For naked mole-rats, social status correlates with this organ’s size

Size does matter, it turns out.

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A naked mole rat walks through a tunnel during a press preview of the Aquazoo at the Loebbecke Museu...
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Rufus, the naked mole-rat sidekick from the cartoon “Kim Possible,” could plausibly still be alive today, and he might have a really big spleen.

Naked mole-rats (Heterocephalus glaber) are size queens, it turns out. That doesn’t only refer to the actual queen, who is the breeding and dominant female in the colony. But new research published this week in the journal Open Biology from the Max Delbruck Center for Molecular Medicine in Berlin examines a surprising correlation between one crucial organ’s size and a social factor within naked mole-rat colonies.

What’s new — While completing other research on naked mole-rats, this team from the Max Delbruck Center for Molecular Medicine in Berlin unintentionally found a strong correlation between social status and spleen size in healthy rats.

All vertebrate animals have a spleen, an organ that is part of the lymphatic system whose job is to fight infections and keep the body’s fluids in check. The human spleen is fist-sized, and resides above the stomach and under the left-side ribs. Spleens are known to swell, a phenomenon called splenomegaly, when the body is fighting an infection because this organ houses a swarm of disease-fighting white blood cells.

What’s peculiar about naked mole-rats, however, is that some perfectly healthy adults had enlarged spleens, and these adults were more likely to rank highly in the social hierarchy. A big chunk of this paper is dedicated to evidence showing that large-spleened mole-rats were healthy, and not simply fighting infection.

While there appears to be a clear relationship between social standing and spleen size, there’s a chicken-egg question about cause and effect.

“The million dollar question is what develops first?” study supervisor and lab leader Gary Lewin tells Inverse. “Does the animal get a larger spleen and then get a higher rank? Or does it only get a larger spleen when it actually attains a higher rank?”

As the queen continues to birth litters, her spine grows longer. This is another example of morphoelasticity in naked mole-rats.

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Why it matters — This finding is a new example of something called morphoelasticity, in which the size and composition of an organ changes after it’s been developed. The spleen not only grew in size, but it also changed in molecular composition. Larger spleens contained more of a type of cell known to battle infections than did smaller spleens.

Humans, as far as Lewin knows, don’t have examples of morphoelastic organs. While it’s also true that humans of different socioeconomic status have varying degrees of health and lifespans, Lewin didn’t think resources bore any importance on naked mole-rats. Wealthy humans can afford regular healthcare and nutritious food (and wealth corresponds with a suite of factors like race and gender), so wealthy people are more likely to be healthy than those with less money. But, naked mole-rats live together in a colony of about 30, and they’re known for counterintuitive altruism. When one comes across a tuber that can be food, it doesn’t eat the whole thing, but brings it to the colony or leaves half of it to grow back for future harvesting.

That’s not to say naked mole-rats live a peaceful, egalitarian existence. The queens are the only ones who breed, and there are designated males to breed with her. Worker mole-rats dig extensive tunnels and scavenge for food. The higher ranked a mole-rat is, the less work it does. “Like in human societies,” quips Lewin. In fact, queens come to power through what Lewin describes as a “coup d’etat,” which is primarily how specimens in his labs perish.

The spleen may come into play in that it will make some mole-rats stronger and more able to recover from infections received in battle. If the large-spleened beings are destined for glory, it may be because they have a stronger immune response. Or, those who fight their way to the top are able to grow a larger spleen.

Lewin is inclined to believe the latter. He says behavioral success, not a larger spleen, will allow a rodent to attain a higher rank. “I reckon it's something to do with social success of the animal, then that then leads to morphological changes that help the animal in its new rank,” he says.

In an experiment, Lewin and his team determined a naked mole-rat’s social ranking by placing two mole-rats in a tube and observing which one climbed over the other; the ascendant was the dominant one.

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Digging into the details — Lewin and his team utilized data from research they’d already completed, looking at 34 spleens from over the last four years. The spleens came from randomly selected healthy mole-rats between 1.3 and 5 years old, excluding breeding males and queens. Spleen mass and length varied throughout the critters, and on average the spleen comprised 0.26 percent of a naked mole-rats body mass.

Then, the researchers grouped mole-rats together based on whether the spleen accounted for more or less than 0.25 percent body mass, which separated large and small spleens.

The fun part is how Lewin’s group determined a mole-rat’s rank. “We put two animals in a tube together, and if they encounter each other, the animal, if it goes over the top, is the dominant animal. The animal going under is the subdominant animal,” he says. Since naked mole-rats all live in a tunnel network, crawling over and under each other is the way they get around, so these interactions happen all the time.

What’s next — Right now the only way to measure spleen size is with a dead naked mole-rat, so Lewin wants the future of these tests to include non-invasive spleen monitoring.

To be clear, since these data were collected retrospectively, Lewin’s lab wasn’t killing naked mole-rats solely to observe the spleen. Rather, over observing colonies over four to five years, they surveyed many aspects, and happened to notice the difference in spleen size.

He hopes to see how spleen size changes as some of these fleshy rodents rise through the ranks (or not). Ultrasound could be the key to keeping tabs on a growing spleen. He also hopes continuous monitoring could help his team see if one factor is a predictor of another.

“It could be that they already have a large spleen when they're born, and they rapidly move through the ranks because of that,” he says. “Or we can test this idea of whether it comes later.

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