Since it's just a hobby, I dont know if I'd spend the money right now on anything that will pigeon hole you in one category. If you were to spend the money on a 105macro, a fine lens indeed, then that lens is somewhat useless for many other things you might like to shoot.
Your lens you have now is a good mix of semi-wide angle and a short telephoto, so I I was in your shoes and looking for my next lens, I'd probably go with a mid to medium telephoto. Something that is going to get you closeup to the wild life you say you like to shoot photos of. Best bang for the buck is the Sigma 50-500 that many of the members on here can comment about. I'm looking at this lens myself as soon as I can save up the dough.
One thing to keep in mind, if you want to try some macro stuff, buy a set of Kenko Tubes, or a Canon 500D close-up filter. This way any lens you have can now be a macro when you want it to. Saves you a ton of money, and boy are they great fun to play around with, especially if you enjoy any of Dalantech's work!!
Those are my thoughts on the subject