The lifestyles of insects, such as bees and wasps, range from simple solitary arrangements to more recently evolved, highly complex family social structures. One major factor that allowed social complexity to evolve may have been insects’ ability to defend their colonies from the spread of disease, according to a recent study in Proceedings of the Royal Society B.
As hives grew from a few individuals to dense and populous family groups, they became more vulnerable to outbreaks. Cooperative disease defenses, termed “social immunity,” include social grooming and cannibalism of infected individuals. Such specialized cooperative defenses might have helped insects evolve into large eusocial groups with separate and permanently reproductive and sterile castes, hypothesizes coauthor and behavioral ecologist Michael Taborsky at Switzerland’s University of Bern.
To test whether social immunity could have evolved as a stepping-stone to eusociality, Taborsky and his coauthors conducted experiments on fruit-tree pinhole borers, a species of ambrosia beetle at an evolutionary stage before eusociality. The beetles live in cooperative family groups, but are not divided into breeding and nonbreeding castes. New adult females briefly act as workers, lingering in the nest for several weeks to babysit their larval siblings before leaving the nest to breed.
Taborsky’s team injected deadly mold spores into some beetle nests in glass test tubes. Other, control nests only received an injection of harmless buffer solution. After the injections, females in moldy nests displayed social immunity behaviors, such as grooming each other and cannibalization of infected beetles, more frequently than females in control nests. New adult females also lingered several days longer in moldy nests than in controls before dispersing to breed, likely to boost their own genetic success by caring for their siblings. In a later experiment, the researchers injected a second set of nests with either mold or harmless control solution, then cut each nest in half with a sterile scalpel. They removed all the beetles from one half of each nest, and left beetles in the other half. After three days, the researchers sampled both halves of each nest for mold, and found about twice as much in the halves where beetles had been experimentally removed, suggesting their hygienic behaviors reduced the infection intensity.
Combined, these findings suggest “that social immunity is not a consequence of eusociality, but rather a step that enables the evolution of eusociality,” Taborsky says. By keeping disease at bay, cooperative defenses may have helped large nest populations to survive and evolve, opening the door for social specialization to care for large numbers of offspring, he says.
Behavioral ecologist and evolutionary immunologist Sylvia Cremer applauds the study and agrees that pinhole borer ambrosia beetles present an interesting new case of social immunity. But Cremer—who published a conceptual framework for social immunity in 2007, defining it as the cooperative disease defenses protecting the colonies of eusocial species—doesn’t share Taborsky’s evolutionary conclusions. Their disagreement comes down to murky definitions of eusociality.
“I can think of at least four different things people mean when they say ‘eusocial,’” says evolutionary biologist Jacobus Boomsma at the University of Copenhagen in Denmark. Taborsky defines eusocial species as those with permanently sterile castes. Cremer, who’s based at the Institute of Science and Technology Austria in Klosterneuburg, thinks species with temporarily celibate workers are eusocial, such as pinhole borer ambrosia beetles. So Cremer wasn’t convinced that the recent findings implied social immunity evolved before the genetic incentives of eusociality. Perhaps ambrosia beetles show a crude expression of social immunity, Boomsma offers, which more evolved social insects have taken further.
Future work could help resolve the uncertainty, Taborsky and Cremer say, by repeating similar experiments across a range of social insect lineages, including bees, wasps, cockroaches, termites, and even ambrosia beetle species whose social systems run the gamut from solitary to debatably-eusocial. Comparing behaviors across a group of species that range from simpler, more primitive social structures, to more newly evolved and socially complex societies might elucidate the evolutionary progression of cooperative group defenses. “To understand whether social immunity only occurs in the eusocial insects or not,” Cremer says, “I think it would be really interesting to look for it across this gradient.”