We tend to think of parasites as creatures that attach themselves to their hosts or worm their way inside, consuming the hosts' resources directly from their bodies. But there are other parasites that steal from their hosts simply by freeloading off them. The classic example is the cuckoo, which lays eggs in the nests of other birds, who then happily feed the cuckoo's offspring as if they were their own.

A successful strategy like that is hard for evolution to pass up. So it really wasn't a surprise to find out that there are also parasitic species of ants, ones that breed within the nests of other ants and raise their offspring using the resources provided by the hosts. Now, researchers have developed evidence that at least one of those species evolved within the nests that they now occupy.

The parasitic ant in question has the evocative name Mycocepurus castrator. It lives off the hard work of a related leaf-cutter ant named Mycocepurus goeldii. Although the host species is distributed widely within South America, M. castrator has a much narrower range—a single stand of eucalyptus trees conveniently located on the campus of Sao Paulo State University in Brazil.

The close relationship between the freeloader and its host is typical of the parasitic ants and may be necessary for the colony to not recognize them as foreign and expel them. But in the case of M. castrator, the relationship is probably a result of how the species originated: researchers have used DNA similarities to show that it probably evolved from the species it now parasitizes.

They also supply a plausible means for this to happen. Most of the descriptions of ant and bee colonies suggest that they have a single queen that is the only individual to lay eggs. As with most things in biology, however, it's rarely quite so well defined. For the host in this example (M. goeldii), having a single active queen is optional. In some colonies that have been studied, there is more than one active queen.

This, the authors note, is a recipe for "cheating" in social insects, where some individuals start reproducing to pass their genes on at the expense of the queen's. They suggest this is how the parasitic species, M. castrator, got its start—simply as a group of cheaters within the main colony. If the cheating behavior was inherited, then it could be passed on stably within the colony. But that wouldn't create any reproductive barriers; the cheaters could still breed with any of the colony's ants.

What the researchers found, however, is that reproductive barriers seem to have formed. In the host species, new queens typically leave the nest by flying, and they mate during that flight. By contrast, the parasite species mates within a nest, and new queens leave simply by walking. In addition, there have been anatomical changes to the species' genitalia that make mating much more difficult between the two. These differences are part of a larger set of anatomical differences that now separate the two species, which includes a significant reduction in size among the parasites.

While it's not clear how these anatomical and behavioral differences developed within the cheating population, the authors make a strong case that genetic differences started piling up while the two species were not only still capable of interbreeding but located in close physical proximity. This would make it an example of what's called "sympatric speciation." (Allopatric speciation occurs when there's some barrier between mating that allows two species to go their separate ways in reproductive isolation.)

The existence of sympatric speciation has been somewhat controversial within the evolutionary biology community. And the existing paper doesn't make an airtight case; it's possible that the two species separated before one of them adopted the freeloading lifestyle, then moved back in to the nests of its relatives. What may shed light on the situation is a bit of genome sequencing, which will provide a clearer picture of the genetic changes that separate the two species.

Current Biology, 2014. DOI: 10.1016/j.cub.2014.07.048 (About DOIs).