There should have been areas that were not affected directly by the Death Wave. But which became unlivable due to repercussions such as acid rain and smaller amounts of Radiation. These would be lowlands which are sheltered by mountains all around, or high places on top of the mountains. These can regrow after some time. The main problem would be that the fruit and water might still have radioactive traces which might be lethal to humans, but maybe not to all animal.
We know for a fact that animals with shorter life span can remain alive, just that their lives would be shorter. One Example was the fact that in Hiroshima the cockroaches remained alive.