The Animals Have Shorter Lifespans
One effect of the high levels of radiation is that the animals who live there seem to have much shorter lifespans. They still live long enough to reproduce, which enables their species to continue to flourish. However, they live significantly shorter lives than other members of their species outside of the exclusion zone.
One reason is that radiation has permeated every level of the food chain. The plants grow in toxic soil, feeding radiation to the herbivores. When bugs bite them or carnivores eat them, they also ingest radiation. So they just aren’t living as long.