Evolution of first aid and social wound care in an army ant society

Injuries and infections pose a significant threat to fitness. Animals cope with this problem by performing self-medication or receiving social care from members of the group. While the benefits of care in increasing survival are clear, the forces leading to the evolution of these helping behaviours are not fully understood. Contrary to the current hypothesis that helping behaviours, such as social wound care, are more likely to evolve in small groups due to the high relative value of an individual, we demonstrate that social wound care evolved in the army ant Eciton burchellii, a species that hunts pugnacious prey and has colonies of ~1 million ants. Our theoretical model shows how injury rate and lethality are fundamental drivers for care rather than group size. Wound care significantly increased survival in ants with infected wounds through two distinct care phases: a first aid at the hunting site through wound grooming, and another at the bivouac with antimicrobial secretions. This species is the first to provide care at the hunting site, thereby minimising the time to receive help, a critical variable in lowering mortality and accelerating the colony's reproductive rate. By rejecting the notion that high individual value is required for the evolution of helping, our study reveals that injury-driven care can emerge as a vital and widespread feature in even the largest insect societies.