title: Antimicrobial strength increases with group size: implications for social evolution.
abstract: We hypothesize that aggregations of animals are likely to attract pathogenic micro-organisms and that this is especially the case for semisocial and eusocial insects where selection ultimately led to group sizes in the thousands or even millions, attracting the epithet 'superorganism'. Here, we analyse antimicrobial strength, per individual, in eight thrips species (Insecta: Thysanoptera) that present increasing innate group sizes and show that species with the largest group size (100-700) had the strongest antimicrobials, those with smaller groups (10-80) had lower antimicrobial activity, while solitary species showed none. Species with large innate group sizes showed strong antimicrobial activity while the semisocial species showed no activity until group size increased sufficiently to make activity detectable. The eusocial species behaved in a similar way, with detectable activity appearing once group size exceeded 120. These analyses show that antimicrobial strength is determined by innate group size. This suggests that the evolution of sociality that, by definition, increases group size, may have had particular requirements for defences against microbial pathogens. Thus, increase in group size, accompanied by increased antibiotic strength, may have been a critical factor determining the 'point of no return', early in the evolution of social insects, beyond which the evolution of social anatomical and morphological traits was irreversible. Our data suggest that traits that increase group size in general are accompanied by increased antimicrobial strength and that this was critical for transitions from solitary to social and eusocial organization.
authors.affiliation: Department of Biological Sciences, Macquarie University, New South Wales, Australia.
authors.forename: Christine
authors.initials: C
authors.lastname: Turnbull
authors.forename: Stephen
authors.initials: S
authors.lastname: Hoggard
authors.forename: Michael
authors.initials: M
authors.lastname: Gillings
authors.forename: Chris
authors.initials: C
authors.lastname: Palmer
authors.forename: Adam
authors.initials: A
authors.lastname: Stow
authors.forename: Doug
authors.initials: D
authors.lastname: Beattie
authors.forename: David
authors.initials: D
authors.lastname: Briscoe
authors.forename: Shannon
authors.initials: S
authors.lastname: Smith
authors.forename: Peter
authors.initials: P
authors.lastname: Wilson
authors.forename: Andrew
authors.initials: A
authors.lastname: Beattie
mesh_terms: D000818:Animals; D000890:Anti-Infective Agents / Q000302:isolation & purification / Q000378:metabolism / Q000494:pharmacology*; D001522:Behavior, Animal; D005075:Biological Evolution; D007113:Immunity, Innate; D007313:Insecta / Q000378:metabolism / Q000382:microbiology*; D008826:Microbial Sensitivity Tests; D011157:Population Dynamics; D012919:Social Behavior; D013210:Staphylococcus / Q000187:drug effects*
chemical_list: D000890:Anti-Infective Agents