Naked mole-rats: A case study in biological weirdness

post by eukaryote · 2019-05-19T18:40:25.203Z · score: 95 (38 votes) · LW · GW · 13 comments

This is a link post for https://eukaryotewritesblog.com/2019/05/19/naked-mole-rats-a-case-study-in-biological-weirdness/

Contents

  1. They don't age
  2. They don't get cancer
  3. They don’t feel some forms of pain
  4. They’re eusocial
  5. They don’t have fur
  6. They’re able to survive ridiculously low oxygen levels
  7. Their front teeth move back and forth like chopsticks
  8. They have no regular sleep schedule
  9. They’re cold-blooded
    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.
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13 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.

13 comments

Comments sorted by top scores.

comment by Raemon · 2019-06-11T20:02:30.710Z · score: 13 (6 votes) · LW · GW

Curated.

I like this post both because it's got both a lot of cool facts about an animal that were just fun to learn, as well as illustrating a good rationalist thought process – looking at a collection of surprising facts and taking a stab at generating a hypothesis as to why.

I think i'd have liked to see an attempt to generate multiple hypotheses, or exploration of reasons why the 'lives underground explains everything' hypothesis might not be true. This post seemed like a good illustration of a couple particular rationality virtues (curiosity, simplicity [LW · GW], noticing confusion [LW · GW]), but could have used more looking into the dark [LW · GW].

comment by philh · 2019-05-22T13:05:55.916Z · score: 11 (6 votes) · LW · GW

Wikipedia says the damaraland mole rat (closely related, but in a different family) is also eusocial, and apparently it too spends its whole life underground. https://en.wikipedia.org/wiki/Damaraland_mole-rat

They can also survive low oxygen levels. (I can't immediately see directly comparable numbers.) But the other weirdnesses either don't seem to apply to them, or are unmentioned.

(Fun fact: autocomplete aptly gave me "the damaraland mole rat is also ridiculous".)

comment by eukaryote · 2019-05-23T15:03:49.228Z · score: 7 (6 votes) · LW · GW

Oh, huh - I thought the Damaraland mole-rats were basically sister species of the naked mole-rats, the two most closely-related species, and so didn't consider them much. But it looks like that isn't true - they're not even the same genus. Maybe they evolved eusociality independently? Going to have to look into this, thanks!

comment by Mary Chernyshenko (mary-chernyshenko) · 2019-06-13T05:42:48.524Z · score: 9 (4 votes) · LW · GW

Asked an entomologist about "comparably weird invertebrates"; his version includes parasitic crustaceans, Strepsiptera, echinococci, Myxozoa, and Ascidiidae.

comment by Spiracular · 2019-06-14T17:00:19.383Z · score: 10 (5 votes) · LW · GW

Adding a couple of mine to this list, will probably add more as I think of them...

Invertebrates: Sacculina (parasitic barnacle that hormonally manipulates crabs), Sawflies (primitive hymenopterans with caterpillar-like larvae, some love forest fires), Ticks, Phengaris arion

Vertebrates: Chameleon, Hoatzin, Bats (just... bats)

Plants: Amorphophallus genus (The entire stalk is a single giant leaf. It sheds that leaf every few years, puts up a giant flower, then sheds that and goes back to growing another single giant leaf?), Orchids, Magnolia (ancient monocots that convergently evolved to look like dicots, flowers that predate bees)

I'm giving partial-points to: scale insects, fig wasps, termites, Neotrogla curvata, Pandoravirus, Neuroptera

comment by Mary Chernyshenko (mary-chernyshenko) · 2019-06-15T09:08:10.487Z · score: 5 (4 votes) · LW · GW

"Ticks" is a diverse group, but I agree they are strange. Like the occasional males in parthenogenetic species? I mean, why do it?..

And my own favorite, if we are permitted to name large groups (can't say "taxa" here) will be, of course, Fungi Imperfecti. I know it's a cop out, though, in many senses relative to the OP.

comment by Spiracular · 2019-06-17T17:15:19.994Z · score: 4 (3 votes) · LW · GW

Ticks just repeatedly break my intuitions. How does something that small and R-selected have a 2-year-long lifecycle?

Oh goodness yes, fungi. Two-nuclei sexual stages startled me when I first learned of it. It's a highly-diverse and successful clade, filling in an array of niches all across the specialist/generalist spectrum, and ranging anywhere from unicellular to syncytial to organisms the size of a city. Plus, many of them seem to manifest that evolutionary pseudo-"inventiveness" that I usually associate with bacteria.

comment by Mary Chernyshenko (mary-chernyshenko) · 2019-06-22T14:18:49.822Z · score: 1 (1 votes) · LW · GW

Pseudo-"inventiveness"?

comment by Benquo · 2019-05-20T01:28:50.545Z · score: 9 (5 votes) · LW · GW

This seems like a really great kind of question to ask - "what's the weirdness generator?" - in response to an intuition that's important - many unrelated surprising deviations from the norm is suspicious, and probably there's a single cause.

comment by Spiracular · 2019-06-14T19:36:05.278Z · score: 9 (4 votes) · LW · GW

The strange pressures of their subterranean lifestyle, which eukaryote described somewhat, probably covers most of it. Inbreeding/isolation is probably the other half of the puzzle.

I'll try to show how some of these traits tie in with low-oxygen and subterranean living, in those places where it wasn't already covered.

  1. A lot of these do bottom out to the pressures of creating a large nest, and dealing with an underground low-oxygen environment.

Eusociality and large protective communal nest-building mesh together really well, which I think fed into a lot of the items in the Richard Alexander list of predictions mentioned above (the accuracy is really impressive!).

Lack-of-hair and weird teeth seem pretty obviously developed for digging/crawling lifestyle. Acid-pain immunity has been proposed to be a consequence of having to handle an otherwise-intolerable level of lactic acid buildup ('sore muscles') while digging in low-oxygen zones. The strange metabolic properties (which probably feed into cancer resistance considerably) also seem to be a way to handle their lower-oxygen-availability lifestyle; endothermy can be surprisingly energy-intensive.

Really, living a low-temperature low-sugar careful-energy-usage low-ambient-cell-replication lifestyle tends to defend against cancer and improve lifespan quite a bit in general (ex: calorie restriction for mammals often extends lifespan, raising fruit flies at low temperatures can full-on double it). Molerats seem to have been under heavy pressure to biologically enforce a strict energy-usage regimen, and they take this to an incredible extreme. So you'd expect to see some cancer resistance, although it's still crazy in terms of degree.

(I'd totally buy that they probably have some additional nutty things going for them. I think I've heard a theory that they have unusually-stringent cell checkpoints pre-division?)

  1. Inbreeding and genetic isolation

No, really. It's both a strong push towards kin-selection (a basis of eusociality if there ever was one), and an exacerbator of genetic drift. The changes might not always be anywhere near this favorable, but isolation still tilts things towards the weird, and the faster rate to saturation increases your ability to build adaptations on top of adaptations [LW · GW]. (see also: high weirdness for species living in islands, caves).

comment by Svyatoslav Usachev (svyatoslav-usachev) · 2019-06-24T19:49:04.151Z · score: 6 (2 votes) · LW · GW

The "common weirdness generator" hypothesis seems quite off to me.

If the individual weird characteristics are independent as random variables, that would correlate with rareness of them occuring in a single species, which we do observe in this case.

If those weird characteristic were actually correlated, that would increase the likelihood of their cooccurance, and make creatures as weird as naked mole rats more common.

So, observation of uniqueness should lead us to believe that they are more likely to be uncorrelated.

comment by shminux · 2019-05-20T01:17:05.800Z · score: 4 (2 votes) · LW · GW

I wonder if there are other mammals like that, and if not, what would explain this version of the Fermi paradox.

comment by Mary Chernyshenko (mary-chernyshenko) · 2019-06-12T10:51:17.128Z · score: 1 (3 votes) · LW · GW

Given how little we know about even such a small subset as... insects... I would say that "weirdness" of some invertebrates is not yet a thing. We might come to appreciate it in the future, but not soon.

In tiny spheres of melted snow Around the tips of mosses, Where creatures dream, and freeze, and thaw, And never count their losses;

In deepest oceans' private wells, Where life still crawls about, Defying water's crushing spells Within just as without, -

Wherever people have cared to look, They saw, and cheered, and cursed The fighting claw, the winning hook, The glorious eggshell burst.