Hmmm, with wild reptiles a longer focal lenght might be better. I guess you'll be buying from a store? Or from the internet?
If its a store, try to see what fits your needs. Most stores don't mind you testing out the lense in the shop

Sigma 105 might be a good choice. I think canon has a 100mm as well. But that costs a pretty penny
*edit* price of a canon 100mm is okay I guess...*