Egg Shape is Constrained More by Width than Length, Evidence from Double-yolked Duck Eggs

Abstract

Background and Objective: While avian egg shape is species specific there is evidence for intraspecific variation and this variation may be especially great in domestic ducks where selection for egg production was not as intense as in the domestic fowl. Egg shape (visually assessed) and shape index (calculated from egg dimensions) were compared in single-yolked (SY) and double-yolked (DY) duck (Anas platyrhynchos domesticus) eggs. Methodology: The SY and DY eggs were collected from a flock of Aylesbury ducks and their dimensions were measured. Shape index was calculated (length divided by width) and egg shape was visually assessed. Results: There was a significant positive relationship between egg shape and shape index both in SY and DY duck eggs (both p<0.001) with the more elongated egg, having a higher shape index. The DY eggs were more elongated than SY eggs in all egg shapes (all p<0.001). When compared to SY eggs, the significantly greater length, which is disproportionately more than the significantly greater width, is associated with presence of a second yolk in DY eggs. Further, length had greater variance than width in both SY and DY eggs. The shape index of SY and DY eggs differed significantly between the various egg shapes (all p<0.05) validating the use of egg shape as an egg categorization tool. However the ranges of shape indexes of different egg shapes overlapped considerably. Conclusion: The shape index is of limited value and thus the visually assessed egg shape should also be used when describing an egg. Further, the egg shape may have important functions during incubation and hatching and the elongated nature of the DY eggs may act as handicap to successful hatching and in part explain why the production of twins from DY eggs has not evolved in avian species.