The independent evolution of the enlargement of the principal sensory nucleus of the trigeminal nerve in three different groups of birds.

In vertebrates, sensory specializations are usually correlated with increases in the brain areas associated with that specialization. This correlation is called the 'principle of proper mass' whereby the size of a neural structure is a reflection of the complexity of the behavior that it subserves. In recent years, several comparative studies have revealed examples of this principle in the visual and auditory system of birds, but somatosensory specializations have largely been ignored. Many species rely heavily on tactile information during feeding. Input from the beak, tongue and face, conveyed via the trigeminal, facial, glossopharyngeal and hypoglossal nerves, is first processed in the brain by the principal sensory nucleus of the trigeminal nerve (PrV) in the brainstem. Previous studies report that PrV is enlarged in some species that rely heavily on tactile input when feeding, but no extensive comparative studies have been performed. In this study, we assessed the volume of PrV in 73 species of birds to present a detailed analysis of the relative size variation of PrV using both conventional and phylogenetically based statistics. Overall, our results indicate that three distinct groups of birds have a hypertrophied PrV: waterfowl (Anseriformes), beak-probing shorebirds (Charadriiformes), and parrots (Psittaciformes). These three groups have different sensory requirements from the orofacial region. For example, beak-probing shorebirds use pressure information from the tip of the beak to find buried prey in soft substrates, whereas waterfowl, especially filter-feeding ducks, use information from the beak, palate, and tongue when feeding. Parrots likely require increased sensitivity in the tongue to manipulate food items. Thus, despite all sharing an enlarged PrV and feeding behaviors dependent on tactile input, each group has different requirements that have led to the independent evolution of a large PrV.