Understanding the implications of the evolution of the psittaciform vocal organ on biological function and bioacoustic parameters in extant parrots
Creators: name: Muhammad, Janna
Publisher: name: The University of Texas at Austin
Year: 2022
Subjects: Syrinx
Vocal organ
Fundamental frequency

description: Vocal capabilities and vocal organ morphology vary greatly across bird species. Most birds only have extrinsic muscles in the syrinx and two sound sources that produce F0, or the fundamental frequency. However, Psittaciformes (parrots) have evolved a syrinx with complex intrinsic muscles and only one sound source. It has been proposed that complex intrinsic muscles may affect properties including fundamental frequency range in Passeriformes (songbirds). Psittaciformes and Passeriformes both have intrinsic muscles in their syrinx. but Passeriformes have two sound sources, which is the condition seen in the sistertaxon to the parrot songbird clade, Falconiformes. Psittaciformes and Passeriformes also show more species diversity and are more social than Falconiformes. While there have been some studies on select Psittaciformes and Passeriformes species that examine complex vocal capabilities and morphology, there have been no phylogenetic comparative studies of variation in the morphology and acoustic characteristics among these two clades and their closest outgroup in the Eufalconimorphae, Falconiformes. Here we investigated variation in, and evolution of, syrinx morphology in parrots. We further investigated the potential effects of the presence of complex intrinsic muscles on fundamental frequency (min, max, mean) by comparing F0/ body mass relationships for 70 psittaciform species and 34 falconiform species. Diffusible iodine-based contrast-enhanced computed tomography (diceCT) was used to visualize the syrinx morphology of three parrot species in Psittacidae and two falconiform species. Morphological descriptions from the literature were used to contextualize these data. These included illustrations and descriptions of taxa from the two other major Psittaciformes subclades (Cacatuoidea and Strigipoidea), additional early diverging and well-nested songbirds. We find a well-mineralized cartilage element, a tracheal shield, that is connected to the bronchial edge of the lateral tympaniform membrane (LTM), to be an evolutionary innovation within Psittaciformes associated with changes in the single sound source seen within the clade. Possible sexual dimorphism was seen in the green parakeet, Aratinga holochlora, with the male showing a visibly slightly larger intrinsic muscle (m. syringealis profundus; SP) than the female. We also find some evidence that the (m. syringealis profundus; SP) shows a similar attachment to an extrinsic muscle (m. tracheolateralis; TL) in Falconiformes and some basally-divergent Passeriformes. The position of this intrinsic muscle and its relationship with fibers of the TL may suggest a shared ontogeny, but this needs to be investigated further. The relationship of F0 (min, max, mean) to body mass was compared using phylogenetic generalized least squares (PGLS) regressions for Psittaciformes and Falconiformes. The expected negative allometric relationship between F0 and body mass (recovered for other vertebrates) was observed in both clades. Psittaciformes show more variance than Falconiformes in all measures of F0, and maximum F0 was recovered as the acoustic parameter that is the least explained by body size in Psittaciformes. This observed difference in parrots could be attributable to complex intrinsic muscles, an explanation proposed for similar trends in oscine songbirds. However, the effects of transition from two sound sources to one in parrots and acquisition of neural control traits in these clades cannot be ruled out. Additional functional studies are needed to investigate this hypothesis further. Nonetheless, these data may inform specific traits of the fossilizable cartilaginous rings that can indicate the timing of acquisition of these traits in psittaciforms and add to our understanding of the evolution of avian vocalization more generally.