THE lifestyle of truly social insects, for example, termites and honey bees, is well known. However, until very recently there was no account in scientific literature of any truly social behaviour among vertebrates.
This position has changed dramatically since 1976, when details of the lifestyle of an inhabitant of the semi arid deserts of east Africa, a rodent called the naked mole rat, came to light.
This animal lives like a social insect. Truly social (eusocial) species, such as various insects, live in colonies containing hundreds of thousands of individuals. Only one female within each colony reproduces - the queen. Most of the other members of the colony are workers that spend their lives feeding, rearing and defending the offspring of the queen.
Such selfless behaviour poses a significant problem in understanding evolutionary biology. Natural selection is the mechanism of Darwinian evolution, and the winners are those most adept at the business of reproduction. Natural selection would therefore seem to distinctly favour the tactic of selfishness. So what could motivate the hoards of insect workers to surrender their reproductive privileges?
Eusocial animals such as bees and ants have an unusual genetic system called haplodiploidy. In this condition, females may be more closely related to their sisters than to their offspring.
It was therefore proposed in 1964 that this close relatedness between sisters would favour individuals that forgo producing their own offspring in order to help the queen produce more sisters. It became widely accepted that eusociality evolved in insects because of haplodiploid genetics.
However, not all eusocial species are haplodiploid. Richard D. Alexander of Michigan University rejected the haplodiploid hypothesis and argued that the primary selective factors favouring eusociality are ecological, not genetic. He specifically proposed that the care and feeding of the young by their parents and the construction of a nest safe from predators promoted the eusocial way of life. These ecological factors are associated with eusocial insects, but Alexander noted they are also present in many vertebrates.
If Alexander's ideas were correct, then evolution should have produced some examples of eusocial vertebrates, but no such examples were known. In 1974, Alexander published a landmark paper in which he imagined the lifestyle of a eusocial vertebrate. The colony would care for the young, and inhabit a secure nest which would be expandable to accommodate future generations. The nest would lie near abundant easily available food and there would be little competition between individuals.
Ideally, the nest would be subterranean, and Alexander proposed the eusocial vertebrate would be a mammal, specifically a rodent, many of which have the ability to burrow underground. He guessed it would primarily eat large roots that grow in the seasonally wet tropics.
Alexander delivered many lectures on this topic across the US. After one such lecture a member of the audience told him that his hypothetical eusocial mammal sounded very like a little known rodent that lives in underground burrows in Kenya, Ethiopia and Somalia. This is Heterocephalus glaber - the naked mole rat.
These animals are no more than six inches long at maturity. Their pinkish, wrinkled skin is almost hairless. They have prominent incisors, pinhole eyes and scrubby whiskers. They are no beauties one observer described them as sabertoothed sausages. They use their incisors to make networks of tunnels and chambers which can house up to 250 animals.
An intense programme of study of the naked molerat began in 1976. It was fairly quickly established that this is a eusocial animal. In each colony of naked molerats, only one female, the queen, gives birth to offspring and nurses the young. The queen is easily identified by her larger size, well developed nipples and enlarged vagina.
The queen is domineering and protects her position by biting, shoving and charging at her subjects. She rarely participates in the maintenance or the defence of her colony but frequently travels around the tunnels monitoring the activities of the colony.
The queen gives birth to three or four litters a year, each of about 10 pups. She nurses them for three to four weeks, after which they quickly become independent. Most of the parental care of the young is carried out by the queen and the small number of males with which she mates.
No female in the molerat colony, except the queen, engages in sexual activity. The other females never behave as if they are in heat, and non breeding males never engage in mating. Physiological studies have shown that the ovaries of the nonbreeding females never mature. On the other hand, up to threequarters of a colony's males produce sperm, although only a very small fraction engage in sexual behaviour.
HOW does the queen exert such control over the reproductive capacities of the non breeding females?
An early suggestion was that she does this by secreting chemical messengers called pheromones which suppress the reproductive machinery of the other females. If a non breeding female is taken from a colony and housed in isolation, she will mature and become reproductively active. But this happens even if she is exposed to soiled bedding or urine from her colony.
It appears, therefore, that the queen does not exert control by chemical messages alone. Perhaps her aggressive behaviour in itself is enough to block the release of reproductive hormones in subordinate female colony members.
No more than three males ink a colony copulate with the queen. It is not known how these males are chosen. A surprising observation is the apparent lack of competition between the chosen males even though the queen is in heat for less than 24 hours.
Apart from the queen and the few males who mate with her, what do the other members of the colony do? These are the principal participants in the maintenance and defence of the colony.
When a breeding male dies, a nonbreeding worker takes over his reproductive role. The transition from nonbreeder to breeder is relatively peaceful. The same transition in females, however, can turn violent. This aggression suggests that, at least to females, the benefits of directly reproducing are worth the risk of trying but failing to become a breeder.