A new study published Thursday in the journal Science explains why bird eggs come in different shapes and sizes. It says that this happens because eggs are shaped according to the birds’ flight behavior.
The research was conducted by a team of evolutionary biologists, physicists and applied mathematicians, who said that eggs are shaped depending on the bird’s flight behavior. For example, stronger fliers, such as swallows, have pointy or elongated eggs; while birds that can’t fly as far or faster have rounder and more symmetric eggs.
Mary C. Stoddard, an author of the new study and a professor at Princeton University, said that eggs aren’t just something we buy at the grocery store to cook up in an omelet, as she believes that the story of eggs is the story of vertebrate life on land.
Scientists previously thought that egg shapes were caused by lack of calcium or nest space
Stoddard says that before hard eggs, creatures lived semiaquatic lives, which means they returned to the water to procreate. However, amniotic eggs –embryos forming into fluid-filled shells-, proved to be mobile spawning pools, which caused the creatures to procreate in dry lands.
Over 360 million years after that revolutionary hard shell, eggs still pose a mystery to scientists. The diversity of bird eggs has puzzled biologists, engineers, and mathematicians, according to Stoddard. Some of them theorized that eggs were shaped depending on the amount of calcium in the birds’ diets. According to that hypothesis, birds with calcium deficiencies would lay eggs with as little shell as possible.
Other scientists theorized that certain egg shapes fitted better into a nest. Another theory said that, for example, peaked eggs from cliff-dwelling birds were shaped like that because they would spin like a top when jostled, rather than tumble down to their death.
Douglas G.D. Russell, a curator of egg and nest collections at London’s Natural History Museum who was not involved in the new study, noted that Aristotle believed that cocks hatched from a pointed chicken egg, while hens hatched from the rounder. Stoddard and her colleagues took a more accurate approach than Aristotle, though.
Filmy membranes beneath the shells are responsible for shaping the eggs
The team photographed over 50,000 eggs representing 1,400 bird species, all specimens found at the Museum of Vertebrate Zoology at the University of California at Berkeley. They classified the bird eggs on a spectrum, ranging from the spherical and symmetrical to the elongated and pointy.
One of the eggs proved to be more ovum-shaped than the others –even the chicken’s -, and its laid by a small warbler known as the graceful prinia. Stoddard noted that prinia eggs are slightly more oblong but “substantially more asymmetric.”
The team made a significant discovery while analyzing the eggs. They found that egg shapes really aren’t about the shell, but rather about the filmy membrane just beneath the shell, which is what actually dictates the overall shape of the egg.
When a bird is creating an egg, it pumps the egg through an oviduct, which is a passageway of glands much like a factory line. The bird secretes the filmy membrane first, and then when the bird’s guts apply certain pressures to that membrane, that pressure sculpts the final shape of the egg.
“If you dissolve away a calcified shell you are left with a membrane-encased blob,” said Stoddard, according to The Washington Post. “This blob still has an egg shape.”
Egg shapes shift depending on the birds’ flight abilities
Once the scientists knew the part that organ played in shaping the final form of the egg, they scoped out the relationship of eggs across the bird family tree.
“In this final mega-analysis, we were able to test for the first time, on a global scale, these different hypotheses,” said Stoddard, according to The Washington Post.
These hypotheses included the effects of flight abilities in birds or cliff-dwelling behaviors. The study found that adaptations for an aerodynamic body had a “knock-on effect” for egg shapes, according to Stoddard. Meaning, pointy or elongated eggs don’t help a gravid bird fly. However, the analysis also revealed that the demands of powerful flight restructured the birds’ internal organs. For example, the abdominal cavity of the superior fliers became smaller, which forced egg shape to change.
Russell believes that the theories in the new paper are fascinating and significant. He told The Washington Post in an email that the evidence that egg shape may be linked with morphological traits associated with birds’ flight ability across all of these animals opens a new chapter in research. The new study leaves room for other evolutionary egg influences, but those influences must act on a smaller scale, noted Stoddard.
“Our study challenges some of the old assumptions about why eggs come in a variety of shapes,” said Stoddard. “On a global scale, across birds, we find that it’s not nest location or clutch size that predicts egg shape – it’s flight ability.”
Source: The Washington Post