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.
A suite of superpowers
The naked mole-rat's unusual traits are not limited to pain insensitivity. They exhibit negligible senescence, meaning their risk of death does not increase as they age. They can live for more than 30 years, compared to the 3-year lifespan of a similarly sized mouse. This longevity is linked to resistance to cancer. Their cells produce an extremely high-molecular-mass hyaluronan (HMM-HA), a substance that is over five times larger than the version in humans. This HMM-HA makes the tissues very elastic and prevents cancerous cells from proliferating through a process called early contact inhibition, which is activated by the p16 gene.
Their underground life has also equipped them for extreme hypoxia. A naked mole-rat can survive for at least five hours in an atmosphere with only 5% oxygen and can endure 18 minutes of total oxygen deprivation with no apparent neurological damage. They accomplish this by switching their primary energy source from glucose to fructose. This metabolic pathway, previously thought to be used only by plants, allows them to power essential organs like the brain and heart without oxygen. Their heart rate drops dramatically from around 200 to 50 beats per minute, entering a state of suspended animation until oxygen becomes available again.