Adaptations for flight may have driven egg-shape variety in birds

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Eggs

One of the most perplexing questions in natural history is how and why do we find so much of diversity in eggs? An international team of scientists believes it has found an answer by combining methods and ideas from mathematics, physics and biology.

In their study published in journal Science, scientists characterized the shape of eggs from about 1,400 species of birds to come up with a model that they claim explains how an egg’s membrane determines its shape. Using an evolutionary framework, the researchers found that the shape of an egg correlates with flight ability, suggesting that adaptations for flight may have been critical drivers of egg-shape variation in birds.

Researchers first plotted the shape of some 50,000 eggs, representing 14 per cent of species in 35 orders, including two extinct orders. The researchers found that egg shape was a continuum — with many species overlapping. The shapes ranged from almost perfectly spherical eggs to conical-shaped eggs.

Scientists say the variations of shape come from the variation in the membrane’s thickness and material properties and the ratio of the differential pressure to the stretchiness of the membrane.

Further into the study the team looked at correlations between egg shape and traits associated with the species of bird, including nest type and location, clutch size (the number of eggs laid at a time), diet and flight ability. Scientists found that flight may influence egg shape. As lead author of the study explains, “To maintain sleek and streamlined bodies for flight, birds appear to lay eggs that are more asymmetric or elliptical. With these egg shapes, birds can maximize egg volume without increasing the egg’s width — this is an advantage in narrow oviducts”.

So an albatross and a hummingbird, while two very different birds, may have evolved similarly shaped eggs because both are high-powered fliers.

“It’s clear from our study that variation in the size and shape of bird eggs is not simply random but is instead related to differences in ecology, including the amount of calcium in the diet, and particularly the extent to which each species is designed for powerful flight” says coauthor Dr. Joseph Tobias from Imperial College, UK.

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