Jonathan Peterson ‘23
Staff Editor
If Dr. Evil was to own a rodent, it would be the naked mole-rat. Science’s vertebrate of the year in 2013, the naked mole-rat is perhaps most well-known for it’s hairlessness. However, this mammal has a variety of other characteristics which are less well-known, yet each are crazier than the last. Ranging from being one of the only eusocial mammals to being nearly completely cancer resistant, these mammals are perhaps more alien than any other mammal on Earth.
Description
Naked mole-rats are found in the drier parts of the tropical grasslands of East Africa, predominantly southern Ethiopia, Kenya, and Somalia.[1] These incestuous and xenophobic mammals are small, with individuals typically ranging between three and four inches long and weighing 1.1 to 1.2 ounces. They have small eyes and poor eyesight as a result of living in low-light tunnels which can stretch up to three cumulative miles. However, their short and thin legs are highly adept at maneuvering in this environment and allow them to move backwards just as quickly as forwards. And, in order to help them dig, over a quarter of the total musculature of the naked mole-rat is devoted to shutting their jaws. Further, their front teeth protrude outside of their mouths, allowing their lips to seal behind their teeth while digging. This prevents the rodent from swallowing much of the dirt that it excavates. These small rodents are, as the name suggests, almost completely hairless and have loose and wrinkly skin. And that’s about everything “normal” about the naked mole-rat.
Society
One of the most bizarre facts about the naked mole-rat is that it is one of the only eusocial mammals in the world. Eusociality describes how a species is structured socially. Common examples of eusocial animals are ants, termites, and some bees and wasps. Some of the essential traits of eusociality is caring for young and labor divisions. Naked mole-rats express both of these characteristics.
Naked mole-rats are divided into roughly two social castes: reproducing and non-reproducing. Each colony has one queen and between one and three reproductive males. Queens are significantly larger than the other members of the colony, clocking in at about 1.8 ounces, with some recorded weighing as much as 2.8 ounces. However, these morphological differences are not pre-existing in naked mole-rats who eventually become queens. Studies done on queens before and after they attained the position reveal that, prior to being a queen, they are the same size as the rest of the colony. So, the size of a queen is not a result of pre-existing morphological differences between individuals, but rather it is the result of the attainment of the position itself. In simpler terms: it isn’t that big naked mole-rats become queens, but that when a naked mole-rat becomes a queen, it gets big.
Queens attain their position either by founding new colonies, fighting for the dominant position, or stepping in without conflict once the reproducing female dies. Because of the potentially violent transitions of power in naked mole-rat colonies, queens are extremely hostile to other females that begin behaving like queens or producing the hormones necessary to become a queen. This is not true of reproducing males, however. These males are selected by the queen herself and may have relationships with the queen that last for many years while the other members of the colony remain sterile.
The second broad group of naked mole-rats are the workers. Worker’s roles are decided along a continuum of behaviors as opposed to distinct groups. For example, larger naked mole-rats typically spend more time defending the burrow from invaders, whereas smaller naked mole-rats will spend more time tunneling, harvesting food, and caring for young. Despite this size-based division of labor, naked mole-rats do not divvy behaviors up into discrete groups, like some other eusocial animals. For example, a naked mole-rat that was tunneling one day might be defending the colony the next and raising young the day after that.
Another division in naked mole-rat society exists within the working class—there are “frequent” and “infrequent” workers. Frequent workers are, as their name suggests, frequently working at tasks like foraging and nest building. Infrequent workers, however have role overlap with frequent workers, however they perform tasks at a much slower rate. So, they’re either lazy or simply inept.
Finally, and in my opinion, most interestingly, there is a third discrete group of naked mole-rats within this social structure: the dispersers. For lack of a better way to say it, I am astonished that a role like the disperser has managed to form in a mammalian culture. Living in a society with a single queen breeding with one to three other males leads to a high statistical chance of experiencing the harms associated with inbreeding. Couple this with the fact that, if a colony does well, this setup will continue down from mothers and fathers to sons and daughters, ideally forever. So, the disperser’s job is the direct opposite: outbreeding. These naked mole-rats are, relative to the other sterile workers in their colony, “[b]ig, fat, lazy, and sexually-charged.”[2] However, despite being the horniest of the working class, these rodents have no desire to mate within their own colony. Instead, they are born with significantly higher fat reserves and are massively different hormonally—both of these factors make them capable of and more likely to depart from their colony in search of, you guessed it, sex. Not only are these naked mole-rats born to leave, but they seem to never fit in while they stay—they are lazy and work poorly with the group. What I find to be most interesting about this is wondering how this mechanism to avoid inbreeding developed.[3] This is because the existence of a disperser has no direct benefit to the naked mole-rat colony the disperser originates from—the disperser simply leaves and benefits a different colony by increasing its genetic diversity. This reaps no rewards for the disperser’s original colony.[4] In fact, the only way the original colony could benefit from the existence of this role is if they are lucky enough to have another colony’s disperser drop by for a little afternoon delight.[5]
Despite this being the end of this article, there are countless other facts about the naked mole rat that I have not covered here. Accordingly, I will be releasing another article, Naked Mole-rats: Mammals or Aliens Part Two—Definitely Aliens. In that article, I will discuss some of the most absurd physical features of the naked mole-rats, including their unique style of thermoregulation, lack of pain sensitivity in their skin, extreme resistance to cancer, ability to survive in obscenely low oxygen levels, and their incredible longevity.
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jtp4bw@virginia.edu
[1] Naked Mole-rat, Wikipedia https://en.wikipedia.org/wiki/Naked_mole-rat (last visited March 12, 2023). Assume that any information in this article comes from Wikipedia unless cited otherwise.
[2] Lindell Bromham & Paul H. Harvey, Behavioral Ecology: Naked Mole-rats on the Move, 6 Current Bio 1082, 1082 (1996) (available at https://www.cell.com/current-biology/fulltext/S0960-9822(02)70671-4?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982202706714%3Fshowall%3Dtrue).
[3] This is incredibly important for naked mole-rats, because two colony mates (i.e., a queen and a reproducing male from the same colony) are more genetically similar than siblings in species that outbreed. In fact, the only way they could be more genetically similar is if they were monozygotic twins—or twins that originate from the same fertilized egg, and thus share identical DNA. Id.
[4] This is not, however, unique to naked mole-rats. Termites, for example, have winged dispersers. I believe that ant colonies also produce winged dispersers. What I find absurd is that this behavior has developed in a eusocial mammalian culture.
[5] This joke shouldn’t be meant to imply that dispersers are roving sex addicts; they depart their colony, find a new one, and remain with it until their death.