Costa Rica Animal Photography - Lens Recommendations for R50

I'm going to Costa Rica this summer and plan on visiting at least two of the major national parks for some hiking and animal photography. I love shooting reptiles/amphibians, but due to where I live, that's typically limited to zoo settings with stable lighting and enclosed environments.

I have a R50 with a 16mm pancake and a 100-400mm telephone lens. I also have the stock 18-45, but don't care for it.

The 16mm is great for landscapes and macros (but only if I can get crazy close up). The 100-400 does very well for larger, stationary animals in brighter lighting at a medium distance, but struggles with low lighting, closer distance, or movement.

My concern is that a lot of the reptiles/amphibians tend to prefer darker areas, get skittish if you get too close, and deserve a respectable amount of distance for safety reasons.

I'm not a pro by any means and do this as a hobby.

Looking to see if the lenses I have will be good enough or if something else may be necessary to take advantage of the opportunity. Not looking to spend thousands, but I'm happy to buy used.