Division of Labor among Mole-Rats
Peter Corning offers a bioeconomic analysis of mole-rat colonies:
One compelling example of a superorganism in nature involves the naked mole-rat (Heterocephalus glaber), a unique African rodent species that lives in large underground colonies (usually numbering 75-80 but sometimes over 200). Naked mole-rats represent a particularly significant illustration of an economic division of labor, because these odd-looking animals — affectionately dubbed “sabre-toothed sausages” — have morphologically-specialized castes and a pattern of breeding restrictions that is both unique among mammals and suggestive of eusocial insects. Typically (but not always), the breeding is done by a single “queen”, with other reproductively suppressed females waiting in the wings. The smallest of the non-breeders, both males and females, engage co-operatively in tunnel-digging, tunnel-cleaning and nest-making, as well as in carrying pups, foraging and the transportation of food (succulent tubers) within the colony’s often extensive tunnel systems. (One investigator, Robert A. Brett, found a tunnel system in Kenya that was more that 3 kilometers long, altogether, and occupied an area equivalent to 20 football fields.) Sherman et al., (1992:75), who have studied these animals extensively, provide the following description of the mole-rats’ co-operative tunnel-building efforts:
The animals line up head-to-tail behind an individual who is gnawing [with it's outsized, powerful front teeth] on the earth at the end of a developing tunnel. Once a pile of soil has accumulated behind the digger, the next mole-rat in line begins transporting it through the tunnel system, often by sweeping it backward with its hind feet. Colony mates stand on tiptoe and allow the earthmover to pass underneath them; then, in turn, they each take their place at the head of the line. When the earthmover finally arrives at a surface opening, it sweeps its load to a large colony mate that has stationed itself there. This “volcanoer” [so-called because its actions appear to an observer outside to resemble miniature volcano eruptions] ejects the dirt in a fine spray with powerful kicks of its hind feet, while the smaller worker rejoins the living conveyor belt.
The vital and dangerous role of defense in a mole-rat colony is also allocated to the largest colony members, who respond to intruders, such as predatory snakes, by trying to kill or bury them and by sealing off the tunnel system to protect the colony. The mole-rats’ “militia” will also mobilize for defense against intruders from other colonies.
Why do mole-rats utilize this highly co-operative survival strategy? Eusociality is relatively rare in nature, and the traditional view has been that a haplodiploid reproductive pattern provides a genetic facilitator. But this is obviously not the case with mole-rats, which are diploid. (Indeed, it seems that haplodiploidy is neither necessary nor sufficient; all species of Hymenoptera are haplodiploid, but most are not eusocial; on the other hand, all termites are eusocial and diploid.) Sherman et al., (1992) provide a bioeconomic (synergy) explanation for the mole-rat strategy: “We hypothesize that naked mole-rats live in groups because of several ecological factors. The harsh environment, patchy food distribution and the difficulty of burrowing when the soil is dry and hard, as well as intense predation, make dispersal and independent breeding almost impossible. By co-operating to build, maintain and defend a food-rich subterranean fortress, each mole-rat enhances its own survival” (p.78). (See also Sherman et al., 1991.) (Although it is not stressed in the mole-rat research literature, another critically important facilitator is a co-operative relationship — and synergy — between the mole-rats and an endosymbiotic bacterium which is able to break down the cellulose in succulent tubers.)
If the bioeconomics — the functional synergies — provide an important part of the explanation for the naked mole-rat survival strategy, the “political” aspects are equally important, and are also well-documented. As is the case with many other socially-organized species, naked mole-rats exhibit a combination of self-organized co-operation (pre-programmed individual “volunteerism”) and orchestrated social controls that are policed by various coercive means. The control role of the breeding queen is of central importance. The queen is usually the largest animal in the colony (size usually determines the dominance hierarchy), and she aggressively patrols, prods, shoves and vocally harangues the other animals to perform their appointed tasks. Indeed, it has been observed that her level of aggressiveness varies with the relative urgency of the tasks at hand. In addition, the queen acts to suppress breeding and reproduction on the part of non-queen females, who are always ready to take over that role. (Occasionally other females are allowed to share the breeding function with the queen; why this is so is not known.) The queen also intervenes frequently in the low-level competition that goes on among colony members over such things as nesting sites and the exploitation of food sources. And when the reigning queen dies, there is a sometimes bloody contest among the remaining females to determine her successor.
All of this control activity is facilitated by an elaborate communication system that includes 17 distinct categories of vocalizations — alarms, recruitment calls, defensive alerts, aggressive threats, breeding signals, etc. In fact, the mole-rats’ communication system rivals that of some primate species in its level of sophistication. Thus, a naked mole-rat colony may be characterized as a superorganism. . .