Vultures as a model for testing molecular adaptations of dietary specialization in birds

Vultures are the only obligate scavengers among extant vertebrates. They provide valuable ecological services in ecosystems through removing carcasses, thus preventing the growth of other scavenger populations and the spread of pathogens. Moreover, their specific diets expose them to various deadly pathogens, which makes them potential candidates for studying molecular adaptations required to survive this extremely specialized scavenging habit. In this review, we summarize the morphological characteristics and behavioral habits, origin and phylogeny, and molecular adaptations to scavenging in both Old and New World vultures. The two groups of vultures share a similar appearance, indicative of convergent evolution. Vultures have experienced different degrees of specialization in their sensory organs; Old World vultures depend on sight, while New World ones depend on both smell and sight. Combined fossil records and molecular data suggest that vultures evolved independently, with distinct phylogenetic positions. We also explored their adaptation to scavenging in facial and intestinal microbiomes, gastric acid secretion and immunity. Compared with the facial microbiome, the intestinal microbiome had a lower diversity, dominated by Fusobacteria and Clostridia. The phages and single invertebrate species Adineta vaga, which feeds on dead bacteria and protozoa, present in the gut suggest a possible alternative defense mechanism. Several genes involved in gastric acidic secretion (including ATP4B, SLC26A7 and SST) and immunity (including BCL6, STING, and TLRs) undergoing positive selection likely have essential roles in eliminating invasive pathogens and initiating an innate immune response. Taken together, this review presents the current research status of vultures and highlights the use of vultures as a model for exploring molecular adaptations of dietary specialization in birds. It also provides a theoretical basis for the study of the genetic mechanisms of vultures to scavenging, and contributes to the formulation of vulture conservation strategies.