Imagine if the zebras instead had watched in horror as the first few "pioneers" got mauled... prodding each other saying "your turn", until all of them were too weak to be able to cross, even as a herd, and the crocodiles simply came out of the river and dragged them into it (which is exactly what would have happened, because the reason they cross rivers is that there is nothing left to eat on their side of it).
Not that I consider humans herd animals, or that I want to other humans who prey on humans so much that I would cast them as a different species. But I still think there is a lesson for us in there, too. Cowardice, as rational as it may seem, simply doesn't pay in the long run. Until we learn that, we're stuck.