Naked Mole Rat Communities

What animal society works in the following way?
There is only one breeding female, the queen, and she’s pregnant more or less continuously, producing young as long as she lives. She’s larger than other members of her horde. The workers and soldiers are kept from breeding by means of scent control wielded by the queen. The colony’s few sexually active males exist only to fertilize the queen. When she dies, one or more of the nonbreeding worker females become fertile and thereby eligible for queen status. If more than one heiress-apparent arises, they fight to the death. The entire colony lives within an elaborate, self-made labyrinth of tunnels and chambers.
What are they? Termites, in their immense concrete castles molded from saliva and soil? Bees in their waxen warrens? Wasps in paper palaces? Leafcutter ants in their underground fungus malls? Not at all. These mystery creatures aren’t insects but mammals and rodents, the naked mole rats of semi-arid northeastern Africa (Ethiopia, Somalia, and Kenya). They’re also the live study subjects of neuroendocrinologist Dr. Ronald Goldman of the University of Connecticut, who keeps a living colony of the animals in his laboratory. He imported the colony from captive breeding groups in South Africa. He also keeps a colony of a related species, Damaraland mole rats.
I had a chance to watch these endlessly busy little mammals, skittering manically through hard transparent plastic tubing like harried in-city workers running for subways. It’s rare for anyone seeing naked mole rats not to gape in awe and wonder at one of nature’s most outlandish experiments.
You’d think naked mole rats would be as homely as their name, but they’re not, at least to me (though opinions differ widely) and their skittering, clumsy-looking, round-the-clock efficiency renders them endearing and even admirable. Naked mole rats are neither moles (which aren’t rodents) nor rats. They are one of twelve known species of mole rats, the other eleven species bearing coats of fur, among them the Damaraland mole rats of South Africa, which has a looser social structure than that found in naked mole rat colonies.
In basic appearance, a naked mole rat looks like a pinkish and greyish, wrinkled, furless mouse-they don’t get very big, four inches being the average size-with stubby limbs, disheveled whiskers, beady tokens for eyes and prominent rodent incisor buckteeth, top and bottom, the entire jaw arrangement like a vague cross between a stapler remover and a set of pliers. Those jaws and teeth look like they mean business, being built for heavy work. Since naked mole rats live their entire lives in total darkness, the creatures’ sight is poor, but its exact acuity is unknown. Studies hint that the eyes may be sensitive to air movements, a unique case of visual apparatus switching duties to that of a tactile organ.
In their native northern Africa, colonies of naked mole rats build elaborate tunnel systems, feeling and smelling their way around. Their food source is tubers and bulbs: starchy, swollen plant roots which their constant mining operations expose. The roots provide all the food and water the mole rats will ever need.
Naked mole rats, as far as anyone knows, rarely or never come above ground (though there’s some argument about this). They bury their feces and dead in “toilet chambers,” small enclosures which they excavate specifically for waste disposal, shutting them off forever when they get too full, while gouging out new ones. The wastes are eventually recycled back into an environment that sorely needs them.
Mole rats dig new tunnels or widen older ones by pickaxing them out of hard-packed, clay-based desert soil with their strong jaws and incisor teeth. Those incisors, which grow continuously throughout life like incisors of other rodents, are positioned permanently outside the animals’ mouths, though the teeth aren’t a handicap to feeding. When working, naked mole rats keep their mouths shut to keep from swallowing dirt. The lead animal in a tunneling project, after chiseling off a load of soil, shoves it with his paws back to a partner behind him, who in turn passes it on to a worker behind him and so on, assembly line style, through a chain of workers until the last animal in line, situated in a shallow burrow opening to the surface, pitches the soil outside for final riddance.
The most amazing, almost unbelievable characteristic of these gnarly little creatures is their complex society, which parallels that of communal insects. There is one breeding female, or queen, her retinue of one to three drone males to keep impregnating her, and a servant staff of up to eighty workers, male and female. The queen produces litters constantly as long as she’s alive, usually every sixty to eighty days, of up to a dozen young. Queen, breeding males and young live in a breeding chamber.
By means unknown, non-breeding worker females and males stay sexually inactive, though not sterile but more like sexually immature juveniles. These workers dig out new tunnels, maintain older ones, care for the young, carry food to the queen, dig out toilet and food storage chambers, and defend the colony.
Theories vary as to how the queen keeps this state of things always in her favor. Hormones in urine, pheromones (airborne biochemicals), or sheer will and physical dominance on the part of the queen have all been suggested. The queen frequently roams the burrows of her colony, ritualistically bumping and shoving colonists to remind everybody who’s boss. Females and males removed from a colony soon become sexually active.
When the queen dies, a few of the worker females begin to ovulate and get aggressive, battling it out for queen, the winner killing her contenders. But what prevents all the females in a colony from coming into estrus and staging a civil war for rulership? Such chaos would soon drive the species into extinction.
Studies show that among the non-queen females of a colony, a few are already near or in the early stages of estrus, even with a healthy queen at the helm. These are more aggressive than their sisters and may even challenge the queen, shoving back when she pushes them. When the queen dies, these few more aggressive females are the ones most likely to go into full estrus and battle it out for queen. The rest of the female population stays out of the fray. In this way, a hierarchy is already in place and only a few females are favored for queenship when the time comes.
Nothing like naked mole rat society exists anywhere else in mammaldom. Their uniqueness and their parallel of insect reproductive behavior has attracted and held the curiosity of biologists, among them Dr. Goldman.
What brought Dr. Goldman and underground communal rodents together in the first place? “It’s a strange story,” says Goldman, and goes on to tell it. When he was an undergrad at Michigan University, one of his favorite classes and teachers was “Evolution of Social Insects” by Dr. Richard Alexander, an evolutionary biologist and entomologist (one who studies insects). Dr. Alexander then had a sort of ongoing feud with the mammalian evolutionists at Michigan U. He hypothesized that somewhere, sometime, some lucky field biologist might stumble upon a species of colonial mammals that were “eusocial,” i.e., patterned along the lines of the social insects, and predicted their likely characteristics. The mammalogists wrinkled their noses at any such notion.
One day, after Alexander gave a lecture on social insects at the University of Arizona, and mentioned his eusocial mammal hypothesis and criteria, a student approached him and told him that he, the student, knew of a mammal species that fit Alexander’s bill. These mammals were-you guessed it-naked mole rats. The student went on to mention Dr. Jennifer Jarvis, a zoologist studying the creatures in captivity in South Africa. Alexander’s phone call to Jarvis confirmed the essentials. Back in Michigan U., Alexander told his class about these strange critters, and young Goldman’s interest was piqued. It still is, and he now maintains thriving colonies of naked mole rats and a related species, Damaraland mole rats. At present, Goldman’s focus of study is the endocrinology of circadian rhythms, i.e., the hormonal regulation of the day-night activity cycle. His query: how do his subterranean subjects deal with circadian rhythms when they seldom or never see day or night? Or do they bother with circadian rhythms at all?
Goldman’s naked mole rats and Damaraland mole rats live in standard lab animal plastic bucket cages with grill tops and shredded wood bedding. Hard plastic tunnels connect the many cages, enabling the residents to skitter about the system as they would a network of natural tunnels. The animals stay active and seem content. Goldman feeds the nakeds carrots and sweet potatoes, and the Damarlands that and hard rodent chow; the nakeds never took to the rodent chow.
One female Damaraland mole rat in Goldman’s colony is seventeen years old, still queen and proud mama last year of two litters of pups. In the plastic tunnel system she’s as active as everyone else in the colony. The Damaraland mole rats spend their days running backwards and forwards through the tunnels, fencing with their sets of teeth to maintain pecking order, even pushing subordinates ahead of them on their rambles through the tunnels.
Why bother with all the elaborate adaptations of naked mole rats? Most likely, says Goldman, for safety. Living underground keeps the creatures out of harm’s way of predation. Small ground-dwelling mammals must constantly be on the lookout for carnivorous mammals, snakes, and birds of prey. Naked mole rats need not worry about predation except for one snake species that occasionally raids the burrows. Mole rats defend the colony by an individual sacrificing him- or herself to the invader, its screeches of death agony alerting the rest of the colony, the workers rushing to plug the tunnel with dirt while the snake is busy with its victim.
Why no hair? Actually, all naked mole rats are covered nosetip to tailtip with scattered, fine, sensitive hairs that serve as tactile guides in the tunnels. The mammalian facial whiskers remain. Fur fills between the toes as an assist in shoving dirt backward in tunnelling projects. Otherwise, the species has lost most of its fur because it just doesn’t need it. The tunnels keep a stable temperature, and the animals never leave them, so there’s little danger of overheating or hypothermia. Parasites like fleas heve no fur to lodge in, so are unlikely to gain a foothold and quickly spread themselves and pathogens through a colony.
As for food, naked mole rats seem to encounter it by luck alone, just digging maniacally in all directions and hoping for the best. When they luck out and strike a tuber, they gorge and carry off food for the rest of the colony, but leave enough of the root and skin behind to enable the plant to repair and reheal itself, to be a future food source in this food-scarce realm.
Smell plays a major role in colonial life. If a scientist removes a worker from a colony for a few hours, then returns him, the others pay no attention. If he’s removed for a day then returned, the others sniff him suspiciously but presently accept him. Keep him out for several days and put him back and the colonists will attack him as they’d attack an intruder from another colony.
Most interesting is a difference in how the two mole rat species, nakeds and Damaralands, replace their queens. The Damaraland males seek a female from another colony, while the naked mole rat queen is recruited from her own colony. Damaralanders outbreed, naked mole rats inbreed.
So, here they are, another trick out of nature’s infinite alchemical laboratory, an almost invisible creature that you’d dismiss as impossible if you heard of it only in travelers’ tales.