Third Tuesday 2018-07: Naked Mole-Rat Event Cache

The naked mole-rat (Heterocephalus glaber), also known as the sand puppy, is a burrowing rodent native to parts of East Africa. It is closely related to the blesmols and is the only species in the genus Heterocephalus of the family Heterocephalidae. The naked mole-rat and the Damaraland mole-rat (Fukomys damarensis), are the only known eusocial mammals, the highest classification of sociality. It has a highly unusual set of physical traits that allow it to thrive in a harsh underground environment and is the only mammalian thermoconformer, almost entirely ectothermic (cold-blooded) in how it regulates body temperature.

The naked-mole rat lacks pain sensitivity in its skin, and has very low metabolic and respiratory rates. While formerly considered to belong to the same family as other African mole-rats, Bathyergidae, more recent investigation places it in a separate family, Heterocephalidae. The naked mole-rat is also remarkable for its longevity and its resistance to cancer and oxygen deprivation.

Typical individuals are 8 to 10 cm (3 to 4 in) long and weigh 30 to 35 grams (1.1 to 1.2 oz). Queens are larger and may weigh well over 50 grams (1.8 oz), the largest reaching 80 grams (2.8 oz). They are well-adapted to their underground existence. Their eyes are quite small, and their visual acuity is poor. Their legs are thin and short; however, they are highly adept at moving underground and can move backward as fast as they can move forward. Their large, protruding teeth are used to dig and their lips are sealed just behind the teeth, preventing soil from filling their mouths while digging. About a quarter of their musculature is used in the closing of their jaws while they dig—about the same proportion that is utilized in the human leg. They have little hair (hence the common name) and wrinkled pink or yellowish skin. They lack an insulating layer in the skin.

The naked mole-rat does not regulate its body temperature in typical mammalian fashion. They are thermoconformers rather than thermoregulators in that, unlike other mammals, body temperature tracks ambient temperatures. However, it has also been claimed that "the Naked Mole-Rat has a distinct temperature and activity rhythm that is not coupled to environmental conditions." The relationship between oxygen consumption and ambient temperature switches from a typical poikilothermic pattern to a homeothermic mode when temperature is at 28 °C or higher. At lower temperatures, naked mole-rats can use behavioral thermoregulation. For example, cold naked mole-rats huddle together or seek shallow parts of the burrows that are warmed by the sun. Conversely, when they get too hot, naked mole-rats retreat to the deeper, cooler parts of the burrows.

The naked mole-rat is the first mammal discovered to exhibit eusociality. This eusocial structure is similar to that found in ants, termites, and some bees and wasps. Only one female (the queen) and one to three males reproduce, while the rest of the members of the colony function as workers. The queen and breeding males are able to breed at one year of age. Workers are sterile, with the smaller focusing on gathering food and maintaining the nest, while larger workers are more reactive in case of attack. The non-reproducing females appear to be reproductively suppressed, meaning the ovaries do not fully mature, and do not have the same levels of certain hormones as the reproducing females. On the other hand, there is little difference of hormone concentration between reproducing and non-reproducing males. In experiments where the reproductive female was removed or died, one of the non-reproducing females would take over and become sexually active. Non-reproducing members of the colony are involved in cooperative care of the pups produced by the reproducing female. This occurs through the workers keeping the pups from straying, foraging for food, grooming, contributing to extension of tunnels, and keeping them warm.

Colonies range in size from 20 to 300 individuals, with an average of 75.