I was lucky enough to spend 5 amazing days in the pantanal in 2006.
I only had a 17-85 IS (i was backpacking hence space was a premium) on my 20D but still managed to get plenty of wildlife photos that i am happy with.
As you are probably already aware the pantanal is generally very open and a 400DO would be great (if it fits in your luggage) for the amazing wildlife. Even with my 17-85 i still got a few photos of beautiful toucans, blue macaws, kingfishers etc but the extra reach would definitely be worthwhile.
You may find that the 400DO is not as useful in the amazon. I spent only 3 days in the peruvian amazon and did not have nearly as many chances (as in the pantanal) at capturing wildlife. This was because of the sheer density of the rainforest, we would often hear or catch a glimpse of a beautiful bird etc but it would be hidden by the dense canopy.
At the end of the day don't worry too much because you are in for one hell of a trip. Take the longest lens you have / will fit / can afford to rent but regardless of what you take you are in for a treat.
I've just thought of the issue of having to change your lenses while in the heat / dust (depending on when you are there) / humidity and 20 million mozzies. That's a tough one because the 400DO will be too long for when a cayman (alligator), giant otter or hopefully the elusive anaconda (we lucked out on that one) pops up next to you. That's the beauty of the pantanal, you are constantly surprised by what you stumble upon.
If I was going back (hopefully one day) I'd probably compromise and just leave a 70-200 lens on for the majority of the time and not try and change lenses every 5 minutes. Although this may not be much help for you.
I hope this helps, have a great trip and feel free to ask anymore questions
i think i asked about experience in the Pantanal a few months ago, but time is now growing short and i can't decide if it's worth renting (and packing) a 400DO. will also be spending 5 days in the Amazon
have had very little luck finding any web sites with useful info
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