Naked mole-rats: A case study in biological weirdness

byeukaryote1mo19th May 201911 comments


Epistemic status: Speculative, just having fun. This piece isn't well-cited, but I can pull up sources as needed - nothing about mole-rats is my original research. A lot of this piece is based on Wikipedia.

When I wrote about “weirdness” in the past, I called marine invertebrates, archaea viruses, and Florida Man stories “predictably weird”. This means I wasn’t really surprised to learn any new wild fact about them. But there’s a sense in which marine invertebrates both are and aren’t weird. I want to try operationalizing “weirdness” as “amount of unpredictability or diversity present in a class” (or “in an individual”) compared to other members of its group.

So in terms of “animals your hear about” - well, you know the tigers, the mice, the bees, the tuna fish, the songbirds, whatever else comes up in your life. But “deep sea invertebrates” seems to include a variety of improbable creatures - a betentacled neon sphere covered in spikes, a six-foot long disconcertingly smooth and flesh-colored worm, bisexual squids, etc. Hey! Weird! That’s weird.

But looking at a phylogenetic tree, we see really quickly that “invertebrates” represent almost the entire animal tree of life.

Image from Telford et al (2015)

Invertebrates represent most of the strategies that animals have attempted on earth, and certainly most of the animals on earth. Vertebrates are the odd ones out.

But you know which animals are profoundly weird, no matter which way you look at it? Naked mole rats. Naked mole-rats have like a dozen properties that are not just unusual, not just strange, but absolutely batshit. Let’s review.

1. They don't age

What? Well, for most animals, their chance of dying goes up over time. You can look at a population and find something like this:

Mole-rats, they have the same chance of dying at any age. Their graph looks like this:

They’re joined, more or less, by a few species of jellyfish, flatworms, turtles, lobsters, and at least one fish.

They’re hugely long-lived compared to other rodents, seen in zoos at 30+ years old compared to the couple brief years that rats get.

2. They don't get cancer

Cancer generally seems to be the curse of multicellular beings, but naked mole-rats are an exception. A couple mole-rats have developed cancer-like growths in captivity, but no wild mole-rat has ever been found with cancer.

3. They don’t feel some forms of pain

Mole-rats don’t respond to acid or capsaicin, which is, as far as I know, unique among mammals.

4. They’re eusocial

Definitely unique among mammals. Like bees, ants, and termites, naked mole-rats have a single breeding “queen” in each colony, and other “worker” individuals exist in castes that perform specific tasks. In an evolutionary sense, this means that the “unit of selection” for the species is the queen, not any individual - the queen’s genes are the ones that get passed down.

They’re also a fascinating case study of an animal whose existence was deduced before it was proven. Nobody knew about eusocial mammals for a long time. In 1974, entomologist Richard Alexander, who studied eusocial insects, wrote down a set of environmental characteristics he thought would be required for a eusocial mammal to evolve. Around 1981 and the next decade, naked mole-rats - a perfect match for his predictions - were found to be eusocial.

5. They don’t have fur

Obviously. But aside from genetic flukes or domesticated breeds, that puts them in a small unlikely group with only some marine mammals, rhinoceros, hippos, elephants, one species of boar, and… us.

You and this entity have so much in common.

6. They’re able to survive ridiculously low oxygen levels

It uses very little oxygen during normal metabolism, much less than comparable-sized rodents, and it can survive for hours at 5% oxygen (a quarter of normal levels.)

7. Their front teeth move back and forth like chopsticks

I’m not actually sure how common this is in rodents. But it really weirded me out.

8. They have no regular sleep schedule

This is weird, because jellyfish have sleep schedules. But not mole-rats!

9. They’re cold-blooded

They have basically no ability to adjust their body temperature internally, perhaps because their caves tend to be rather constant temperatures. If they need to be a different temperature, they can huddle together, or move to a higher or lower level in their burrow.

All of this makes me think that mole-rats must have some underlying unusual properties which lead to all this - a “weirdness generator”, if you will.

A lot of these are connected to the fact that mole rats spend almost their entire lives underground. There are lots of burrowing animals, but “almost their entire” is pretty unusual - they don’t surface to find food, water, or (usually) mates. (I think they might only surface when digging tunnels and when a colony splits.) So this might explain (8) - no need for a sleep schedule when you can’t see the sun. It also seems to explain (5) and (9), because thermoregulation is unnecessary when they’re living in an environment that’s a pretty constant temperature.

It probably explains (6) because lower burrow levels might have very little oxygen most of the time, although there’s some debate about this - their burrows might actually be pretty well ventilated.

And Richard Alexander’s 12 postulates that would lead to a eusocial vertebrate - plus some other knowledge of eusociality - suggests that this underground climate, when combined with the available lifestyle and food source of a molerat, should lead to eusociality.

It might also be the source of (2) and (3) - people have theorized that higher CO2 or lower oxygen levels in burrows might reduce DNA damage or related to neuron function or something. (This would also explain why only mole-rats in captivity have had tumors, since they’re kept at atmospheric oxygen levels.) These still seem to be up in the air, though. Mole-rats clearly have a variety of fascinating biochemical tricks that are still being understood.

So there’s at least one “weirdness generator” that leads to all of these strange mole-rat properties. There might be more.

I’m pretty sure it’s not the chopstick teeth (7), at least - but as with many predictions one could make about mole rats, I could easily be wrong.

To watch some naked mole-rats going about their lives, check out the Pacific Science Center’s mole-rat live camera. It’s really fun, if a writhing mass of playful otters that are also uncooked hotdogs sounds fun to you.