Sperm bank

Once fertilized, the young female goes on her way; she pulls off her own wings for which she has no further use as she will never mate again. The sperm acquired during her mating flight she keeps in an oval pouch located in her abdomen, known as a spermatheca. In this way, she accumulates a stock of several hundred thousand spermatozoa, sometimes even several million, in a dormant state. For the rest of her life, she will draw upon them to fertilize her ova, eventually giving birth to offspring whose fathers have been dead for many years. The survival of such a natural sperm bank is somewhat mysterious. How do queens manage to keep spermatozoa in their required functional state? It has been suggested—though this remains a hypothesis— that glands close to the spermatheca secrete a substance capable of nourishing the male cells.

Be that as it may, the queens are now primed and ready to found their family. This they do in ways which vary from species to species: the fertilized females of wood ants, for instance, are incapable of establishing their own colony; but they have invented a subterfuge, in that they act as parasites on established colonies, sometimes of another species. The behaviour of others, such as the young queens of the army ant, is even less complicated: having no wings, they never leave the nest; they merely excrete their pheromones and wait for male outsiders to come in and mate with them. If two queens are fertilized in a nest, the colony splits in two.

These wood ants and army ants, however, are exceptions. In the majority of cases, the young fertilized female initiates a new colony without any help (claustral colony founding) after the mating flight. Having to raise her brood entirely from the limited energy obtained from the histolysis of her wing muscles and the fat that she accumulated before the mating flight, she first produces very small workers so as to increase their numbers and thus the chances of success. It is better to go for numbers and to have about twenty undersized offspring than to have larger but fewer workers. By the following year, this first generation has reached adulthood and the queen, surrounded by her earliest workers, which busy themselves with the mundane household chores and the hunt for food, can now quietly devote herself to her primary function—that is, she lays. The foundations of a society are in place and the colony can now develop properly. For three or four years, it will grow exponentially until it reaches its maximum size.

Once the group is well established, the time has come for the queen to start conceiving not just workers, but also males and fertile daughters. Among ants, sexual differentiation is a simple process: any unfertilized egg (a haploid) will produce a male, whereas any fertilized egg (a diploid) will develop into a female. One of the most surprising aspects of the whole business is that the queen appears to have the power of choosing the sex of her young, of 'deciding' whether or not to fertilize an egg by opening or failing to open the aperture of the spermatheca. The 'choices' she makes vary in accordance with the age of the colony; and in the early days, she will produce only workers and no males. Her attitude also depends on the time of year: a great number of her earliest eggs, laid in the spring, will be males, and later on she will fertilize almost all her eggs so as to have mainly daughters.

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