Could you imagine eating a pile of chile peppers or spicy hot mustard and not feeling any pain? The ability to sense pain is physiologically quite important as it alerts us to potentially dangerous or poisonous chemicals.
Many plants (stinging nettles, pungent bulbs, hot chilies) and animals (stinging ants, scorpions, snakes) produce noxious chemicals to protect themselves from predators. In turn, some predators have evolved resistance to these chemicals. An international team of researchers examined how 8 species of subterranean African rodents evolved resistance to pain induced by noxious stimuli.
Noxious chemicals induce pain by binding to specialized sensory neurons called nociceptors. All vertebrates and invertebrates have receptors that can sense noxious chemicals. Similar to people and mice, Emin’s mole rat (Heliophobius emini), Common mole-rat (Cryptomys hottentotus hottentotus), Mahali mole-rat (Cryptomys hottentotus mahali), and Damaraland mole-rat (Fukomys damarensis) are sensitive to various types of noxious chemical stimuli (acid, mustard and chile peppers).
Remarkably, naked mole-rats (Heterocephalus glaber) do not respond to either acid or capsaicin (chile pepper) but they do respond to a chemical found in mustard. In contrast, Natal mole-rat (Cryptomys hottentotus natalensis) had no response to capsaicin, even though they were responsive to mustard and acid. Cape mole-rat (Georychus capensis) and East African root rat (Tachyoryctes splendens) were insensitive to acid whereas highveld mole-rat (Cryptomys hottentotus pretoriae) were insensitive to mustard.
Variations in resistance to these noxious stimuli likely evolved as these animals lived with stinging ants and dined on roots and other plant parts that produce pungent chemicals, which over time resulted in molecular changes in their pain receptors.