The Naked Mole-Rat


You may be familiar with the naked mole rat, but did you know they are insect-like, cancer-immune, poop eating, cold-blooded aliens of the mammalian world? They’re so extremely different from any other animal you’d think they’d stand pretty far apart from their mammalian relatives and you would be correct. Field Museum mammalogist Bruce Patterson, together with Yale post-doctoral researcher Nate Upham have determined these animals are so special, the deserve to not only be in their own species or genus, but to be recognized as a brand-new mammalian family. Before we go talk to Bruce, here are a few reasons why naked mole rats stand apart from any other mammal. Naked mole rats are social mammals in the truest sense of the word. They live in colonies that more closely resemble insect hives similar to bees or ants than any kind of mammalian society There’s one designated queen mole rat who mates with four or five males and gives birth to an entire colony that can number up to a few hundred individuals. A queen can give birth to 20 to 25 pups at a time and have as many as five litters in a year. One captive female was recorded to have more than 900 pups in her 13-year lifespan! Instead of the queen’s offspring going off and starting lives and families of their own, they serve specific working roles within the organization of the family group and do everything from expanding their tunnel systems in search of food to defending the colony from snakes and other invaders, and of course help with the constantly regenerating litter of pups. Being a fossorial animal, which is one that burrows, comes with its perks. You have protection from predators attacking from above, shelter from the harsh desert outside and easy access if your diet consists of things like tubers and roots that also grow underground. The environment in these tunnels is very constant, both in temperature and in humidity, so there’s little need for the naked mole rats to control their own internal body temperature, and they don’t. They’re entirely cold-blooded, meaning they can’t thermoregulate and instead are reliant on the ambient temperature (much like reptiles). If they’re cold they need to huddle together to get warm which may be another reason why they tolerate living in such large colonies. Naked mole rats also have the unique ability to withstand extreme oxygen deprivation, also known as hypoxia. The presence of so many individuals in a closed burrow system depletes the air of most of its oxygen. The naked mole rat can tolerate these extreme levels that are so low that they would cause brain damage and death in other mammals. Understanding the physiological mechanisms that permit this adaptation offers us insights into the treatment and prevention of strokes in people. Like lots of rodents, naked mole rats practice coprophage. That is ingesting their own feces. This isn’t some radical, insane behavior its actually necessary for them to break down tough fibers or cellulose walls that make their food largely indigestable. Big mammals have long digestive tracts that can retain food long enough for the bacteria in their guts to break down plant cellulose. But small mammals like rodents have to re-ingest partially-digested feces as a way to grant that bacteria more time to break down the tubers they eat as their primary source of food. We typically think of the long-lived mammals being those which are massive in size. For instance, in 2007, biologists recovered 115-year-old harpoon point that was found lodged in the shoulder of a bowhead whale. But for their small size naked mole rats are by far the longest-lived rodents with individuals able to thrive for an average of 17 years in the wild and up to 27 years in a lab. A mouse typically only lives three to four years. A mouse like… a lil’, a lil’ mouse. Understanding how these little guys have been able to avoid a number of negative aspects associated with aging such as increasing weakness and susceptibility to diseases, it could offer new insights on human longevity. Part of their long lifespans could be attributed to the fact that naked mole rats are cancer-resistant. No naturally-occurring tumors or prolific cell division have ever been recorded or observed in these animals. So by using naked mole rats as a model organism cancer researchers and biologists alike could potentially unlock mysteries surrounding cancer and aging in humans. So keeping all of that in mind, we went to talk to Dr. Bruce Patterson about what it means for the naked mole rat to be in its own scientific family. Hey! We’re here with Bruce Patterson who is the McCarther curator of mammals here at The Field Museum, and we are going to talk about this guy today. What is this? Not only, like, what animal is it? What specimen is it? Bruce: This is a naked mole rat. It’s a specimen that The Field Museum collected in 1896 in British Somaliland which is now the country of Somalia. And it was collected by our famed taxidermist Carl Akeley. Carl Akeley mounted this specimen. Now Carl Akeley has mounted a lot of famous specimens, but this may be the only naked mole rat he ever mounted and is a prize for us. It shows many of the characteristics that make naked mole rats so interesting. The name itself, heterocephalus, in Latin means different head. And if you look at the skull of this naked mole rat, it’s got great big buck teeth, it has a reduction in the number of molars and different digital formulae that distinguish it from all other mole rats. So it’s a marvel to us at many different levels. Emily: It’s a truly unique creature, and that is why you’ve put it in a new family. Bruce: That’s right. Emily: So what got you interested in learning more or researching or looking into the naked mole rat, specifically? Bruce: Well, a graduate student at the University of Chicago Nate Upham and I were studying a big group of South American rodents that include the chinchillas, they include the porcupines, they include Guinea pigs and their relatives and this big collection of rodents have their nearest sister group (their closest relatives) in Africa. And their African relatives include the African mole rats In the course of our studies it’s always important to look at species outside your group of interest because the characters they share in common tell you a lot about how evolutionary sequences have developed and how to interpret them. Emily: So this project started by looking at South American rodents, just kind of wanting to draw some more conclusions about them and then all of the sudden the more you’re looking at the relationship of African mole rats you realize you have this one outlier, which is the naked mole rat. Bruce: Exactly. Emily: So how closely is the naked mole rat related to other mole rats? Bruce: Uh, not very closely. One of the reasons we’d studied the South American group is that it has a beautiful fossil record that goes back 50 million years, and most of the record is in Argentina and Bolivia and Southern Brazil. But the record provides a detailed time-calibrated resolution of how this beautiful radiation of rodents unfolded in South America. And that record allowed us to take our molecular phylogeny, our DNA insights into how all these species are related to one another and calibrate the dates of divergence for each of the branches in our tree. And the dates we came up with for the naked mole rat relative to the other African mole rats was 31 and a half million years. They last shared a common ancestor 31 million years ago at a time when the common ancestor of baboons and humans and apes was running around the African plains. These guys were already a distinct lineage. Emily: So just knowing that information, going back through the fossil record and being able to pinpoint, “Wow, this animal has nothing to do with the other animals it was grouped with.” That’s how you kind of determined that it wasn’t not only its own species, not only its own genus, but its own family. Bruce: Exactly. When we discovered–and we had information for a long time that this was an oddball group genetically. But never before had we had the fossil evidence to tell us exactly how long it had been off on its own. So we went back and looked at the biological characteristics of naked mole rats in comparison to other African mole rats and they’re in a class by themselves in terms of all sorts of ecological, morphological, physiological characteristics. They’re just a group unto themselves. So that it really does make sense to elevate them to family status rather than have them be simply a genus within the family of African mole rats. Emily: For sure. So when we think about this new classification, when was the last time a new family of mammals was named? Bruce: Well there are comparable instances for example the aardwolf. The aardwolf, evolutionarily, is an oddball hyena. But its so distant from the other hyenas that it was broken out into a family of its own. Basically what we’ve done with the naked mole rat relative to the African mole rats follows the same reasoning and logic that here’s something whose relationships are so remote that it really doesn’t belong in that family, but in a family unto itself. Emily: Yeah. But it’s not very common. It’s not like people are just going around and naming new families of mammals like they’re naming new families of beetles. Bruce: No, maybe every five or ten years a genus gets transferred from one family to another. Emily: That’s cool.

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