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

Abstract

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 wound care in increasing survival are clear, the forces driving the evolution of these behaviours are not fully understood. Contrary to the current hypothesis that social wound care is more likely to evolve in small groups due to the high relative value of an individual, we demonstrate that group size is not necessary for its evolution or retention. Instead, our theoretical model formalises how injury rate and lethality are fundamental drivers of care rather than group size. This is further evidenced by the presence of wound care in the army ant Eciton burchellii, a species with colonies of [~]1 million ants that hunts pugnacious prey. Here, wound care significantly increased survival in ants with infected wounds, occurring in two phases: direct care at the raiding site, and treatment with antimicrobial secretions in the bivouac. This species is the first to provide on-site care, thereby minimising the latency to receive care and thus potentially the likelihood of lethal wound infection. Our findings reject the notion that small group size is necessary for the evolution or retention of social wound care, revealing instead that injury-driven care can be a widespread and adaptive feature of even the largest insect societies. Significance StatementInjuries and infections threaten survival, but animals can counter these dangers through self-medication or social care. It has been suggested that helping behaviours directed towards vulnerable or injured individuals, such as social wound care, are more likely to evolve in small groups. This study in an army ant with [~]1 million workers shows otherwise, with injury frequency likely being the key component for the evolution or retention of wound-care behaviours. The unique strategy of on-site care at the raiding site, followed by antimicrobial care inside the bivouac, significantly improved survival outcomes. Our theoretical model further shows that even at low rates of care, fitness benefits are high, suggesting that social wound care may be far more widespread than previously thought.