Birds’ egg shapes, fly­ing abil­ity linked

Northwest Arkansas Democrat-Gazette - - STYLE - STEPH YIN

Owls’ are spher­i­cal, hum­ming­birds’ are el­lip­ti­cal and sand­pipers’ are pointy.

All bird eggs have the same func­tion — to pro­tect and nour­ish a grow­ing chick. But they come in a bril­liant ar­ray of shapes. This va­ri­ety has puz­zled bi­ol­o­gists for cen­turies. Now, in the most com­pre­hen­sive study of egg shapes to date, pub­lished June 22 in Sci­ence, a team of sci­en­tists seems to have found an an­swer.

The re­searchers cat­a­loged the nat­u­ral vari­a­tion of egg shapes across 1,400 bird species, cre­ated a math­e­mat­i­cal model to ex­plain that vari­a­tion, and then looked for con­nec­tions be­tween egg shape and key traits of birds. On a global scale, the au­thors found, one of the best pre­dic­tors of egg shape is flight abil­ity, with strong fliers tend­ing to lay long or pointy eggs.

“This pa­per is re­mark­able be­cause it cre­ates a won­der­fully uni­fied the­ory for the va­ri­ety of egg shapes we see in na­ture,” said Claire Spot­tis­woode, a bird ecol­o­gist at the Univer­sity of Cam­bridge and the Univer­sity of Cape Town who did not par­tic­i­pate in the re­search.

In the new study, the au­thors con­ducted a mul­ti­step in­ves­ti­ga­tion that brought to­gether bi­ol­ogy, com­puter sci­ence, math­e­mat­ics and physics. They first wrote a com­puter pro­gram, named Eg­gx­trac­tor — who says sci­en­tists have no sense of hu­mor? — that clas­si­fied eggs based on their el­lip­tic­ity and asym­me­try.

El­lip­ti­cal eggs are elon­gated and round on both ends, like cu­cum­bers, and asym­met­ric eggs are pointier on one end, like man­goes.

With Eg­gx­trac­tor, the re­searchers plot­ted nearly 50,000 eggs, rep­re­sent­ing all ma­jor bird or­ders, from a data­base of dig­i­tal im­ages by the Mu­seum of Ver­te­brate Zool­ogy in Berke­ley, Calif.

“We could see then that egg shapes var­ied from spher­i­cal, to el­lip­ti­cal, to very pointy, to al­most ev­ery­thing in be­tween,” said Mary Caswell Stod­dard, an as­sis­tant pro­fes­sor of ecol­ogy and evo­lu­tion­ary bi­ol­ogy at Princeton Univer­sity and the lead au­thor of the study.

Next, the re­searchers at­tempted to an­swer how eggs might ac­quire vary­ing shapes. Rather than look­ing at the shell, as one might ex­pect, they fo­cused on the egg’s mem­brane (the film you see when peel­ing a hard-boiled egg), which is es­sen­tial to the egg’s shape.

The sci­en­tists iden­ti­fied two pa­ram­e­ters that could in­flu­ence egg form: vari­a­tions in the mem­brane’s com­po­si­tion and dif­fer­ences in pres­sure ap­plied to the mem­brane be­fore the egg hatches.

By ad­just­ing these two pa­ram­e­ters, “we were able to com­pletely re­cover the en­tire range of ob­served avian egg shapes” — a good test of the model, said L. Ma­hade­van, a pro­fes­sor of ap­plied math, bi­ol­ogy and physics at Har­vard Univer­sity and an au­thor of the study.

Fi­nally, the re­searchers looked into why egg shapes might be so spec­tac­u­larly di­verse. One pop­u­lar hy­poth­e­sis cen­tered on nest lo­ca­tion: Cliff-nest­ing birds, it was thought, lay pointy eggs so that if the eggs are bumped, they spin in a cir­cle rather than rolling off the cliff. An­other sug­gested that birds lay eggs in shapes that pack to­gether best in dif­fer­ent-size clutches.

But when the au­thors re­lated egg shape to these and other vari­ables, they were sur­prised to find that none of them fit on a global scale (though they may still play im­por­tant roles on smaller scales). In­stead, egg shape was strongly cor­re­lated with a mea­sure of wing shape, called the hand-wing in­dex, that re­flects flight abil­ity.

So what con­nects flight to egg shape? In gen­eral, birds want to pack as many nu­tri­ents as pos­si­ble into their eggs. But, in order to fly, they must main­tain sleek bod­ies — mean­ing their eggs can’t be too wide.

Com­mon mur­res, for in­stance, are fast, pow­er­ful fliers and have asym­met­ric eggs, as do sand­pipers, which mi­grate long dis­tances.

Wan­der­ing al­ba­trosses are one of the most far-rang­ing fliers — some have been known to cir­cum­nav­i­gate the Antarc­tic Ocean three times in a year — and have el­lip­ti­cal eggs.

East­ern screech owls rarely move be­yond their small ter­ri­tory, where they tend to fly in short, low-pow­ered glides, and have al­most spher­i­cal eggs.

“Per­haps, evo­lu­tion­ar­ily, birds stum­bled upon this very nat­u­ral, geo­met­ric so­lu­tion, which is to in­crease the el­lip­tic­ity and asym­me­try of their eggs,” Ma­hade­van said, since do­ing so al­lows for greater vol­ume with­out in­creas­ing girth. This ex­pla­na­tion re­quires fur­ther re­search, he added.

Ul­ti­mately, this study shows that “we can chal­lenge old as­sump­tions,” Stod­dard said. “In some­thing as fa­mil­iar and com­mon as a bird egg, we are still dis­cov­er­ing new truths.”

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