Avian egg shape is generally explained as an adaptation to life history, yet we currently lack a global synthesis of how egg-shape differences arise and evolve. Here, we apply morphometric, mechanistic, and macroevolutionary analyses to the egg shapes of 1400 bird species. We characterize egg-shape diversity in terms of two biologically relevant variables, asymmetry and ellipticity, allowing us to quantify the observed morphologies in a two-dimensional morphospace.We then propose a simple mechanical model that explains the observed egg-shape diversity based on geometric and material properties of the egg membrane. Finally, using phylogenetic models, we show that egg shape correlates with flight ability on broad taxonomic scales, suggesting that adaptations for flight may have been critical drivers of egg-shape variation in birds.
Bibliographical noteFunding Information:
M.C.S. was supported by Princeton University, with additional support from the L'Or?al USA For Women in Science Fellowship, the L'Or?al-UNESCO International Rising Talents Fellowship, the Harvard Society of Fellows, and the Milton Fund. E.H.Y. was supported by Start-Up Grant no. M4081583 from Nanyang Technological University, Singapore. C.S. was supported by the Oxford Clarendon Fund and the U.S.-U.K. Fulbright Commission. Collection of biometric, life history, and environmental data was supported by a Natural Environment Research Council grant (NE/I028068/1) to J.A.T. L.M. was supported by fellowships from the MacArthur Foundation and the Radcliffe Institute. We thank H. Eyster, D. Swindlehurst, and J. Damore for contributing to data collection; C. Cicero and R. Bowie for specimen access, including digitization of the egg collection at the Museum of Vertebrate Zoology, Berkeley (funded by NSF grant 0646475); and the referees for valuable comments. Egg-shape data reported in this paper are included in the supplementary materials. This study was conceived by M.C.S. and L.M. and designed by M.C.S., J.A.T., and L.M.. The digital morphometric analyses and coding were conducted by M.C.S., E.H.Y., and D.A. M.C.S., C.S., and J.A.T. compiled biometric, life history, and environmental data for birds. The phylogenetic comparative analyses were led by C.S. and J.A.T. L.M. linked the projective geometry parameters to the asymmetry-ellipticity morphospace, and L.M. and E.H.Y. constructed and solved the mathematical model for egg shape. M.C.S., J.A.T., and L.M. coordinated the research, and all authors contributed to writing of the manuscript.
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