Sigma makes a 120-400mm OS lens that is physically similar in size. There is also the Sigma 150-500mm OS and Tamron 150-600mm VC, but both are considerably larger lenses -- I would expect you want to keep the weight and size of your equipment down on such a trip.
I'll also throw my hat in on recommending a 2nd body. Not only does it give you a backup, but it will also allow you to keep both a telephoto and wider lens mounted at the same time, thereby greatly reducing the number of lens changes needed in a harsh environment and allowing you to be ready for almost any shooting situation. For these types of trips, I travel with a 5D3, 7D, 24-105L, 100-400L, 16-35L f/4 IS (just got it to replace the 17-40L), and 40mm pancake. The 100-400L stays mounted on the 7D all the time for wildlife, while the 16-35 and 24-105 alternate on the 5D3. The pancake is for a light weight faster walk around on the 5D3 -- it takes up almost zero space in the bag.