One of the most unusual, bizarre-looking and fascinating animals known to science is found in the arid earth of sub-Saharan Africa. It lives in subterranean colonies, with a single breeding queen. It has a worker caste that takes care of the young animals and soldiers that defend the colony: It is “eusocial” — truly social, like ants in a nest or bees in a hive. But this is not a social insect — it’s one of us, a mammal.
The naked mole-rat is a rodent, the only known vertebrate that lives in such a “hive” society, with a caste system more commonly associated with insects than mammals.
Farmers in Kenya, Ethiopia and Somalia have been aware of naked mole-rats for years. They saw the mounds of earth pushed up in molehills and knew that some kind of animal was eating the succulent tubers and roots of the plants that grow in the hard soil. But the unique social system of the naked mole-rat was not guessed at until 20 years ago, when an evolutionary theorist was put in touch with a biologist working in Nairobi.
Richard Alexander, an entomologist from the University of Michigan, was on a lecture tour that had taken him to Northern Arizona University at Flagstaff. In his lecture there, he outlined what a hypothetical social vertebrate would look like and the sort of place it would live. A mammologist in the audience put his hand up and said that Alexander had just described the naked mole-rat and suggested that he contact Jennifer Jarvis, who was working on the mammal in Africa.
Over the next few years, the details of this remarkable animal emerged, and it turned out that Alexander’s hypothetical animal was actually very real. Adapted to a life spent entirely underground, naked mole-rats have lost all of their fur. Sensory whiskers cover their skin and tail, allowing the rats to navigate while traveling forward or backward through their tunnel system. They have poor eyesight, but their hearing is well-tuned to the low frequencies that can be transmitted through soil.
The tunnels are dug with massive, ever-growing buck teeth that are permanently outside the mouth. The lips close behind the teeth so the rat can dig through the hard-packed soil without getting dirt in its mouth. An estimated 25 percent of the rat’s total musculature is devoted to the jaws, and the incisors have the unprecedented ability to move independently of each other. So as well as for digging and feeding, naked mole-rats can use their teeth for manipulating objects. The teeth are also thought to be involved in social interactions, such as grooming and fighting.
Naked mole-rat colonies are headed by a queen who mates with a few of the colony’s males. She reproduces up to five times a year, giving birth to an average of 12 pups per litter (litter sizes of up to 27 pups have been recorded, the largest known of any mammal). All the other individuals in the colony help the queen — by caring for the pups, foraging for tubers, maintaining the burrows and defending the colony against foreign rats or snake predators. Colonies average 70 individuals, but some containing almost 300 animals have been observed.
The queen is thought to use hormones to prevent the other females from attempting to breed. Colonies are long-lasting, and most individuals are likely to be siblings, but when the queen dies a few females will fight to the death to replace her. On attaining queenhood, the winning female grows longer than the usual 8 cm (even though she is already adult), by increasing the space between her vertebrae.
Now scientists have found that as well as having all these specialized behaviors and adaptations, naked mole-rats have equally specialized and unusual brains.
Kenneth Catania and Michael Remple, of Vanderbilt University in Nashville, Tenn., looked at the naked mole-rat’s somatosensory cortex — the part of the brain related to the sense of touch. In mammals, this part of the brain is organized in such a way that adjacent areas of the body are represented by adjacent areas of the brain. In humans, most of the somatosensory cortex is devoted to the touch-sensitive hands and lips, but in naked mole-rats a whopping 31 percent of the cortex is dedicated to the teeth. Moreover, the somatosensory cortex of the naked mole-rat is 50 percent larger than that of closely related lab rats.
Writing in this week’s issue of Proceedings of the National Academy of Sciences, Catania and Remple conclude that the brain of the naked mole-rat has been reorganized over evolutionary time, remodeled to accommodate the important role of the teeth in the life of the rat.
“These findings,” the authors write, “indicate that major cortical remodeling has occurred in naked mole-rats, paralleling the anatomical and behavioral specializations related to fossorial [digging] life.”
Cornell biologist Paul Sherman, a leading mole-rat researcher, described them fondly as “overcooked hotdogs with buck teeth,” but he would be first to enthuse about how this ugliest of beasts beautifully demonstrates many facets of evolution.