Beneath the hot, dry grasslands of East Africa lives an animal whose strangeness and hardiness are legendary. The naked mole rat, a hairless rodent with a small pig snout and studded 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 that can reproduce. It’s a hard 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 past 30 (much longer than other rat species), and are impervious to acid burns. Now a new study in Cell reports reveals a secret behind these rats’ abilities. Evolutionary adaptations 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 rule for more than 15 years. Successful colonies grow very large, with workers excavating tunnels and chambers with their teeth. Although their nests are large, many individuals are still crowded together, which undoubtedly causes some discomfort.
Under these conditions, the naked mole rat is likely susceptible to a condition called thermal hyperalgesia. Humans have the same condition, which we generally call heat sensitivity. It’s what happens when your skin is already burned or injured in some way, so a regular level of heat feels unbearably hot (think running hot 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 fire signals to your brain that register as pain.
When naked mole rats dig in rough soil and come into constant contact with other members of their colony, their skin is irritated. They get thermal hyperalgesia, but it doesn’t cause pain. To find out why, a group of researchers isolated naked nerve cells from mole rats and exposed them to minute amounts of the hot chili chemical capsaicin. What they found was that a few small changes in the amino acids on the animals’ TrkA receptor (a receptor on sensory neurons) prevented them from feeling pain through heat sensitivity.
Max-Delbruck Center for Molecular Medicine physiologist Gary Lewin, lead author of the Cell paper, said there are many reasons why this force could have evolved:
We think evolution chose this adaptation just subtle enough that the pain signaling becomes non-functional, but not strong enough that it becomes a danger to the animal. They live underground in desert areas and have to do a lot of work to get their food. They have the lowest metabolic rate of all mammals. Evolution has shut down everything that isn’t 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 age. This process likely rules out discomfort and helps them conserve more energy as they age. What’s intriguing is how such a small chemical change can result in a huge transformation. “Although the version of the naked mole rat’s TrkA receptor is almost identical to that of a mouse or rat, it has a very significant effect on the animal’s ability to feel pain,” explains Lewin.
In the naked mole rat, we see a fascinating example of an evolutionary adaptation that spares an animal extreme discomfort. Pain doesn’t always make us stronger. Indeed, in some cases, feeling pain is such a risk that animals have adapted to not experience it at all.
Cell Reports, 2016. DOI: 10.1016/j.celrep.2016.09.035