Antimicrobial strength increases with group size: implications for social evolution.

Area of Science:

• Insect behavior and evolution
• Chemical ecology
• Social biology

Background:

• Animal aggregations can attract pathogens.
• Social insects, termed 'superorganisms', evolve massive group sizes.
• Antimicrobial defenses may be linked to group size evolution.

Purpose of the Study:

• To investigate the relationship between innate group size and individual antimicrobial strength in thrips.
• To determine if increased group size in insects correlates with enhanced pathogen defense mechanisms.
• To explore the role of antimicrobial strength in the evolution of sociality.

Main Methods:

• Analysis of antimicrobial strength per individual in eight thrips species (Insecta: Thysanoptera).
• Comparison of antimicrobial activity across species with varying innate group sizes, from solitary to eusocial.
• Correlation analysis between group size and measured antimicrobial strength.

Main Results:

• Species with larger innate group sizes (100-700) exhibited stronger antimicrobial activity.
• Species with smaller groups (10-80) showed lower antimicrobial activity.
• Solitary species displayed no detectable antimicrobial activity; social species showed activity above certain group size thresholds (e.g., 120 for eusocial).

Conclusions:

• Antimicrobial strength is significantly determined by innate group size in thrips.
• The evolution of sociality, increasing group size, likely required enhanced defenses against microbial pathogens.
• Increased antimicrobial strength accompanying larger group sizes may have been critical for the irreversible evolution of social insect traits.