In the lab, the cause of death is usually hard to find; the main issue that shows up in necropsies, Buffenstein said, are mouth sores, indicating the animals weren’t eating, drinking or producing saliva well in their last few days and infection set in.
“We really don’t know what’s killing them at this point,” Buffenstein said.
From https://www.livescience.com/61568-naked-mole-rats-no-aging.html: