Anatomical description of neornithine stomach with implications on neornithine stomach morphology

Abstract

Neornithines, the most diversified extant tetrapods, have been a classic example for understanding form-function relationships, particularly in the context of the interaction between dietary ecology and neornithine phenotypic evolution. While the previous studies have primarily focused on beak morphology, the significance of the neornithine stomach, which serves as a functional analog of mammalian dentition, is expected to play an important role as well. However, current knowledge on neornithine stomachs is predominantly biased toward poultry and birds of prey, leading to a significant underappreciation of its impact on macroevolution. Here, we provide detailed descriptions of neornithine stomachs represented by 115 species of major orders and test if and how neornithine stomachs are related to their dietary ecology. We identified four morphotypes among neornithine stomachs, which are strongly constrained phylogenetically. While the neornithine diet demonstrates strong associations with stomach morphotypes, the associations are small or absent when accounting for the phylogeny in statistical evaluations. Similarly, the neornithine diet has negligible effects on their ventriculus mass under the analyses with phylogenetic correction. The results resemble a recent finding that a neornithine diet has no effect on intestine length when accounting for phylogeny, but rather flight performance does. Thus, the present study further supports the previous findings that dietary specialization in neornithine birds closely follows phylogeny, making functional convergence across taxa difficult to detect.

Keywords: diet; neornithine; stomach.

FIGURE 1

Representatives of type A stomachs. The stomach of Anas acuta in (a) left and (b) right lateral views. (c) The internal cavity of the Anas acuta stomach. (d) Transverse section of the ventriculus at the level of the pyloric region. Note that the pyloric region does not form a distinct chamber but is a groove in between two large gastric folds. (e) Transverse section of the ventriculus at its maximum width. (f–g) Stomach of Dromaius novaehollandiae in (f) left and (g) right lateral views. The stomach of Struthio camelus in (h) left and (i) right lateral views. ct, centrum tendineum; d, duodenum; e, esophagus; gi, gastric isthmus; k, koilin layer; l, lumen; M. c cd, M. crassus caudodorsalis; M. c cv, M. crassus cranioventralis; M. t cd, M. tenuis craniodorsalis; M. t. cv, M. tenuis cranioventralis; pr, pyloric region; pv, proventriculus; sca, sulcus caudalis; scr, sulcus cranialis; v, ventriculus.

FIGURE 2

Morphotype B (a–d). The stomach of Strix uralensis in the left (a) and right (b) lateral views. Transverse sections of Picus awokera (c) and Milvus migrans (d) ventriculus. Morphotype C (e–j). The stomach of Phalacrocorax carbo in (e) left and (f) right lateral views. (g) The internal cavity of the Phalacrocorax carbo stomach. The stomach of Ardea cinerea in (h) left and (i) right lateral views. (j) The internal cavity of the Ardea cinerea stomach. Morphotype D stomach Puffinus tenuirostris (k–n). Left lateral view of the whole stomach (k). Puffinus tenuirostris ventriculus in left lateral (l), right lateral (m), and transverse (n) views. ct, centrum tendineum; d, duodenum; e, esophagus; gi, gastric isthmus; k, koilin layer; l, lumen; M. c cd, M. crassus caudodorsalis; M. c cv, M. crassus cranioventralis; M. t cd, M. tenuis craniodorsalis; M. t. cv, M. tenuis cranioventralis; pr, proventriculus; pv, proventriculus; sca, sulcus caudalis; scr, sulcus cranialis; v, ventriculus.

FIGURE 3

Graphical representations of diet–stomach morphotype–gizzard muscularity relationships. (a) A mosaic plot of diet against stomach type. (b) A scatter plot of ventriculus mass (VM) against body mass (BM). (c–l) Violin plots showing proportions of diet against stomach morphotype. The red dot represents the median value. The thick vertical line represents the interquartile range, and the thin vertical line represents the whisker range.