Animal medical systems from Apis to apes: history, recent advances and future perspectives

Animal medical systems encompass a wide range of behaviours aimed at maintaining or improving health. It has become clear that these behaviours are not limited to animals treating themselves (self-medication) but also include the treatment of group members, resulting in the adoption of the more inclusive term "animal medication". Behaviour with the intent to avoid, reduce the impact, or otherwise treat disease transmission, rather than the use of medicinal substances, can be described as medical behaviours. However, most behaviours described here involve the ingestion or application of items with medicinal properties to oneself or the application of items to others or their temporary or permanent communal living spaces like nests or burrows, hereto named medicinal or medication behaviours. This review begins with a historical overview of the field, showcasing an increasing awareness of the wide diversity of taxa exhibiting animal medication and elucidating the development of criteria used to define and categorise such behaviours across the animal kingdom. A thorough synthesis of recent research is presented, by providing critical reflection that challenges conventional notions and emphasises the significance of sociality and ecological context. To this end, medical systems are explored by using numerous examples, thus highlighting the diverse strategies animals employ to maintain health and improve fitness, ranging from honey bees foraging on antimicrobial resin to control hive disease to apes ingesting small amounts of toxic secondary compounds to control parasite infection. The understanding of how animals maintain their health through medical strategies offers valuable insights into the evolutionary origin and complexity of the drivers behind these behaviours. Evidence suggests that advanced cognition is not necessarily a prerequisite because innate mechanisms are likely involved in the expression of these behaviours across the animal kingdom. By highlighting the importance of life-history traits and ecological context in predicting animal medical systems, we reassess the presumed primary drivers of these adaptations. Finally, this review raises important questions about animal medical systems, including the universality of the mechanisms involved, the evolutionary significance of parasite pressure, and the ecological implications of this suite of behaviours. By addressing these complexities, this review provides a nuanced understanding of animal medical systems and highlights avenues for future research in this field.