When it comes to ant immunity, the colony is the body. Much like the billions of immune cells that protect us from disease, the horde of ants in a colony coat their nest in antibacterial resin to ward off pathogens. Now, scientists have found more intriguing parallels. For the first time, they have observed ant colonies removing infected larvae to prevent the spread of sickness, a behavior analogous to white blood cells ridding the body of dangerous pathogens and previously seen only in bees. And just like inbred humans and purebred pets, inbred ant colonies have weaker "immune systems": they're slow to detect disease and remove infected larvae, putting the entire population at risk.
Researchers already knew that ant colonies where queens mate with close kin are more vulnerable to attacks from fungi, microscopic worms, and bacteria. But previous studies couldn’t find problems with individual ants. So evolutionary biologist Sylvia Cremer of the University of Regensburg in Germany focused on how inbreeding influenced the whole colony.
Cremer's team produced two lineages of Cardiocondyla obscurior ants, a species native to Brazil: inbred ones created by mating queens with their brothers for 10 generations and genetically diverse colonies in which queens were allowed to mate with nonsiblings. To test the colonies’ susceptibility to disease, the researchers infected larvae of both colonies with a parasitic fungus, Metarhizium anisopliae, which eats insects from the inside out.
Within the first day, the noninbred colonies began removing brood exposed to the fungus from the nest, despite the fact that none showed any sign of infection. In contrast, the inbred colonies waited 3 days before getting rid of diseased larvae, at which point the fungus had already started developing inside the brood’s bodies. Although both types of colonies successfully dodged the disease, the genetically diverse ants saved a larger percentage of their brood, the team will report  online tomorrow in the Proceedings of the Royal Society B.
Cremer says that the ants must have a way of sensing when larvae are infected—and that inbreeding disrupts this sense. “We think these inbred ants cannot smell the pathogen and have to rely on some other signal coming from the larvae once they are really infected,” she says.
Entomologist Marla Spivak of the University of Minnesota, Twin Cities, who researches disease in honeybee colonies, says she was “really glad to see an example of this behavior in ants.” The study bolsters the idea that ant colonies have social immune systems, she says, and adds a novel behavior to the colony’s known repertoire.
The original version of this article stated that inbred ant colonies waited 24 hours before removing diseased larvae. They actually waited 3 days.