as the artistic media, but nothing is getting it quite right. Any advice/tips would be highly appreciated.

Much Thanks :D