A life without burn
In the arid grasslands of Kenya, the naked mole-rat (Heterocephalus glaber) lives a life that defies many mammalian conventions. One of its most studied adaptations is its insensitivity to certain types of pain. Specifically, these rodents do not react to acid or to capsaicin, the chemical that gives chili peppers their heat. This is not a learned tolerance but a fundamental difference in their neurobiology. The sensory neurons in their skin lack a neurotransmitter called Substance P, which is critical for sending inflammatory pain signals to the central nervous system in other mammals.
The evolutionary reason for this adaptation appears to be their unique habitat. Naked mole-rats live in large, crowded underground colonies of up to 300 individuals where oxygen is scarce and carbon dioxide levels are exceptionally high. The respired CO2 creates carbonic acid when it comes into contact with moisture in the skin, which would cause a constant burning sensation for most animals. To cope, the naked mole-rat's voltage-gated sodium channel, NaV1.7, has a mutation that prevents neurons from firing in response to acid. A small change to the TrkA receptor, which is activated by Nerve Growth Factor (NGF) during inflammation, also severely diminishes the signaling cascade that leads to thermal hypersensitivity, or the heightened pain from heat at an injury site.