The naked mole rats, found in African countries has evolved to feel pain differently, and are mainly unaffected by pain from carbonic acid. This troll like rodent is highly resistant to cancer can live 10-30 years longer than mice and rats. They are small, hairless, and virtually blind and live in colonies up to 300, tunneling underground serving a common queen. The naked mole rats expel carbon dioxide, which turns into carbonic acid when mixed with water. They have evolved to be insensitive to the pain from carbonic acid burns due to its acidic environments. When injected with nerve growth factor, mice and rats became sensitized to certain kinds of pain yet the mole rat doesn’t. The mole rats resistance to pain is due to a specific pain receptor known as TrkA. Animals that hibernate have the same genetic variation; they too release carbon dioxide while buried in the winter. A rare genetic condition in humans called congenital pain insensitivity is caused by a mutation in the TrkA receptor and results in the failure to feel pain.
https://www.linkedin.com/pulse/naked-mole-rats-just-dont-feel-pain-randy-bilesky?published=t
http://www.delta-optimist.com/opinion/blogs/blog-naked-mole-rats-just-don-t-feel-the-pain-1.2365549