A comparative description of the maxillary and mandibular divisions of the trigeminal nerve in birds

Abstract

For many animals, touch is one of the most crucial senses, as it allows an animal to assess its surroundings, develop properly, and socialize. Remote touch is an essential part of avian survival, as it allows some families of birds to identify prey through changes in pressure. Some birds possess a sensitive bill tip organ filled with a large number of mechanoreceptors to perform remote touch sensation. This implies that they possess a complex trigeminal nerve system. The trigeminal nerve has three divisions (ophthalmic, V1; maxillary, V2; and mandibular, V3) that supply somatosensory information from the face and head. Birds from the families Apterygidae, Scolopacidae, Anatidae, Threskiornithidae, and Psittacidae are known to have a sensitive bill tip organ supplied by the trigeminal nerve, whereas other birds use vision, hearing, and other touch to identify prey, potentially resulting in less overall dependence on the trigeminal nerve. Here, we created nerve maps of birds from a range of orders including Anseriformes, Gruiformes, Pelecaniformes, Strigiformes, Accipitriformes, and Passeriformes. We find that species with a remote touch organ possess more observable nerve fiber bundles associated with the maxillary and mandibular trigeminal nerve divisions than species not possessing a remote touch organ. Our results indicate that birds with foraging or prey capture techniques not relying on mechanoreception through the bill possess maxillary and mandibular divisions of the trigeminal nerve that are less robust as they enter the beak.