Problems with males

If one thing is certain about ants, it is that, apart from a very few rare species like those in the sub-family of the Ponerinae, workers cannot ever mate. For they have no spermatheca, the pouch that enables queens to keep a stock of males' spermatozoa throughout their lifetime. However, there is nothing stopping workers from laying eggs. Even if unfertilized they can always serve as a source of food for the colony or, if they develop, hatch into males (which it should be remembered always come from unfertilized eggs). In monogynous nests that have lost their queen, the workers actually succumb to the desire for motherhood. Because there is no more future for their colony, their best option is then to reproduce on their own.

When the queen is present, however, the workers generally remain sterile. This is especially remarkable since, according to the theory of kin selection, they would be better served by reproducing, thereby passing on more copies of their genes. If a queen has mated only once, the workers will share with their own sons half of their genetic inheritance, whereas their degrees of kinship with their brothers (who are male offspring of the queen) or their nephews (offspring of other workers) will drop respectively to 0.25 and 0.375. It is conceivable that, as they are closer to their nephews than to their brothers, it would be to their advantage to help their sisters reproduce rather than their mother. But the queen, too, has more to gain in the matter of heredity if she gives birth to her own male offspring, with whom her degree of kinship is 0.5, than if she is content to have grandsons (0.25). In this area as in others, the interests of the queen and the workers diverge, heralding new conflict between mothers and daughters.

But even this situation can change if the queen has mated with several partners. The workers are then genetically closer to their mother's sons than to those of their sisters and half-sisters.

So they are not going to let their equals reproduce. They keep each other under surveillance, functioning within the colony as what entomologists call 'police workers'. This system of monitoring works very effectively, as has been shown by studies of bees, who also do such constabulary duty. If a worker suddenly develops its ovaries, her nestmates lose no time in attacking her or in destroying any eggs she may have laid. In either case, the offender has little chance of succeeding in her designs. Myrme-cologists have concluded that the setting up of such a system of policing was essentially an outcome of the kinship ties within the colony.

But this behaviour can be explained in a quite different way, because it is likely that worker reproduction incurs costs for the colony. If all the workers started procreating, they would expend a great deal of energy in laying, which would have a detrimental effect on their daily work. There would be many fewer legs and mandibles available for protecting the nest and feeding all its inhabitants, and eventually the whole colony would be worse off.

Which of these two hypotheses, the one based on kinship, the other on productivity, is the more tenable? With the aim of resolving this question, we analysed in collaboration with Rob Hammond all the data available, which meant we could compare the reproductive behaviours of workers in different colonies belonging to fifty species of bees, wasps, and ants. Our first finding was that, in 90 per cent of these species, the majority of the males are offspring of queens and not of workers. This proves that workers make little use of their ability to give birth to their own descendants. We also found that the workers who do reproduce are no more numerous in colonies where there is high genetic diversity. Even in nests where they are more closely related to their nephews than to their brothers, and where procreating would therefore be in their interest, they still produce very few males. All this supports the case for productivity being the main factor explaining why workers do not usually reproduce. In a more recent study, which adds more data, Tom Wenseleers and Francis Ratnieks confirmed our finding that worker reproduction is rare, but they also found a tendency of workers to be less likely to reproduce when the queen has mated with several partners. Thus, variation in kinship across colonies probably also plays a small role in explaining variation in the reproduction of workers.

Anything goes

Close observation of ants shows that even in reproductive matters they like to display originality. Some queens have no objection to mixed marriages and will mate with males from outside the family. Others enjoy having it both ways and go in for sexual and asexual reproduction. There are even some who use cloning to reproduce descendants in their own image. Their males, not to be outdone, do the same. Among ants, it seems, anything goes.

Pregnancy And Childbirth

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