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Paper No. 221-5
Presentation Time: 2:30 PM
PELVIC COEVOLUTION WITH EGG SIZE AND SHAPE IN MODERN BIRDS: IMPLICATION FOR REPRODUCTIVE BIOLOGY OF EXTINCT DINOSAURS
Pelvic evolution from extinct dinosaurs to modern birds is often assessed biomechanically, with functions ranging from weight-carrying, ventilation, and locomotion/flight. Recently, reproduction has been hypothesized to have shaped pelvic morphology along this lineage. Opening of the pelvis by splitting the pubes allowed an increase in relative egg size and changes in the egg shape. Moreover, pre-laying egg rotation (180° yaw rotation), as observed in the chicken, possibly separates the ischia. Since a wider pelvic canal would facilitate the egg-laying process, we hypothesize that reproductive advantage for females partly drives the evolution of pelvic morphology. We investigated pelvic coevolution with egg size and shape in modern birds (28 representative species from major avian groups) using a multiple regression and found that female pelvic canal width correlates with egg length (pMCMC=0.01) and diameter (pMCMC=0.07). Given no support for phylogenetic dependence, egg rotation probably causes the correlation with egg length. Further, we will test whether egg rotation exists across birds using a comparative radiography and incorporate fossil taxa in our analyses. Pelvic sexual differences (dimorphism) are present in non-avian reptiles and mammals but has not been detected in birds. We examined dimorphism using a multiple regression and found no difference in pelvic width after accounting for body size (pavg>0.05). Thus, egg rotation might have modified pelvic structure but not enough to produce dimorphism. Nevertheless, this study elevates reproduction as one of the factors in avian pelvic morphological evolution and elucidates how selection on reproductive biology modifies skeletal anatomy in dinosaurs.