Naked mole-rats can be immune to pain.

Naked mole-rats are burrowing rodents with an unusual ability to withstand certain types of pain. Their crowded, poorly ventilated underground colonies can have extremely high levels of carbon dioxide and other chemical irritants, which causes acidosis in the body’s tissues and normally activates pain-sensing nerves in mammals. Naked mole-rats, however, are essentially insensitive to acid-induced pain: Scientists have found a species-specific genetic mutation prevents their pain-sensing neurons from responding normally to acid.

These creatures are also behaviorally insensitive to the pain caused by capsaicin, the compound that makes chili peppers hot, although this appears to involve a different mechanism than their resistance to acid. Naked mole-rats aren’t immune to all types of pain, however; they respond normally to acute mechanical pain,such as pinching, as well as to the pain caused by heat.