It’s a weird one this week, folks.
I was preparing for a lab presentation when I stumbled across a paper looking at pain insensitivity in mole-rats. I was absolutely taken with this paper – who knew that naked mole-rats (Heterocephalus glaber) didn’t feel pain from peppers or citrus? (more on this later). Why are they like this? And why did other species of mole-rats also evolve pain insensitivity??
But after some research, it turns out that pain insensitivity isn’t the weirdest thing about naked mole-rats. They also have an extremely muscular mouth, have the social structure of ants, and live a surprisingly long life.
A face only a mother could love
Naked mole-rats aren’t the most attractive animals out there. They’re basically pinkish, wrinkly sausages with tiny eyes, no ears, and gigantic teeth. The ‘naked’ part of their name comes from the almost complete lack of hair on their bodies; however, they do have sensory whiskers on their face and tails, and hair between their toes. This toe-hair serves as a broom to sweep soil out of their burrow.
These adaptations help the naked mole-rat survive in their underground homes. This species lives exclusively in underground tunnels in eastern Africa. The lack of ears means they don’t have to worry about their ears filling up with dirt as they dig these tunnels. And the wrinkly skin means that while it might snag, it won’t tear on pebbles in their burrows.
Other adaptations help naked mole-rats get the most out of the little resources available underground. For instance, they are basically cold-blooded mammals: they can’t regulate their body temperature internally and instead move to warmer tunnels or huddle together when they need to warm up. This strategy helps cut down on the amount of energy the animals need to survive.
Naked mole-rats also don’t seem to need much oxygen, which is useful when you’re underground. One study put naked-mole rats in a chamber with only 5% oxygen (about a quarter of the oxygen that’s in our air). This little oxygen would kill a mouse in 15 minutes, but the naked mole-rats were doing just fine 5 hours later. They can even survive with no oxygen for 18 minutes (although they pass out after 30 seconds).
A mouth made for chomping
A naked mole-rat’s mouth is extremely powerful. Approximately 25% of a naked mole-rat’s muscle mass is in their mouth; in contrast, humans only devote 1% of their muscle mass to their jaws. Their jaw is so powerful that naked mole-rats have been known to chew through concrete! Naked mole-rats can also move their front teeth independently from each other, kinda like chopsticks.
In another neat adaptation, naked mole-rats can close their lips behind their front teeth. This is important because naked mole-rats use their teeth to dig their underground burrows. Obviously, if they left their mouths open, they would end up swallowing a lot of dirt; by closing their lips behind their teeth, they can burrow without fear of accidentally eating a mud pie.
To build a burrow, naked mole-rats work together. The individual in front breaks up the dirt with their powerful incisors. Then, an assembly line of naked-mole rats sweeps the dirt with their feet through the tunnel system and to the surface.
The Queen’s in charge
Naked mole-rats are eusocial, which means they live as a group with a single, reproducing female queen who is supported by male and female workers. This is the same social structure that is found in insects like ants and termites. Naked mole-rats are in fact one of just two eusocial mammals (the other being the Damaraland mole-rat).
The average naked mole-rat colony is on average 70 individuals, but they can get larger (up to 295 individuals). The queen is the largest individual and the only one who breeds; the other females are reproductively suppressed. She can live for many years, and all her children will stay in the colony as workers. She maintains her position as queen by actively surveying her kingdom and bullying her subjects to remind them who’s in charge. When the queen dies, a few females will fight amongst themselves (sometimes to the death) to decide who will be queen. The winner will grow physically larger and sexually mature so that she can begin reproducing.
Worker naked mole-rats spend their days doing a variety of tasks, including taking care of the pups, foraging for food, and digging new burrows. Digging is especially important in the quest for nutrition since naked mole-rats don’t travel above ground. Instead, they dig through the earth in search of tubers.
Solder mole-rats do what you would expect: defend the colony from threats. These come in the form of snakes and foreign naked mole rats.
Can eat all the peppers they want
Naked mole-rats can heat a hot pepper with no problem. The small rodents don’t react when capsaicin, the active ingredient in chili peppers, is applied to their paws. They also don’t react to acids, which is particularly useful in a carbon dioxide-rich environment that could lead to carbonic acid in a naked mole-rat’s lungs (aka, the underground).
This insensitivity to pain is due to the naked mole-rat’s nerve cells. The receptor in our neurons that relays the ‘this is painful’ message, TrkA, doesn’t bind to cells as effectively in the naked mole-rat as it does in rats. Basically, their cells just aren’t as good at passing along the pain signal. This could be a good adaptation for the naked mole-rat: they can still feel pain when it gets really bad, but they are able to have fewer pain nerve cells. This would reduce the amount of energy needed for their brain to successfully function.
How long can they live???
Naked mole-rats are the longest-living rodent that we know of. Given their size (slightly bigger than a mouse), we would expect them to live slightly longer than a mouse. So, while mice and rats live 4 to 5 years, we would expect naked mole-rats to live about 6 years.
But they don’t.
Instead, naked mole-rats can live at least 30 years.
In addition, naked mole-rats defy Gompertz’s law. This ‘law’ is a mathematical equation that can describe aging. Basically, the risk of dying increases exponentially as you age; for us humans, it doubles about every 8 years after we turn 30. This law should apply to all adult mammals. But not the naked mole-rat! Instead, one study found that their rate of dying stayed the same after they reached sexual maturity.
Naked mole-rats also seem very resistant to cancer. Both this cancer-resistance and slow aging may be due to naked mole-rat DNA: they have very active DNA repair and, partly as a result, have very low levels of DNA mutations that could lead to cancer. Naked mole-rats also have a molecule in their cells called high-molecular-mass Hyaluronan, which helps prevent cells from overcrowding and forming tumors.
Of course, further research needs to be done regarding naked mole-rat aging and cancer resistance. But if and when we crack the naked mole-rat code, we could use this information to improve our own lives: living longer, avoiding cancer, and even treating pain better.
Not too shabby for a wrinkly sausage rodent.
Smithsonian’s National Zoo and Conservation Biology Institute
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