Of the 26 genera of subteranian rodents, only the Bathyergidae display a range of social behaviors that ranges from solitary independent living to cooperative breeding and eusociality. 1 Of the social Bathygergids, Naked Mole Rats are some of the most social. Eusociality is a term that refers to the social structures of animals that fulfill three requirements: 2

Reproductive Division of Labor.

Social overlap of the generations.

Cooperative care of the young.

As explained more fully below, Naked Mole Rats fulfill all three of these requirments.

Naked mole rats live in colonies that have an average of 80 members, although colonies with nearly 300 mole rats have been found. 3 This makes H. Glaber colonies the largest of the social Bathyergids . H. Glaber share a burrow system and live togeather throughout the year, and they share the common needs of food foraging, defense, reproduction, burrow maintainence and care of the young. Source: http://www.flickr.com/photos/ipuzzled/93802741/

Tunnel systems are closed off from the outside world, causing the whole system to be oxygen deprived. 4 Mole rats primarily expand their burrow system just after the rain when the dirt is soft and capable of being tunneled through. 5

The burrow system has several communal chambers. There is a single sleeping chamber that is used both for breeding and everyday sleeping. There is a feeding chamber where food collected during burrowing is stored in and communally consumed. There is also a communal defication area. 6

The biomass of the entire colony is generally around 2000g, which is about the weight of an individual mole rat of one of the larger solitary species, such as the Cape Dune mole-rat. 7

Naked mole rats display limited division of labor between different castes. The largest and most substantiated caste division is between the reproductives and the non-reproductives. Generally only one reproductive female is found per colony, although there have been instances where two females share the position. 8 In contrast, there are up to three breeding males, who often share paternity over the broods of the Breeding female.

Source: http://www.flickr.com/photos/artbandito/18105386/in/set-429942/ Divisions within the nonbreeding caste are less well substantiated. Jarvis (81) reported observing three distinct worker castes in her mole-rat colonies. She labeled these frequent workers, infrequent workers ad non-workers. Frequent workers were smaller, and did the lion's share of the work moving food, transporting soil and carrying children. Infrequent workers spend more time resting in the sleeping den, but do work occasionally. Non-workers however spend most of their time in the communal area helping to take care of the young.

However, other studies have not been able to confirm these findings. Lacey and Sherman (91) saw behavioral variation amongst non breeding animals to be strongly corrleated with body weight, but not distinct castes. Their findings were similar to jarvis findings, that smaller workers were most frequently involved in food carrying, soil tranport, and collection of nest materials, with larger non-workers appearing lazier. However, larger animals are the colony members that were most likely to be involved in volcanoing and new tunnel construction. 9 Furthermore, when presented with intruders like snakes, larger nonbreeders were the primary participants in defensive activities. 10 As volcaoing is the most likely time for predation events to occur, this suggests that larger individuals frequently are found not working because they are saving their strength for the more energy demanding activities involved with burrow defense and expansion.

Assuming there are castes, there is some evidence suggesting that H. glaber displays age polyethism. Jarvis ('81), still working in the assumption of distinct castes, suggested that all newborns, once weaned, enter into the frequent worker caste. Faster growing mole-rats would quickly attain larger size and move into the infrequent worker caste, whereas slower growing mole-rats would continue in the frequent-worker caste, permanently. Faster growing mole-rats also may go on to become breeders when the current queen dies. This is similar to what has been found about Damarland mole-rats.

Determining social status associated with a host of factors is also difficult. Very little agressive behavior has been observed within mole rat colonies, with the exception of shoving behavior by the queen. However other cues have been used to derive theoretical social heiarchies from the availible data. One such cue is the passing behavior of mole rats in tunnels. Almost always one rat will pass over the another rat, rather than passing side by side. It has been suggested that the dominant individual in a pair will be the one that passes on top. 11 Domiance patterns derived from this behavior have a high correlation with domiance patterns developed by watching agonistic interactions, and are strongly linear. Furthermore, the domiance rank of an individual has a strong negative correlation with the mass of an individual, and a positive correlation with age and concentration of urinary testosterone levels. 12

Other researchers have found less clear cut answers. Using different tests to determine dominance, such as tugging on food, some researchers have come up with differnt domiance patterns, that are very non-linear. 13 However it has been suggested that these differing findings may be a result of the different tests used to determine dominance. One thing that is clear is that an individuals social status is not fixed once individual attains adulthood. The queen is almost always considered to be the most domiant individual in the colony, however replacement of the queen generally comes from within the colony, so it is clear that the social rank of adults changes changes.

Generally in species where social position is variable, animals dont develop morphological distinctions that specialize them for their role. 14 Naked mole rats appear to be a slight exception to this rule, as some morphological specializations can be seen in some of the majro roles held by individuals. The most elaborate and distinctive change happens in the the Breeding female, whose body lengthens and becomes much larger when she dominates the rest of the colony. 15 This extention of the body has been shown to be a result of the vertebra lengthening. The method by which this happens is currently unknown, however it has been suggested that it is an adaptation to allow the breeding female to become pregnant without getting stuck in the tunnels. 16

O'Riain has identified other changes that happen to a division of mole rats that he terms the defender mole rats. 17 The defender mole rats undergo several morphological specializations, including an increase in incisor width, an increase in jaw musculature, and the developement of a heterotrophied temporal musculature resulting in what is described as a ``square shaped head.'' 18 Source: http://www.flickr.com/search/?q=naked&w=81222110%40N00

These adaptations make sense in the light of the fact that 1) the morphological changes undergone by those in the defender group presumably dont impact the ability of an animal to change social role and 2) the role of breeding female is the last role a individual will ever attain, so the morphological changes that she undergoes will not affect her ability to hold other social roles.

In H. glaber colonies, nonreproductives contribute to the reproductive effort of the breedign female in direct and indirect ways. Indirect fashions in which they assist include foraging for food, digging new sections of burrow, defense of the colony, maintnence and repair of the burrow system and the collecting and transpotation of nesting material. They also contribute directly to the reprouctive efforts. Males and females increase time spent huddling within the nest from the time just before the pups are born till four weeks later when they are weaned. 19 Non-breeding males and females will also frequently nudge and groom the newly born pups; Source: http://www.flickr.com/photos/45088087@N00/83174691/ retrieve them when they wander astray or are carried along by the movements of workers traveling through the central nest; and carry the pups away in the case of alarm calls being sounded in another part of the nest. 20 Additionally, mole-rats practice allocoprophagy, or the repeated consumption of feces. Pups do not have the bacteria yet living in their digestive tract to allow them to digest food. So they eat the pre-digested feces of the other colony members. 21

Various studies have found evidence of generational overlap in H. glaber. In a study by Braude (1991), it was found that 21-80% of marked colony members were captured one year after marking, 15% for two years, and 2% for more than three years. H. glaber individuals are known to live up to 30 years, 22 and some individuals remain queen for as long as 18 years. The queen meanwhile can have up to five litters a year, the gestation period is 70 days. 23 Thus there is distinct evidence that subsequent generations coexist side by side. Most recruitment into the colony is from the young of the colony, so few colony members come from outside the extended family. 24

Eusociality probably developed via the subsocial route of social formation. In this route, the offspring delay dispersion until they are sexually mature, usually choosing to remain in their family group through their adulthood. The choice is usually made by weighing the risks of leaving versus the benefits of staying with the family. The risks include: 1) being able to find suitable territory, 2) being able to find a mate and 3) successfully reproducing alone without helpers. However, by remaining at home, the individual may never gain the opportunity to mate. The choice of remaining at home does provide inclusive fitness benefits, meaning they are benefiting by helping their close relative reproduce. Naked mole-rats have a society that is built upon this subsocial route. They risk much more by leaving the colony than remaining as a non-breeder. Those that leave the colony will have trouble finding new territory to settle in, as there is a high habitat saturation, and they will have trouble reproducing without helpers (females lack the body fat to be able to produce enough milk for her litter, so she must be fed by the rest of the family). Leaving home also would result in increased predation. The non-breeders, by remaining, contribute to the reproductive success of their queen by sacrificing their own chances, but they do benefit by being closely related to her. Therefore, naked mole-rats do benefit by remaining in their family nests, creating a eusocial society. 25

The common ancestor (Bathyergidae) of the mole rats was probably solitary, given that most subterranean mammals are, but there is some argument over this. The trait of being social could just as easily have been lost in many of the modern day species that are solitary. Comparative studies have been inconclusive as to whether this trait was gained by the mol-rats, or if it is left over from their ancestor. Eusociality must have been developed at some time, though. The original thought was that it appeared in naked mole-rats due to inbreeding which resulted from the lack of dispersion. But the Damaraland mole-rat, which is an out-breeder, also displays eusociality. Instead, eusociality seems to arise from the environmental constraints of high habitat saturation.



1. Bennett and Faulkes, 2000, Ch. 42. Jarvis, 19813. Jarvis, 19914. Milus, 20065. Milus, 20066. Jarvis, 19817. Bennett and Faulkes, 2000, Ch. 48. Jarvis, 19919. Braude, 199110. Lacey and Sherman, 199111. Clarke and Faulkes, 199712. Clarke and Faulkes, 199713. Bennett and Faulkes, 2000, Ch. 414. Bennett and Faulkes, 2000, Ch. 415. Jarvis, 199116. Jarvis, 199117. Bennett and Faulkes, 2000, Ch. 418. O'Riain, 199619. Lacey and sherman, 199120. Jarvis, 199121. Jarvis, 198122. Bryner, 200623. Ciszek, 199924. Braude, 1991.25. Bennett and Faulkes, 2000, Ch. 8