Pear-shaped eggs evolved to maximize the surface area-to-volume ratio, increase metabolism, and shorten incubation time in birds

Bird eggs can be spherical, ellipsoid, ovoid, or pear-shaped (pyriform), the latter being the most complex. There is however no unambiguous evolutionary/adaptive explanation for this final, exotic shape. We hypothesized that pyriform eggs have a larger surface area-to-volume ratio (S/V) that may be a criterion for increased embryo metabolism. By integrating mathematical approaches, we confirmed this to be the case and developed a model of the egg metabolic rate defined as the ratio of S/V to its maximum possible value, depending on egg length. We found this to be inversely proportional to the egg incubation period and concluded that the complex pyriform shape is most likely due to embryo metabolism increase and, as a result, a reduction in the incubation period and shortened hatching time. As a result of this study, we conclude that some avian eggs are pyriform as this may attain a larger S/V ratio making them grow and hatch quicker.