In naked-mole-rat societies, the royals do not wield scepters or don crowns. But that’s not to say that their majesty is subtle. The toothy, pruney rodents live in close-knit underground communities of up to about 300 members apiece, ruled by a tyrant queen that refuses to be mistaken for just another balding pleb. She ascends the social ladder through a series of brutal battles, assuming her throne as the only fertile female in the group; she spends the rest of her sometimes decades-long life reminding her battered kin who’s boss. “She is always shoving and pushing the other animals around” and will literally walk over any subservients she meets, says Gary Lewin, a neuroscientist at the Max Delbrück Center for Molecular Medicine, in Germany. It’s her way of keeping the colony’s lesser mortals in check—lest a lower-ranked female get any bright ideas and attempt a bloody coup.
Such prestige is hard-won, but the perks seem well worth the pain. Naked-mole-rat societies, from matriarchs to the lowliest subservients, are arranged in spectrums of status. The further up the hierarchy the rodents go, the better chance they have at being brawny and living to an ultra-old age; at bearing offspring and eschewing the grunt work of nest cleaning, food gathering, and territorial defense; and, it seems, at growing an unusually large spleen. Among naked-rat nobles, the blood-filtering organ, which houses, nurtures, and manufactures a menagerie of immune cells, tends to be quite buxom and elongated, as if pulled lengthwise like taffy. The spleen-social-status link is just as odd as it sounds. “This isn’t the case in any other animal we know of,” Lewin told me. And perhaps even stranger still, the engorgement seems to be an advantage.
In the eyes of most doctors and veterinarians, an enlarged spleen is not a good thing. The organ swells up when a person or animal is injured or infected, and is in dire need of immune reinforcements. “Saying ‘bigger spleen’ is another way to say, ‘Okay, your body is fighting something,’” says Dana Lin, an evolutionary biologist at Duke University. That’s true in naked mole rats too: The spleen balloons out when nasty microbes are afoot, then shrinks back down.
But Lewin and his colleagues have found that higher-ranked mole-rat spleens may be larger at baseline, too, and may play an outsize role in defense. Perhaps colony elites acquire more than classist chops when they move up the chain of command. They might also gain supersize organs better poised to keep pathogens at bay.
The findings are still at an early stage. “I wouldn’t put it in the bank quite yet,” says Jenny Tung, an evolutionary biologist at Duke who supervises Lin’s work; neither was involved with Lewin’s new study. But if the results pan out, they’ll join a growing body of research pinpointing social status as a strong sculptor of immunity in all sorts of animals. A few years ago, Tung led a team of scientists in demonstrating that rhesus macaques, which also live in hierarchical groups, harbor their own status-sensitive immune system; lower-ranked monkeys, the frequent target of bullying, experience above-average levels of inflammation. Similar patterns seem to play out in people with low socioeconomic status, whose bodies seem to experience far more stress and end up less shielded against sickness.
The naked-mole-rat narrative is likely to carry its own quirks, given how bizarre these rodents are. Naked mole rats can live upwards of 30 years—about 10 times as long as a typical laboratory mouse—and are rarely troubled by the cancers that plague humans in old age. They are impervious to certain types of pain and can survive without oxygen for a whopping 18 minutes; they eat copious amounts of their own poop. All of these traits, Tung points out, somehow intersect with immunity and, by association, immune organs, such as the spleen.
Valérie Bégay, a cancer biologist who led the spleen work in Lewin’s lab, first noticed the potential link between rodent rank and spleen size a few years ago, while dissecting the animals for another project. Some of the animals on her table, she found, had a much larger spleen than others, sometimes double or triple the size of their more petite counterparts’, as a percentage of total body mass—a difference she couldn’t detect in other rodents such as mice. And yet, all of the mole rats on Bégay’s table were healthy, not an infection in sight. That “made no sense,” she told me.
So began the search for an explanation. The beefed-up spleens weren’t a product of some stealth sickness, or a wild genetic anomaly; they were, by all accounts, fairly normal organs, just oddly immense. Bégay also found no evidence that the animals’ liver—another organ that gets recruited into anti-pathogen battles—was shrinking or swelling in the same way, a hint that the spleen was somehow unique. The clearest link she and her colleagues could pull out, she said, was between a naked mole rat’s spleen size and its rank: The more highly regarded the naked mole rat was in its community, the bigger the organ tended to be. When the researchers split the spleens into two groups, large and small, roughly 75 percent of the tiny ones belonged to their respective colony’s lowest societal tier. Bégay and her colleagues also analyzed the contents of the trimmer spleens, and noticed that the organs weren’t just smaller, but also appeared to be less equipped to launch a vigorous immune defense.
The researchers don’t know what comes first: the strapping spleen, the social cachet, or a third X factor that might trigger both. Untangling these relationships “is the million-dollar question,” and will require experiments that track spleen size over time, perhaps as young rodents vie for a place atop the social pyramid, says Melissa Holmes, a naked-mole-rat expert at the University of Toronto who wasn’t involved in the study. In one scenario, some mole rats might be born with a chunkier spleen that, through extra immune juice-ups, helps its host reach higher social echelons. In another, the one Lewin favors, the mole rats first scamper up the colony ladder, unlocking an engorged-organ achievement at each step along the way. How exactly that would happen is a mystery. Maybe repeat tussles for dominance—especially if they turn a touch savage—rev up a mole rat’s immunity so often that the spleen starts to bloat. Or perhaps it’s a funky hormonal phenomenon. That wouldn’t be unusual for mole rats: After the queen takes her throne, all the other females’ genitals shrink spectacularly, rendering them infertile. In the process, some lowly laborer spleens might get shrimpy too.
The differences in spleen size may have especially big impacts in naked mole rats, whose immune systems are sort of wimpy and weird. In humans and other rodents, nearly all immune cells are manufactured in the bustling hub of the bone marrow. Mole-rat marrow, meanwhile, looks oddly empty and dormant. And though the tissue churns out plenty of blunt, fast-acting innate immune cells that can beat back most foreign bugs, it devotes far fewer resources to the more precise adaptive fighters, such as antibody-producing B cells, that other mammals rely on to quell the worst infectious threats.
That leaves the mole rats super vulnerable to certain viral blights. Outbreaks in their colonies, which aren’t so great at physical distancing, can result in mass death. In laboratories, the few survivors that rise out of these infectious frays tend to be—surprise!—higher-ranked. A lot could explain that bias, given that colony royals have so many health benefits on their side. But it’s hard to imagine, Lewin said, that a bougie spleen isn’t at least partly to thank.