Mole-Rats

Little is known about courtship and mating in these animals, or about how new burrow systems become established. Individuals of solitary species will defend a territory, fighting viciously with any others it may encounter (perhaps under a rich food patch). Male-female pairs will tolerate each other in the same burrow for a day or so to accomplish mating, but they soon separate. The solitary female cares for the young until weaning, at about eight weeks; after this, antagonism and aggression build and the young disperse.

Biologists are especially interested in the highly social naked mole-rat, because it is one of the rare mammals that exhibits eusociality. Eusocial animals, such as many bees and wasps, live in colonies where only one or a very few individuals produce all the offspring, and the rest serve as sterile helpers; thus, we observe a division of labor. In naked mole-rats, one large female, known as the queen, is the only female to undergo reproductive cycling (i.e., to have an active estrus cycle) and produce offspring. In addition, a colony generally contains only 1-3 breeding males; reproduction in all other individuals is effectively suppressed, apparently by means of aggressive behavior and olfactory cues from the breeding adults, who appear to carefully sniff and monitor the physiological condition of the others. Non-reproductive adults carry out the tasks of tunnel digging and maintenance, foraging for and storing food, and caring for the young, which of course are not their own. If the queen should die, violent fights may erupt as potential successors try to assert themselves over their competitors. Ferocious defensive behavior has also been observed when the burrow systems of two separate colonies become linked by a common opening; mole-rats will fight to the death in defense of their home burrow.

Eusociality presents a problem to evolutionary theorists, which was recognized by Darwin. How could an animal evolve by means of natural selection (in which organisms with particular traits survive and reproduce better than their competitors), if they forsake their own reproduction and devote themselves to helping others reproduce? How can a trait spread, if its bearer fails to produce offspring? Darwin's answer was that if an individual contributes enough to the reproduction of closely related individuals who carry but do not necessarily express the trait, it can spread by natural selection. This is known as inclusive fitness. This explanation appears to work for naked mole-rats, in which the non-reproductive workers help the related queen produce offspring, who go on to recreate the same social structure at home or in another burrow. Evidently, this system suits them well; perhaps this kind of social group can collect more food than a single animal foraging alone. Perhaps the danger of traveling above ground means staying at home is more desirable than emigrating; suppressed reproduction may be necessary to keep colony numbers down and avoid starvation. It may be that their highly social population structure has enabled naked mole-rats to inhabit the hot, arid regions in Africa, where the solitary species are not found.