Research Article

Avian egg shape: Form, function, and evolution

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Science  23 Jun 2017:
Vol. 356, Issue 6344, pp. 1249-1254
DOI: 10.1126/science.aaj1945
  • Fig. 1 Morphospace of avian egg shape.

    Average egg shapes for each of 1400 species (black dots), illustrating variation in asymmetry and ellipticity, as defined in the text. Images of representative eggs are shown alongside their associated points in morphospace (colored red) to highlight variation in shape parameters. [Egg images (shown on the same relative scale) are copyright of the Museum of Vertebrate Zoology, Berkeley. Image details and sources are given in Data S2.]

  • Fig. 2 Partitioning of egg variation among avian orders.

    Black dots (one per species) (n = 1400) show the morphospace of egg shape in two dimensions (asymmetry and ellipticity). Colored polygons show the minimum convex hull plotted for all species within a subset of avian orders (Strigiformes, Apodiformes, Suliformes, Passeriformes, and Charadriiformes). Overlap in egg shape is extensive, even among ecologically and phenotypically dissimilar orders. However, some orders (e.g., Charadriiformes) occupy a much larger region of egg morphospace than others. For silhouette details, see Fig. 4.

  • Fig. 3 Biophysical model of egg shape.

    (A) An axisymmetric egg is described by a planar curve C that, when revolved around the axis of symmetry (z axis), yields the surface of the egg, with radial and angular coordinates Embedded Image parametrized by a curvilinear material coordinate σ defined relative to one of the poles of the egg. There is a uniform pressure difference P across the membrane, a thickness-integrated axial stress Embedded Image and a thickness-integrated azimuthal stress Embedded Image that together characterize the three principal stresses at every location along the axisymmetric egg membrane. Here, Embedded Imageis the unit vector in the normal direction and Embedded Image and Embedded Imageare the two orthogonal tangent directions pointing in the axial and azimuthal directions along the membrane surface. The stiffnesses in the axial and azimuthal direction are given by Embedded Image and link the stresses to the elastic strains driven by the pressure difference across the membrane (see text and supplementary materials for details). O is the Cartesian origin. (B) Schematic representation of the reparameterization/growth process. The initial reference shape Embedded Imagegrows to Embedded Image after one growth step, driven by the scaled pressure Embedded Image and the ratio of azimuthal to axial stiffness Embedded Image (see text and supplementary materials for details). This new shape is then used as a reference shape in the next growth step, yielding an iterative approach to morphogenesis. (C) Experimentally observed variations in egg membrane thickness taken from Fig. 3 of Balch and Tyler (25) allow us to fit a simple power law and parametrize the functions Embedded Image (see supplementary materials) and therefore Embedded Image, Embedded Image. (D) A spherical egg grows into a classic chicken egg that is both elliptical and asymmetrical in 45 discrete growth steps following the protocol in (B). (E) The full avian morphospace can be generated using different functional forms for the scaled ratio of azimuthal to axial stiffness and the scaled pressure, Embedded Imagerespectively (see text and supplementary materials for details).

  • Fig. 4 Evolutionary patterns of egg-shape diversity.

    A phylogeny of 1209 species in our sample for which molecular sequence data exist, based on (26) and (27). For each species, the average egg length (light blue), asymmetry (medium blue), and ellipticity (dark blue) are represented by line lengths at branch tips. Silhouettes for representative species in each order are shown (details and image sources in Data S2).

  • Fig. 5 Egg shape, egg volume, and flight ability.

    (A and B) For a given egg girth, birds increase egg volume by increasing asymmetry and/or ellipticity. In our data set, eggs that are more (A) asymmetric and (B) elliptical tend to have larger volumes for a given egg girth (width). Average egg girth (x axis) is calculated directly from images; egg volume (y axis) is derived from Baker (11). Eggs with high (upper 50% of all species) asymmetry (A) or ellipticity (B) are shown in turquoise; eggs with low (lower 50% of all species) asymmetry (A) or ellipticity (B) are shown in red. For clarity, we omitted eggs with the highest volumes (top 5% of species). (C and D) Comparison of egg asymmetry (C) and ellipticity (D) between species with high and low flight ability in the 12 most speciose orders in our sample. We divided extant species in each order according to HWI to directly compare between those with low HWI (lower 50%) and high HWI (upper 50%). Increased HWI (i.e., flight ability) is often associated with increased asymmetry or ellipticity (solid lines) and less often with decreased asymmetry or ellipticity (dashed lines).

Supplementary Materials

  • Avian egg shape: Form, function, and evolution

    Mary Caswell Stoddard, Ee Hou Yong, Derya Akkaynak, Catherine Sheard, Joseph A. Tobias, L. Mahadevan

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Figs. S1 to S16
    • Tables S1 to S5
    • References
    Data S1
    Egg shape by species
    Data S2
    Figure details
    Correction (10 September 2018): This revised supplement includes updated phylogenetic analyses based on 1211 species rather than the 1209 reported in the original manuscript, plus additional references. The taxonomic nomenclature in Data S1 has been corrected for typos.
    The original PDF is accessible here and the original Data S1 file is here.

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