MONTPELLIER, FRANCE—“Which came first?” might be the question most often asked about eggs and birds. But a close second is “Why are eggs shaped the way they are?”
Now, two research groups have weighed in with an answer—and it challenges a different explanation of egg shape that received a lot of attention last year.
Eggs come in a dizzying array of sizes and shapes, including nearly symmetrical spheres and more elongated, conical forms. And researchers have long debated which biological forces help shape shells. They’ve proposed a number of ideas: Certain egg shapes help more of them pack into a nest, others let oxygen more effectively reach developing chicks, and still others prevent them from rolling away from nests.
Last year, a study led by Princeton University evolutionary biologist Mary “Cassie” Stoddard of 50,000 eggs from 1400 species from across the bird family tree came up with another answer: The demands of flying led to the evolution of longer, more asymmetrical eggs. Headlines around the world heralded the discovery.
But some researchers were skeptical. The findings “touched a nerve” and raised questions, says Tim Birkhead, an evolutionary biologist at The University of Sheffield in the United Kingdom who has studied birds for decades and spent years working with mathematicians to come up with a better way to measure egg shape. Buried in the study was the finding that flight ability accounted for just 4% of the variability in egg shape. Such a low percentage is typical of large, complex studies and is still significant, Stoddard says. But for Birkhead, it was a signal that his team should intensify its search for another explanation.
Now, it is offering one: that incubation conditions—including where nests are located and how parents incubate the eggs—are far more important than flight in explaining the evolution of egg shapes.
Birkhead made his case here this week in a talk at the Second Joint Congress on Evolutionary Biology that focused on studies of the common murre (Uria aalge), a bird known in the United Kingdom as the guillemot. Its eggs have been coveted by collectors since the 1840s for their beautiful variation in color and pattern.
The common murre’s pear-shaped egg “is an extreme in that it has both a most asymmetrical and most elliptical” shell, explains Mark Hauber, a behavioral ecologist at the University of Illinois in Urbana who studies the bird. It nests on narrow rock ledges in densities of up to 70 individuals per square meter, laying a single bluish, variegated 100-gram egg directly on the ledge.
During a visit to one of these nesting ledges, Birkhead was struck by the possible role of shell shape in preventing murre parents from losing their eggs. To test the idea, he stashed two kinds of eggs—a common murre egg and an elliptical egg laid by a close relative called the razorbill—in different places on the cliff. Then he observed which was less likely to roll away.
The murre eggs proved much more stable than the razorbill eggs, especially on steep slopes. And his team got similar results when they tested the idea back in the laboratory, timing how long it took murre and razorbill eggs to topple off ever-steeper surfaces, covered with sandpaper to mimic the cliff’s rough surface.
Overall, the more pointed the egg (defined by a longer distance between the widest and narrowest parts), the longer it resisted rolling, Birkhead reported at the meeting and today in The Auk. Such a shape doesn’t slow eggs that are rolling away; instead, it keeps eggs from starting to roll in the first place, he emphasizes. By lying on its long axis, a murre egg makes contact with more of the ground, so it’s more likely to remain on the ledge, even as parents switch places and temporarily stop holding the egg with their legs, Birkhead adds. “For [common murres], egg shape allows them to be on those steeper slopes,” says Michael Antolin, an evolutionary biologist at Colorado State University in Fort Collins who was not involved with the work.
In a second study, Birkhead and his colleagues assessed the relationship between egg shape and incubation site in 30 species including murre relatives and penguins, which produce a wide variety of egg shapes and sizes. They also evaluated the potential role of egg and clutch size, the mode of embryo development, and the parent’s posture while “sitting” on the nest. They concluded that the incubation site explains a whopping two-thirds of the variation in egg shape, although incubating posture also plays a role, they reported Tuesday in Ibis.
Meanwhile, at the City University of New York, Hauber and graduate student Ian Hays at Hunter College in New York City were also evaluating the protective role of egg shape in common murres and their relatives. They first devised a way to precisely assess more complex aspects of shell shape, such as curvature and change in pointedness. Then they designed model eggs based on 380 real eggs from 11 species, including common and thick-billed murres—all of which they filled with a silicon “yolk.” Finally, using a 3D printer, they made 27 eggs of various shapes and tested how they moved on slopes of different steepness.
On fairly flat slopes, conical eggs were less likely to roll in a straight line. Thus, on steep slopes, a moving egg was more likely to circle and stay on the ledge, Hauber and Hays report today in the Journal of Experimental Biology. In contrast, elongated eggs rolled relatively easily on steep inclines, as did more symmetrical eggs.
“Using 3D-printed eggs is a clever idea,” Birkhead says, although he’s skeptical that models truly mimic a real egg’s behavior.
Both groups appear to arrive at the same conclusion: that the need to stay put is a “more feasible” explanation for egg shape than flight ability, says Patricia Brennan, an evolutionary biologist at Mount Holyoke College in South Hadley, Massachusetts, who was not involved with the work. “It’s critical [that birds] do something so the egg doesn’t tumble down,” she notes.
Citing the broad sampling in her work, Stoddard stands by her conclusions. However, she adds that smaller taxonomic groups like the murres and their relatives “do not always follow the rules that emerge at the global scale.”
And Brennan gives a lot of credit to Stoddard for “firing up” research on egg shape. “It’s been a long-standing puzzle for centuries” that’s still not completely solved. And who knows, she adds, how the egg question might be answered next year.