Research from North Carolina State University finds that among eusocial insects - like ants, bees and termites - the more individuals there are in a typical species colony, the weaker the species' immune response. The finding strongly suggests that hygiene behaviors, and not just immune systems, play a key role in keeping eusocial insects healthy.
Eusocial insects live in groups. And living at close quarters with many other individuals would appear to increase their risk of contracting disease. Yet eusocial insects are incredibly successful, raising the question of how they are able to thrive.
The "social group hypothesis," argues that the eusocial lifestyle has given eusocial insects stronger immune systems. A second hypothesis, the "relaxed selection hypothesis," argues that eusocial insects have evolved specific behaviors that reduce the risk of disease transmission.
"We wanted to test the social group hypothesis to see which of these hypotheses was accurate," says Margarita López-Uribe, a postdoctoral researcher at NC State and lead author of a paper on the work.
To test the social group hypothesis, the researchers tested the "encapsulation response" in 11 different insect species: six eusocial insects, including ant, eusocial bee, eusocial wasp and termite species; and five non-eusocial insects, including non-eusocial bee, non-eusocial wasp and cockroach species.
The encapsulation response is an immune response in which hemocytes in an insect's hemolymph engulf and immobilize any foreign substances that enter the insect's body. Hemocytes are roughly analogous to white blood cells, while hemolymph is roughly analogous to blood in mammals.
For this study, the researchers inserted a probe into the insect samples and measured how strong the encapsulation response was.
They found that eusocial insects generally had a less pronounced immune response than their non-eusocial counterparts. They also found that the larger the colony size associated with a species, the weaker its immune response.
For example, honey bees (Apis mellifera) form large colonies and exhibit complex group behaviors - and they had a significantly less pronounced encapsulation response than sweat bees (Halictus ligatus), which live in much smaller groups.
"This tells us that the behaviors we see in eusocial species - like grooming each other or bringing antifungal materials into nests or hives - are playing an important role in colony health," López-Uribe says. "And this argues in favor of the relaxed selection hypothesis."
The paper, "Reduced cellular immune response in social insect lineages," was published online March 9 in the Royal Society's Biology Letters. The paper was co-authored by Warren Sconiers, Steven Frank, Robert Dunn and David Tarpy of NC State.