The ability of bats to act as natural reservoir hosts of zoonotic viruses has been attributed to aspects of their innate immune systems. In particular, bats appear to detect and respond to pathogens differently compared to humans, allowing them to tolerate viruses that are harmful to other mammals. Studies to date have identified several lineage-specific mechanisms responsible for dampened immune and inflammatory responses in bats; however, these have mainly focused on a few putative reservoir species and their relatives, representing ~1% of bat species. It is therefore not known whether the vast majority of bat species (which span >60 million years of evolution) also possess immune mechanisms for tolerating viruses, and, if so, whether these might predispose them to viral infections and potential involvement in future zoonotic spillovers. We will conduct the first large-scale comparative study of bat immune adaptations by screening key innate immunity genes across hundreds of species, spanning this group's full evolutionary and ecological diversity. To test how putative molecular adaptations (amino acid changes) alter immune responses to viral infection, we will run cell-based assays, focusing on 3 key proteins (STING, NLRP3 and MyD88) that represent different effector pathways in innate immunity. Finally, we will examine whether the presence of impactful molecular adaptations in these and other loci can explain known variation in bat-virus associations.