Last time we discussed eusocial insects. The two known species of truly eusocial mammals are both members of the Bathyergidae family of subterranean African mole rats. Naked mole rats (heterocephalus glaber) inhabit the horn of Africa (parts of Somalia, Ethiopia and Kenya) and are small (3-3.5 inches in length), pink, wrinkled and nearly hairless. Meanwhile the Damaraland mole rat (cryptomys damarensis) is larger and furrier and lives around the Kalahari desert in Namibia, Botswana, Zimbabwe and NW South Africa. Here's some more facts about the naked mole rat which seems to be the species scientists have studied more.
Their caste system
In the wild, naked mole rat colonies range in size from 20 to 300 members with the average size being around 75. At the top of the hierarchy is the queen, the single reproductive female, whose elongated body allows her to bear large litters of offspring. A typical litter consists of 12 pups although there can be as many as 27, and a queen may carry 4 or 5 litters a year. Insect queens do nothing but lay eggs, but among naked mole rats the domineering queen is actually a leader. She is able to devote most of her time and energy to reproduction and caring for her pups, but she also leaves her nest to push around subordinates and keep them in line.
Below the queen is the harem of 1-3 males with whom she chooses to mate. Then there are the soldiers (both male and female) whose job it is to defend the colony from predators (their main predators are snakes) and to attack any interloping mole rats from outside the colony. The lowliest mole rats are the workers who are the smallest in size and whose responsibilities include maintaining the colony, digging new tunnels, foraging for food and caring for the queen and her pups. Rather than being intrinsically sterile, soldiers and workers are thought to be reproductively suppressed. Its been theorized that henpecking by the queen and other superiors and/or the lack of non-relatives within a colony might prevent these mole rats from becoming fertile.
Naked mole rats have evolved in many way so as to adapt to their harsh environment. They spend their lives in dark underground tunnels and as a result they have tiny eyes and are nearly blind. Scientists think their vestigial eyes might serve mostly to sense changing air currents which, together with their sensitivity to vibrations, would be useful in alerting them to the presence of predators. Since they don't use their eyes much anyway, mole rats can run backwards through tunnels as quickly as they can move forward.
The naked mole rat also breaks the rule that all mammals are warm blooded as it cannot internally regulate its body temperature (they don't sweat and don't have fat for insulation). Fortunately, the temperature in their burrows tends to be 85-89°F. To keep warm, mole rats might bask in the heat of tunnels close to the surface, and they sleep huddled together.
The world of the mole rats is also characterized by a scarcity of oxygen and an overabundance of carbon dioxide. They are able to survive in their low-oxygen environment thanks to their slow metabolism (less than half that of the typical rodent) and their blood which is unusually efficient in absorbing oxygen. Moreover, the skin of the naked mole rat lacks a neuropeptide involved in transmitting pain sensations to the brain. This makes them impervious to the acidic sting resulting from the build up of carbon dioxide.
In order to expand their colonies and search for food, workers are constantly digging new tunnels through the earth. To do this they use their big, prominent incisors, which keep on growing throughout the mole rat's life, and their strong jaw muscles, which comprise 25% of their total muscle mass. Naked mole rats' incisors are actually located outside of their mouth, this way they can close their lips behind them while digging to avoid ingesting dirt. To further facilitate swift excavation, mole rats can line up like a chain gang and quickly sweep the displaced dirt out of the way.
Naked mole rats eat roots and tubers. They actually have no way of detecting where new food sources may be located so they dig tunnels at random and discover them by chance. Many plants in the mole rats' arid habitat have fleshy tubers that can grow larger than a soccer ball: the mole rats will eat out the center of these roots while leaving the rest so that it will grow back providing them with a renewable food source. Naked mole rats also re-ingest feces in order to further stretch the food available to them.
As soon as a worker discovers food, he immediately runs back to share it with the other members of his colony and to communicate the location of the food source to the others. Sociality and communal living are so ingrained for naked mole rats that if one is kept isolated from others of his kind he will die. Scientists think that eusociality evolved for these two species because searching for food in groups and sharing the spoils represented a big advantage in the harsh, arid lands they inhabit. Indeed, thanks to all their unique adaptations, naked mole rats (or "sand puppies" as they're also called) thrive in their native habitat and are very numerous. Naked mole-rats also enjoy unusually long lifespans for rodents and have been known to survive in captivity for 27 years or more. Scientists think their longevity has something to do with their slow metabolism, and during lean times mole rats might also go into hibernation further slowing down their vitals.
When the queen dies, civil war breaks out in a colony of naked mole rats. Several of the larger female soldiers may try to gain weight and step up to fill the power vacuum and become the new reproductive female. Peace is only restored when one of the females has achieved dominance, probably killing all her rivals. Scientists have also found that within a colony there are sometimes a few members who are fat and not into the whole working thing. These rebels may eventually wander away from the colony in search of a not-so-closely-related mole rat of the opposite sex to mate with and maybe start a brand new colony.
Want to Learn More?
When I went to the National Zoo a month or so back I was actually super psyched to see that they had naked mole-rat and damaraland mole-rat colonies in the small mammal house. Perhaps even more surprisingly, I was not the only zoo-goer who seemed to be excited about these odd, ugly rodents. Sadly, I do not have a photo to share (I'm between cameras at the moment, but working on it), but apparently the National Zoo has a naked mole-rat webcam although I've been checking the last few days and its been offline.
There's lots more information on these guys available on the web: I would suggest you start out with the National Zoo's extremely informative page on mole rats as well as the other links included in this post.
Images: Damaraland mole-rats photo by T. Jackson found on NPR website; pregnant naked mole-rat queen photo by Rochelle Buffenstein/City College of New York found on annotated budak; feeding naked mole-rat photo from NEFSC photo archives found on abolitionist.com.