Network morphospace of the hindlimbs of diving birds

DE MENDOZA RICARDO S.; CARRIL JULIETA; DEGRANGE FEDERICO JAVIER; TAMBUSSI CLAUDIA P.

Congreso; 9th International Meeting on the Secondary Adaptation of Tetrapods to Life in Water; 2021

Diving has convergently evolved many times within birds. The underwater propulsion can be performed by using the forelimbs, as in penguins and auks, or the hindlimbs, as in cormorants, loons, and grebes. Furthermore, the hindlimbs in penguins, and highly-diving foot propelled birds (grebes and loons) are proximally included within the abdominal wall, with only the foot emerging from the body. Unquestionably, this characteristic has structural and functional consequences. In order to study their hindlimb musculo-skeletary architecture, we compared musculo-skeletal networks of wing propelled divers (Alcidae and Sphenicidae), non-divers (Numididae and Anatidae) and foot-propelled divers (Phalacrocoracidae, Podicipedidae, and Gaviidae). The anatomical networks are undirected multigraphs where bones and muscles were considered as nodes and the physical junctions among them as the edges. From each network we obtained different parameters, as the plain number of nodes and connections, or connectivity parameters as the density of connections, average path length, diameter, average cluster coefficient, average degree, parcellation, and heterogeneity. With the different parameters as variables, we performed a principal component analysis (PCA), with the three first principal components accounting for 98% of the variation. In theresulting morphospace, the first PC (73.9%) segregates the wing propelled divers, with the penguins clustered in the lowest values and the auks scattered in the lower portion of the second PC (16.9%). Non-divers and highly and lesser foot-propelled divers occupy the positive quadrant delimited by the two first PC, with the highly diving grebes and loons segregated from the rest by having higher values for both components. The PCA results show that in foot propelled birds the paths in the network are longer, and the heterogeneity and parcellation are higher, while in wing-propelled divers the networks are denser and with higher cluster coefficient and degree. According to these results, the musculoskeletal structure in the hindlimbs of both highly diving foot-propelled divers and wing-propelled divers, is different from non-divers and lesser foot-propelled divers, probably related to theinclusion of the proximal part of hindlimbs within the abdominal wall. Nevertheless, the musculoskeletal structure in the hindlimb of wing-propelled divers is more integrated, with fewer nodes and a denser architecture, while in foot-propelled divers is less integrated, with longer paths and homogeneous modules.