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loyalties to their own colony and will attack intruders that smell like they come from a foreign nest. If a new colony is established too near an existing colony, fights between the workers of the two groups can result, and several scientists have suggested that this generally pugnacious behavior could have evolved to be directed at ants of other species as well. If the two species were closely related, and hence shared a more recent common ancestor, the likelihood of them also becoming tolerant of a captured pupa or larva is increased, because the captive would smell more familiar.

Jeannette Beibl, a researcher at Regensburg along with Foitzik, examined the DNA of numerous slave-making species. They and colleagues R. J. Stuart and J. Heinze determined that the practice evolved independently several times in different groups of ants, some relatively recently, at least by evolutionary standards. This variation suggests that different selection pressures might have caused slave making to evolve in the different species.

Six-Legged Constables

IF THE army ants aren't a real fighting force and the slave makers are just parasites with an uncanny resemblance to their hosts, do treachery and aggression exist at all for the social insects? The answer is a resounding yes. The carnage is subtle, but far more devastating in its after effects than even the most formidable slave-taking raid. In evolutionary terms, loss of life is not nearly as injurious as loss of reproduction. The social insects, with their suicidal colony defense and sterile workers, have perplexed evolutionary biologists since Darwin. While biologists have mostly explained the benefits of such extreme cooperation for the individual colony members, those busy little virgin bees and ants still turn out to show some enterprising forms of rebellion.

Although worker ants, bees, and wasps cannot mate, they often possess functional ovaries and can produce their own eggs. These unfertilized eggs develop into males, because throughout this group of insects and a few others, daughters have two copies of each chromosome, like humans and other vertebrates, and develop from fertilized eggs, but sons have only the mother's genes and, in effect, have no father. (I often give my animal behavior students an exam question that asks whether it is true that a honeybee male has a grandfather but no father. The ones who get it are triumphant, often hammering the point home in far more text than necessary, while the ones who don't flounder in ever-widening circles of confusion; one ended up declaring in apparent despair, "No, every animal has a father, if they didn't have a father they wouldn't have a mother and then what would happen?")

This genetic oddity means that workers are often more closely related to each other than they are to their own offspring, although the exact proportion of shared genes varies depending on how many males have mated with the queen. The real genetic payoff comes, as I pointed out in an earlier chapter, not from the workers helping to rear their sterile sisters, but from production of the future reproductives, the queens and drones that will leave the colony and found a nest of their own.

Under some circumstances, therefore, it is beneficial to an individual worker to lay some eggs that will become male reproductives. But the other workers would pass on more of their genes by investing in their brothers, the sons of the queen, rather than their nephews, the sons of their sisters. So you might expect that workers would sabotage each others' efforts to slip a few of their own eggs into the hive. Indeed, Francis Ratnieks and Kirk Visscher documented exactly such behavior, termed worker policing, in honeybees. The bees are able to tell which eggs are laid by the queen and which by their sister workers and will remove the latter and prevent them from developing. Visscher and Reuven Dukas discovered that the workers can even detect the degree of ovarian development in their sisters and act more aggressively to the ones that are on the verge of producing their