Naked mole-rat

Naked mole-rats are eusocial, meaning they live in large colonies in which only one female breeds and the majority of workers (both males and females) spend their entire lives working for the colony. This is an unusual characteristic for mammals. Colony size averages 70 individuals, but colonies of up to 295 have been observed.

The animals in these groups are very closely related. Workers are generally raising their siblings, because a single queen may reign for many years. Dispersal is quite rare and inbreeding is common, which results in a high degree of genetic similarity among colony members. If a queen dies or is removed from a colony, a few females may fight to the death in order to become the new queen.

Naked mole-rats live in complex underground burrow systems. The tunnels are about 1.5 inches (4 centimeters) in diameter. Some tunnels run just under the surface of the ground, while others can be up to 6.5 feet (2 meters) deep. There is a great deal of branching and interconnection of tunnels, with the result that a colony's total tunnel length can total up to 2.5 miles (4 kilometers). Tunnels connect nest chambers, toilet areas and food sources. Burrowing is the only way these animals find food, because they do not travel above ground.

Workers dig through the hard-packed soil with their powerful, ever-growing incisors, aided by the fact that a quarter of an individual's muscle mass is involved in jaw closure. Their lips actually close behind their incisors, enabling them to dig without getting dirt in their mouths. They work assembly-line style: the front animals break through the dirt while a string of workers sweeps the soil through the tunnel system to an opening at the surface, where another worker kicks the dirt up onto the ground above its head, forming a mole hill. Mole-rats seem to do the majority of a year's digging just after the rainy season, when the normally hard ground is softened.

These eusocial rodents also cooperate to thermoregulate. Unlike most other mammals, they cannot maintain a steady body temperature. Their temperatures fluctuate with the ambient temperature, making naked mole-rats essentially cold blooded. By huddling together in large masses, they slow their rate of heat loss. They also behaviorally thermoregulate by basking as needed in their shallow surface tunnels, which are warmed by the sun.

Scientists have found that the somatosensory cortex, which is involved in the sense of touch, is unusually large. Nearly a third of it is dedicated to the animal's four, large incisors. This suggests that naked mole-rats use their teeth to sense the world around them. As they evolved unusual physical and behavioral adaptations for their underground habitat, their brain organization has evolved in parallel, resulting in an equally specialized and unusual brain adaptation.