As if the naked mole rat wasn’t weird enough, now researchers may have finally decoded why the sabre-toothed sausages can’t feel much pain. It’s previously been found that the wrinkly rodents are not only highly resistant to cancer – allowing them to live up to 30 years old – but they also have a reduced sensitively to pain, including being numb to the effects of acid. It turns out that this superhero ability may be down to changes in just three amino acids.

Living in perpetual darkness underground, where they live in honeybee-like caste systems, scurrying around in kilometers worth of tunnels feeding on the tubers of plants, naked mole rats are pretty high up on the list of the oddest mammals around. Their subterranean lifestyle has given rise to many of their bizarre features, including their famed nudity, highly reduced eyes, and inability to regulate their own body temperature. But it may have also helped drive another of their strange abilities: their lack of sensitivity to pain.

It turns out that the mole rats can’t feel acid or capsaicin, the active component of chili peppers. Usually, when a cell is injured or inflamed, it will release a nerve growth factor that will then bind to a receptor on the surface of sensory pain-cells. This receptor, called TrkA, will then send the message to the brain, while also increasing the activity of a particular molecular pore known as TRPV1. This has the impact of increasing our sensitivity to touch or heat, meaning that, for example, we are more likely to try and avoid the initial source of pain.

^{The mole rats live a highly unusal life, more like social insects than mammals. Theron Trowbridge/Flickr CC BY-NC 2.0}

The researchers found that naked mole rats have a small change in their TrkA receptor, and have published their findings in Cell Reports. They discovered that if they created a cell with a normal rat TRPV1 pore, but a naked mole rat TrkA receptor, the nerve cell has a reduced sensitivity to pain. They then compared the gene that coded for the naked mole rats' version to those in 26 other mammals, including five other mole rat species. They were then able to identify that it was an alteration to just three amino acids in the receptor that made it less sensitive.

This change didn’t make the receptor completely dysfunctional, but rather hypo-functional. “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,” explains Gary R. Lewin, who led the study. This could be an advantage to the hairless rodents as they live in tunnels that have dangerously high levels of carbon dioxide, and low levels of oxygen.