Size and number of the hypoglossal nerve foramina in the avian skull and their potential neuroanatomical significance

The cranial openings of nervus hypoglossus, the 12th cranial nerve, are for the first time studied across a broad range of higher avian taxa. This nerve plays an important role in the innervation of the syrinx and exits the skull through a variable number of foramina. Most previous authors described 2–3 foramina nervi hypoglossi (FNH) for neornithine birds, but the number, size, and arrangement of FNH is actually more variable than what is apparent from the literature. In the case of three foramina, there is usually a pair of caudal foramina and a rostral one, but even in closely related taxa, a great variability of the FNH pattern may exist. Many taxa of Neognathae exhibit a quadruple of symmetrically arranged FNH, in others four foramina are strung together in a line. A few taxa show more than four FNH, although in these cases the additional foramina are very small. Of particular interest is the occurrence of a very large caudal FNH in Trochilidae and many species of Passeriformes. This large foramen is suggestive of a correlation with the highly developed vocal capabilities of these birds and may transmit fibers from the tracheosyringeal portion of nucleus nervi hypoglossi, in which case it would be an osteological correlate of vocal capabilities. However, targeted neuroanatomical studies are required to determine how individual hypoglossal foramina relate to hypoglossal roots and their branches, and which of them receive fibers supplying axial, lingual, and syringeal muscles.