Shells in all shades



PIC­TURE A bird egg: Per­haps it’s the co­coa brown of a free-range chicken. Or a robin’s creamy blue-green. If it’s a quail egg, it has inky speck­les. Those colours and vari­a­tions, ac­cord­ing to a new study pub­lished last week in the jour­nal Na­ture, share a com­mon ori­gin. Eg­gshell colour, in all its glory, is a trait that comes from long-ago an­i­mals: the di­nosaurs.

Birds, strange as it seems, are liv­ing di­nosaurs, the last of a lin­eage that oth­er­wise went ex­tinct 66 mil­lion years ago. Be­fore this work, though, many bi­ol­o­gists pre­dicted that mod­ern birds, not their an­ces­tors, de­vel­oped colour­ful eggshells. Eg­gshell the­o­ries were “kind of all over the place,” said study au­thor Mark Norell, a pa­le­on­tol­o­gist at the Amer­i­can Mu­seum of Nat­u­ral His­tory in New

York. Some peo­ple hy­poth­e­sised colours evolved sev­eral times in­de­pen­dently in the his­tory of birds; oth­ers sug­gested it hap­pened once as birds be­came the an­i­mals we know. Colour­ful eggs, this study con­cludes, are much older.

“The dis­cov­ery of a sin­gle ori­gin of eg­gshell colour in di­nosaurs is a won­der­ful re­minder that mod­ern birds in­her­ited many traits from their di­nosaurian an­ces­tors,” said Mary Caswell Stod­dard, a Prince­ton Uni­ver­sity evo­lu­tion­ary bi­ol­o­gist who was not in­volved with this re­search. “We eat eggs for break­fast, but they hold so many clues to the evo­lu­tion­ary past.”

Smash an eg­gshell into its mol­e­cules, and you’ll find only two types of pig­ment among the wreck­age. A mol­e­cule called biliverdin is the source of green (a decades-old ex­per­i­ment linked biliverdin to green splotches that some­times ap­pear in bruises). An­other mol­e­cule, pro­to­por­phyrin, pro­vides the rusts and browns. Th­ese pig­ments mix like wa­ter­colour paints to pro­duce the en­tire colour pal­ette of bird eggs.

A few years ago, Mark Hauber, an or­nithol­o­gist at Uni­ver­sity of Illi­nois at Ur­bana-Cham­paign, and his col­leagues de­signed a pro­to­col to ex­am­ine biliverdin and pro­to­por­phyrin in eggshells, in­clud­ing shells from a gi­ant ex­tinct bird called the moa. The au­thors of the cur­rent study ran with this idea and “did a much bet­ter job than we did,” by in­clud­ing a range of di­nosaurs, Hauber said.

Study au­thor Jas­mina Wie­mann, now a doc­toral stu­dent at Yale Uni­ver­sity, be­gan her search for colour­ful eggs with a di­nosaur named Heyuan­nia huangi, an ovi­rap­tor with a beak like a par­rot’s. Other sci­en­tists told her she was “wast­ing her time,” she said, be­cause they as­sumed di­nosaur eggs lacked pig­ment.

Us­ing Ra­man spec­troscopy, a non­de­struc­tive an­a­lytic tool, Wie­mann looked for the molec­u­lar sig­na­tures of pro­to­por­phyrin and biliverdin in the ovi­rap­tor eggshells. Heyuan­nia huangi, she dis­cov­ered, laid blue­green eggs.

This new re­search ex­pands her pre­vi­ous study. Wie­mann, Norell and Tzu-Ruei Yang, at the Uni­ver­sity of Bonn in Ger­many, ex­am­ined eggs from 18 species. The non-ex­tinct an­i­mals in­cluded an al­li­ga­tor and the do­mes­ti­cated chicken. The di­nosaur crowd was a di­verse bunch: ovi­rap­tors and other bird an­ces­tors, but also sauropods (long-necked di­nos) and hadrosaurs (duck-billed di­nos), which aren’t closely re­lated to birds.

The study au­thors se­lected their shells care­fully so they could know what sort of di­nosaur laid the eggs. “We only ac­cepted spec­i­mens for the re­search where there is ei­ther an em­bryo in­side the egg,” Norell said, “or that there was an adult that was closely as­so­ci­ated with the clutch of eggs.” Though shell colours faded over mil­lions of years, as the eggs be­came fos­sils, traces of the pig­ment mol­e­cules re­mained.

Sauropods and hadrosaurs did not have colour­ful eggs. Theirs were like al­li­ga­tor eggs – white. Di­nosaur rel­a­tives of birds, though, had colour­ful eggs, in­clud­ing spots and both pig­ment types. (If you’re won­der­ing why su­per­mar­ket eggs are white, that’s a hu­man in­ven­tion, the re­sult of farm­ers who specif­i­cally raised chick­ens with genes for white eggs. Jun­gle fowl, the wild cousins of chick­ens, lay brown­ish eggs.)

The di­nosaurs in this study came from the Cre­ta­ceous pe­riod, late in the dino line. Norell said it is pos­si­ble that “more prim­i­tive” an­i­mals, like a Tyran­nosaurus, may have laid colour­ful eggs, but the sci­en­tists lacked older shells to test.

Th­ese re­sults con­firm that Wie­mann’s dis­cov­ery of blue ovi­rap­tor eggs was not a fluke, Hauber said. “Ex­tinct di­nosaurs laid eggs just like the rest of the birds do,” he said.

Egg pat­terns are a win­dow into di­nosaur nest­ing habits. “A pretty easy pre­dic­tion - but we’re the first ones to have real ev­i­dence for it - is that the ori­gin of coloured eggs is as­so­ci­ated with the ori­gin of an open nest,” Norell said. Crocodiles and tur­tles bury their white eggs, which means they don’t need to be cam­ou­flaged. But many birds have coloured eggshells that cam­ou­flage their eggs in nests that are ex­posed to the el­e­ments and preda­tors.

Hauber of­fered three other rea­sons, in ad­di­tion to cam­ou­flage, why birds have colour­ful eggs: One, pig­ments can act like a para­sol or sun­screen, pro­tect­ing the em­bryos within from too much heat. Two, some par­ent birds use spot pat­terns to recog­nise their own eggs if they live in large colonies (or to prevent freeload­ing birds, like cuck­oos, from sneak­ing their eggs into a brood). And three, pig­ment de­posits are like molec­u­lar mor­tars, strength­en­ing the shell’s struc­ture. Th­ese are all pos­si­bil­i­ties for di­nosaurs, too. “We put on our think­ing cap now!” Hauber said.

So lis­ten well, crayon man­u­fac­tur­ers and paint mix­ers: Robin egg blue is fine, but ovi­rap­tor blue is the orig­i­nal.

An as­sort­ment of fresh chicken eggs

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