this site is maintained by Nona Poulton under the supervision of Dr. Verna Case as part of the course requirements of Animal Behavior (Bio 323) at Davidson College, Davidson NC.
The Naked Mole-Rat (Heteroceophalus glaber)
Click on an image to find out more about H. glaber!
Because of the unique social system of the naked mole-rat, many of the topics discussed in the pages below span two or more of the topics (i.e. mating system and eusociality), so there may be instances of repeated information, although repeating the information may be required in order to understand a concept. Thank you for visiting the naked mole-rat website!
Picture credits, left to right: the first two are courtsey of Stan Braude [braude@biology2.wustl.edu] and the last two are courtesy of Roger Segelken of Cornell University.
Basic Information and Interesting
Facts about H. glaber:
The H. glaber, or more commonly known as the naked mole-rat, has been described as being the only truly eusocial mammal. Other eusocial species like the bees, ants, and wasps (hymenoptera) live in much the same way as the naked mole-rat, having sterile castes of up to hundreds of workers and one breeding female. Unlike the hymenoptera, however, the naked mole-rat workers are not haploid and are only sterile because of breeding depression.
Naked mole-rats are about four to five inches long, weighing about 2 ounces each. The exception is the breeding female, who is typically much larger (six to eight inches long). They are virtually hairless, with the exception of whiskers and small hairs on their tail, both aiding in feeling their way around their underground environment. These hairs help quite a bit because they have very small eyes and that are almost useless. They also have small ear pinnae, but their hearing is acute, as evidenced by their large repertoire of social vocalizations. Their sense of smell is very important, and they are also very sensitive to vibrations in the ground and the movement of air currents. The animal appears to be naked at first glance, but a few pale-colored hairs are scattered about the body and tail, there are prominent vibrissae on the lips, and a fringe of fine hairs is present along the edges of the feet. This fringe helps to collect and sweep back loose soil during digging. The wrinkled skin is pinkish or yellowish in color. The hind feet also have five clawed digits. The body lacks sweat glands and the normal mammalian layer of subcutaneous fat (Jarvis 1978).
The naked mole-rat could live its whole life inside the burrow without seeing daylight. Because of this, they are very protected from predators. In fact, the only major predators they have are snakes, especially the rufous-beaked snake. Also because of their underground lair, they have adapted quite well to available food sources, which are roots, tubers, and all kinds of geophytes. They are strictly herbivores, with the exception of dead pups, which they will eat to conserve energy. The burrow has a large communal nest chamber, that is either unlined or partly floored with dry vegetation. Radiating outward are a series of foraging tunnels usually about 3 cm wide and 15-40 cm below the surface. One system was estimated to have more than 300 meters of such passages. There is much branching of the tunnels in the vicinity of food supplies. Bolt holes, reaching depths of about 70 cm, are sometimes present and may serve as emergency refuges. Above the burrow are the mounds of excavated earth, which are uniquely volcano-like in shape and in the presence of a central, unplugged hole. Digging is done primarily with the incisor teeth, and the feet are used to kick and push the loosened earth. An individual near the surface may work alone, but deeper burrowing is done by relay. One animal remains for a while at the earth face, loosening the soil and kicking it backward. Behind this animal is a chain of several others. When enough earth has accumulated, the first animal of the chain pushes the soil backward with its hind feet through the tunnel to a point near the surface and then takes a place at the end of the chain. Meanwhile, the other animals of the chain have advanced in position by straddling the one moving to the rear. Near the surface another individual remains for a while to collect the soil brought up by the others and kick it out through the central hole of the forming mound. At irregular intervals, this individual and the one doing the drilling are relieved by other members of the team.
They obtain all the water they need through their food; they do not drink. They eat primarily geophytes. This diet is high in cellulose, which is difficult to digest. Naked mole-rats have high densities of gut fauna that aid in digestion. They also regularly practice coprophagy, the reingestion of feces, which allows them to maximize their uptake of nutrients from their food.
Heterocephalus has the poorest capacity for thermoregulation of any known mammal. Its body temperature is relatively low, only 32° C. The temperature within its burrow is maintained at 30°-32° C, and the humidity at 90 percent, regardless of external conditions, which is why these mole-rats have evolved this way; to conserve energy, they have lost (evolutionarily) the ability to regulate their own body temperature. The animals avoid extreme temperatures near the surface by restricting burrowing activity to early morning and late afternoon. Most burrowing seems to be done following the rains, when the soil is more easily worked. Therefore, the important task of locating new food supplies is accomplished mainly during energetically favorable periods. The tubers, roots, and corms, which compose most of the diet, are generally left growing in place after discovery and are visited by individuals as the need arises. All animals huddle together to sleep, which helps them conserve heat.
Separate colonies may be located less than 100 meters apart in favorable environments. Colonies are led by a single large female and there are both working and non-working classes. The dominant female and the non-workers spent most of their time in the nest chamber, coming out to urinate or defecate, which are done in specialized areas. The workers cooperate in burrowing, gathering nest materials, and bringing food to the nest for the dominant female and non-workers.
Before full development of the social hierarchy (i.e. when a breeding female dies), females coming into estrus fight and frequently kill one another. Once the dominant female establishes her position, she suppresses breeding by the other females. She is the only one that mates, and she initiates courtship, generally with a breeding male.
These animals have been studied heavily because of their individuality and unique place in the animal kingdom since the mid 50s. The first colony to be captured and held in captivity was in the 70s. Many of those original colony members are still alive in captivity, so it is unknown exactly how long their lifespan is. It is projected to be around 20-30 years, which makes them the longest-living mammal of their size.
H. glaber's basic classification is as follows:
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Bathyergidae
Links:
For inspiration, please visit this Naked Mole-Rat-related site (it is simply amazing): Naked Mole-Rat Cam (updates every minute)
Like what you see? Buy this must-have mole-rat book at amazon.com: The Biology of the Naked Mole-Rat by Paul W. Sherman, Jennifer U.M. Jarvis, Richard D. Alexander. It's a page-turner!
Listen to naked mole-rats online while getting a good basic overview of their social biology.
This link has quite possibly the ugliest picture known to man. Please be seated while viewing. Please take precautionary measures if you have high blood pressure, are pregnant, or are just generally weak-stomached.
This may prove entertaining. It's from the April Fool's issue of Discover Magazine, 1995. And yes, they are actually mole-rats that they are joking about and no, you can't find these creatures in the wild.
Adopt a naked mole-rat at the St. Louis Zoo!
© Copyright 2002 Department of
Biology, Davidson College, Davidson, NC 28035
Send comments, questions, and suggestions to: vecase@davidson.edu