Naked mole-rats: A case study in biological weirdness

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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.