What I've heard was that the sheer brutality of their training made them accustomed to discomfort and the psychological indoctrination they received left them with a fear of the dishonor of failure and a resultant fatalism. The result was that they bore up well under the hardships of the jungle and were expected to do so by their leaders. That didn't make them supermen. But the facts that the jungle was such a hostile environment and that Americans were more humane to their own men and didn't expect them to live in inhumane conditions may have given the Japanesean initial edge. However, when one side is focused on giving their lives for their country and the other side are focused on obliging their enemy more than giving their own lives, it seems to make for a difference in the long run.
Bob