Beneath the hot, dry grasslands of East Africa, there lives an animal whose weirdness and hardiness are legendary. The naked mole rat, a hairless rodent with a tiny, pig-like snout and nubby ears, lives in underground colonies of tunnels and nests that can stretch for miles. As many as 300 of the rodents work together in these burrows, united around a single queen, who is the only member of the colony who can reproduce. It's a hardscrabble life, with little food and even less water.

And yet in this harsh environment, under extremely crowded conditions, the naked mole rat has evolved to be virtually indestructible: these small mammals almost never get cancer, live to be over 30 (much longer than other rat species), and they are insensitive to acid burns. Now a new study in Cell Reports reveals one secret behind these rats' abilities. Evolutionary tweaks to the amino acids in their pain receptors make naked mole rats extremely insensitive to pain after they are born.

Naked mole rats probably wouldn't have evolved this incredible adaptation if it weren't for their unusual habitat and social arrangements. These rodents are one of only two mammals known to be eusocial like ants and bees—as mentioned earlier, they have one reproductive female per colony (the other eusocial mammal is also a mole rat). Females fight, often to the death, for the privilege of becoming queen and can reign for more than 15 years. Successful colonies become very large, with workers digging out tunnels and rooms with their teeth. Though their nests are big, many individuals are still crowded together in them, which no doubt causes some discomfort.

Under these conditions, the naked mole rat is likely prone to a condition called thermal hyperalgesia. Humans have the same condition, which we generally call heat sensitivity. It's what happens when your skin has already been burned or injured in some way, so an ordinary level of heat feels unbearably hot (think of running warm water over a scrape). When this happens, it's because sensory receptors on your skin have been chemically "sensitized" by inflammation or high temperatures. Once those receptors are sensitized, even a little bit of heat will cause sensory nerves to shoot signals to your brain that register as pain.

When naked mole rats dig in rough soil and constantly come into contact with other members of their colony, their skin is irritated. They get thermal hyperalgesia—but it doesn't cause them any pain. To figure out why, a group of researchers isolated naked mole rat nerve cells and exposed them to minute quantities of the hot chili chemical capsaicin. What they discovered was that a few tiny changes in the amino acids on the animals' TrkA receptor (a receptor on sensory neurons) prevented them from feeling pain from heat sensitivity.

Max-Delbruck Center for Molecular Medicine physiologist Gary Lewin, lead author on the Cell paper, said that there are many reasons why this power might have evolved:

We think evolution has selected for this tweak just subtly enough so that the pain signaling becomes non-functional, but not strong enough that it becomes a danger for the animal. They live in desert regions underground, and they have to do a lot of work to get their food. They have the lowest metabolic rate of any mammal. Evolution has shut down everything that is not absolutely necessary—including extra nerve receptors.

Lewin and his colleagues also found that naked mole rats are born with heat sensitivity but lose it as they grow up. This process likely shuts down discomfort and helps them conserve more energy as they age. What's intriguing is how such a small chemical change could result in a huge transformation. "Even though the naked mole rat's version of the TrkA receptor is almost identical to that of a mouse or a rat, it has a very significant effect on the animal's ability to feel pain," Lewin explained.

In the naked mole rat, we see a fascinating example of an evolutionary adaptation that spares an animal from extreme discomfort. Pain does not always make us stronger. Indeed, in some cases, feeling pain is such a liability that animals have adapted to stop experiencing it entirely.

Cell Reports, 2016. DOI: 10.1016/j.celrep.2016.09.035