Hatching a new hypothesis about egg shape diversity

Morgan Kelly ・ Princeton Environmental Institute

The research of Mary Caswell Stoddard, a Princeton University assistant professor of ecology and evolutionary biology and PEI-affiliated faculty, suggests that the shape of an egg for a bird of a given species may be driven in part by features of a bird’s physiology related to its capability for flight.

Mary Caswell Stoddard, an assistant professor in Princeton’s Department of Ecology and Evolutionary Biology, proposes a far-ranging hypothesis regarding how and why bird eggs acquire their shapes. Her research, an interdisciplinary collaboration involving multiple authors, suggests that the shape of an egg for a given bird species may be driven in part by physiological features related to its capability for flight.

In a study published June 22 in the journal Science, lead author Stoddard and colleagues suggest that the correlation may have a mechanistic explanation: it may be that birds with a strong flight capability have developed aerodynamic body shapes that have influenced the configuration of these birds’ internal organs, including the reproductive system.

This would in turn influence the shape an egg acquires as it moves through a bird’s oviduct, the tube through which an egg passes after leaving the ovary. In the scenario Stoddard and her team propose, the egg’s flexible membrane, not the hard shell, determines the egg’s final shape. The egg adopts a signature shape due to the membrane’s properties as well as pressures in the isthmus, the portion of the oviduct positioned just before the shell gland (where the membrane is coated with a layer of calcium carbonate that hardens to form its shell).

An egg that tends toward asymmetry or ellipticity might pass more easily through the narrow oviduct of a bird with an aerodynamically streamlined body, while still having enough volume to hold the nutrients necessary to support an embryo.

Stoddard and her team arrived at their hypothesis through an analysis of the shapes of over 49,000 eggs, which represented approximately 14 percent of the species of birds that exist worldwide.