It’s All In The Family – The Naked Mole Rat

Throughout the history of natural science, zoologists have pottered around happily cataloguing and describing the varied groups of organisms that inhabit our planet. By looking at physical features, behaviour and now DNA analysis this form of taxonomy helps us to classify and clarify how everything we see around us fits into place. This process is relatively straight forward but nature has a habit of occasionally throwing something up in our faces that leads biologists walking away scratching their heads. The Naked Mole Rat is one such creature.

The naked mole rat (Heterocephalus glaber) is found in the dry, tropical grasslands that cover Kenya, Ethiopia and Somalia and has a rather unsightly appearance shaped by the conditions to which is has adapted. They are small rodents with a body length around 8-10cm and their body mass lying somewhere between 25-70 grams; are completely hairless, wrinkled and pale-skinned. Their diet consists mainly of the underground parts of plants, (particularly tubers that are formed by many plants that grow in the arid deserts of East Africa. Like many animals in similar climates they obtain all the water they need through their food and therefore do not drink.

As its name suggests, the naked mole rat resides in deep, underground burrows like moles and indeed, due to their dark habitat, they are completely blind but these are where the similarities end. Their acute sense of hearing and smell compensate for their lack of sight, allowing them to sense vibrations through the dense earth and assess each other’s status by smell. Rather than digging with large burrowing hands, the naked mole rat uses its elongated teeth like a shovel to scrape away earth and shape the extensive tunnels of the den, which when stretched out can reach up to two or three miles in length. In order to keep the dirt out of their mouths when they dig the lips of the naked mole rats close behind their front teeth, which remain permanently exposed, adding to their already rather grisly appearance.

Appearance aside, their oddities are abundant:

Like some other rodents (such as rabbits, chinchillas and hamsters), the naked mole rat practices coprophagy (or the eating of faeces). Like other rodents who eat their own faeces in order to acquire a higher intake of Vitamin B12 (crucial for maintaining a normal functioning of the brain and nervous system and the formation of blood), the naked mole rat has great difficulty in properly digesting tubers and plants due to the high cellulose content. The re-ingestation of faeces (usually their own, or, if they are pups these are provided by other male workers) provides the mole-rat with gut fauna (bacteria), which helps to break down the cellulose and maximise the nutrient uptake during the digestion process.

At school your biology teacher probably gave you the following criteria for defining mammals; give birth to live young, produce milk from glands, and are warm blooded. And your biology teacher would be right…well, almost. Naked mole rats are mammals, but unlike other mammals they are unable to thermoregulate (use their metabolism to maintain a steady body temperature) meaning that they are essentially cold-blooded. Like reptiles, their core temperature fluctuates with the ambient temperature, so to control it they huddle in large groups to slow the loss of body heat when cold or alternatively bask in shallower surface tunnels, which are heated by the sun. When temperatures get too hot they merely retreat deeper into the burrow where it’s cooler.

The naked mole rat is also one of the hardiest and most resilient animals you’ll ever likely meet. Due to the lack of the neurotransmitter Substance-P (responsible for transmitting information regarding tissue damage to the central nervous system), the mole rat startlingly has no skin sensation and is unable to feel pain! It is also one of the only mammals in which cancer has never been observed.

Cancer is caused by a cell or group of cells, which either multiplies beyond normal limits (uncontrolled growth), or intrudes on and destroys neighbouring tissue. The mechanism that checks cancer is a gene called p16 (a tumour suppressing protein), which stops the formation of new cells once a group of cells reaches a certain size. Most mammals have another gene called p27 (a cell-cycle inhibitor protein), which does a similar task, but prevents cellular division at a much later point than p16. The existence of both p16 and p27 genes in naked mole rats creates a double barrier that prevents the development and spread of cancer cells. To top this off, they are the longest living rodent in the world. While most rodents have an average lifespan of 2-5 years, the naked mole rat has been found to live up to 27 year in captivity.

But perhaps what makes the naked mole rat particularly unique is that they are the only known mammals (with the exception of their closely-related cousin, the Damaraland Mole Rat) that live in eusocial societies. Eusociality (or true sociality) is a social state that is defined by scientists in biology as the existence of reproductive altruism and kin selection (strategies in evolution that favour the reproductive success of an organism’s relatives, even at a cost to their own survival and/or reproduction). This contains the following criteria:

  1. Reproductive division of labour (with or without sterile caste systems)
  2. Overlapping generations
  3. Cooperative care of young

While many other mammals’ exhibit one or two of these aspects, only the naked mole rat truly exercises all three traits. Whereas mammals like great apes, large cats and horned or antlered animals like the deer and moose all have the ability to reproduce and are equipped to fight for their right to mate in order to ensure the survival of their individual genes, the naked mole rats sacrifice their own genetic preservations for the good of the colony, resulting in a truly altruistic (self-sacrificing) society.

A female, a queen, will mate with perhaps three or so dominant male rats and will give birth to the entire population of the colony, which can surpass as many as 100 individuals at a time. This breeding system is most closely characterized as polyandry (one female with many male mates) since the queen will copulate with multiple breeding males. Males generally follow an age and size-related hierarchy whereby the oldest and biggest males enjoy breeding rights with the queen. Interestingly enough, when a female becomes a queen she actually grows longer by increasing the distance between the vertebrae in her spine! With a gestation period of around 70 days, the queen can produce a litter every 80 days, 5 litters a year and may produce up to 50 litters in her lifetime (as many as 600 pups). She will not be replaced until her death when the next female in line develops hormones and behaviours similar to the queen. She will then aggressively fight with several other females for her place as queen. Her children (the workers), both male and female, although physiologically able to reproduce, will remain sterile throughout their entire lives in order to carry out duties such as burrowing, searching for food (tubers, roots and seeds) and raising the young in order to ensure the survival of the colony. This refusal to reproduce is known in nature as reproductive suppression. This type of societal structure is most commonly found in the colonies of social insects such as ants, bees, and wasps as well as termites, all with reproductive queens and more or less sterile workers and/or soldiers, but the naked mole rat is the only known mammal to have evolved in this way.

Now, reproductive specialization is found in various organisms in nature and generally involves the production of sterile members of the species, which carry out specialized tasks and care for the actively reproductive members. This can be manifested by the modification of behaviour (and also in the case of many insects, anatomy) of individuals within a group for security and preservation, including self-sacrifice.  Eusociality with biologically sterile individuals characterizes the most extreme form of kin selection in the natural kingdom. But if individuals are not striving to pass on their individual genes, why and how is this form of society effective in terms of natural selection? What’s in it for the naked mole rat? In a world in which every individual competes and strives to outdo one another to pass on his/her genes, why has naked mole rats evolved to be a passive, sterile and altruistic species and why have they been successful?

Well, presumably, the evolution of the mole rat’s kin selection is tied to the subterranean habitiat: by closing the top of their burrow the mole rat’s ancestors found that they could escape some of the dangers of surface predators such as snakes, which often find their ways down burrow entrances.  This enclosed habitat consequently led to inbreeding which produced individuals with high genetic similarity and an environment that found kin selection to be more beneficial than individuality in the colony. All social animals must modify or restrain their behaviour in some way in order to cultivate a harmonious environment when living in a community but what we have here is an extreme form; a society in which every member of the colony is related. They are a family; all of them brothers and sister and sons and daughters and therefore their genetic pool is closed and contained. They have no reproductive competition from the outside; no sexual selection pressures. With a rodent’s resilience to the more common genetic flaws that often occur from inbreeding in other types of mammals, the naked mole rat has found a niche in establishment of a unique kind of mammalian society but one with traits that can be seen in the groupings of apes, human societies and even economic theory; one where ‘altruism’ can cement relationships to ensure both personal and genetic continuity and security. In other words ‘what is good for the colony is good for the individual.’ Could it be that what we are seeing in the societies of the naked mole rats is a remnant; a glimpse into the early evolutionary developments of selflessness and, by extension, perhaps even morality? I’ll have to do more research…

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5 Comments on “It’s All In The Family – The Naked Mole Rat”

  1. Gonzo Says:

    These guys look like something out of Doom, have always liked them, and in a few million years they might end up looking like the bad guys in gears of war! 🙂

    Speaking of scary monsters, even the meanest ones around would need rules to keep their society together. This leads to the question, why would they need to live together in the first place? If living in a group provides benefits that could not be obtained by individuals living isolated from each other, like protection against predators, then it’s in any given individual’s interest to keep the group together. It’s a kind of indirect selfishness, actually. So morality doesn’t seem to come from selflessness but from quite the opposite, which is the struggle of every individual within a society to serve their own interests in the best possible way.

    • Thanks for the comment, Gonzo 😉

      I agree completely with you and touched briefly on the matter here at the end of my article:

      ‘altruism’ (not the inverted commas here) can cement relationships to ensure both personal and genetic continuity and security. In other words ‘what is good for the colony is good for the individual.’

      One can argue that even in human society there is no such thing as an ‘unselfish’ act – even donating to charity or helping out a stranger serves to make us feel better about ourselves. As I’m sure you’re aware, morality serves as an unspoken law to hold societies together – without it they would collapse. These seemingly just and unselfish acts we modern humans carry out most probably have their ancient origins in a form of what we see here in the mole rats ‘altruistic’ society – a self-serving evolutionary development for strengthening, regulating and providing for the members of an incresingly complex society.

      Still, this is where the lines of science and philosophy once again blur into fuzziness and where I depart to leave others to consider and elaborate on over drunken philosophical conversations in the pub…

  2. Love your site man keep up the good work

  3. Saundra Parker Says:

    I like your site and enjoyed reading about the mole rat. Very interesting and well-written.
    Keep up the good work!

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