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Odd bird fathering styles may come from dinos

Dec. 18, 2008
Courtesy Florida State University
and World Science staff

In several species of flightless birds, the males are de­vot­ed fa­thers—and poly­gam­ists. These males serve as the sole in­cu­ba­tors and care­givers for over­sized broods from mul­ti­ple moth­ers.

An artist's con­cep­tion de­pict­ing the Ovi­rap­torid di­no­saur Citi­pati on a nest that was found in the Go­bi des­ert of Mon­go­lia by the Amer­i­can Mu­se­um of Nat­u­ral His­to­ry. It is one of the di­no­saurs stud­ied by Er­ick­son and his group. (Draw­ing by Mick El­li­son, © Amer­i­can Mu­se­um of Nat­u­ral His­to­ry, 2008)

Now, a study published in the Dec. 19 is­sue of the re­search jour­nal Sci­ence con­cludes that this be­ha­vior dates back to the di­no­saur an­ces­tors of these birds.

Sci­en­tists had long won­dered about the ori­gins of po­lyg­a­my and pa­ter­nal care pat­terns among modern-day Pa­le­og­nathes, an an­cient branch of the bird family be­lieved to have orig­i­nat­ed soon af­ter birds evolved from di­no­saurs. The group in­cludes os­triches, emus and tina­mous.

Males con­trib­ute to pa­ren­tal care in less than five per­cent of mam­mal and non-a­vi­an rep­tile spe­cies, the re­search­ers said. And while more than 90 per­cent of bird spe­cies co-parent to some de­gree, it is only among the Pa­le­og­nathes that both po­lyg­a­my and pa­ter­nal care rule. 

Pho­to of the ovi­rap­torid di­no­saur Citi­pati on a nest that was found in the Go­bi des­ert of Mon­go­lia by the Amer­i­can Mu­se­um of Nat­u­ral His­to­ry. (Photo by Mick El­li­son, © Amer­i­can Mu­se­um of Nat­u­ral His­to­ry, 2008)

In the new stu­dy, pa­le­o­bi­olo­gist Greg­o­ry M. Er­ick­son of Flor­i­da State Uni­ver­s­ity and col­leagues con­nect­ed the ev­o­lu­tion­ary dots link­ing the re­pro­duc­tive pat­terns of liv­ing birds to the be­hav­ior of their ex­tinct di­no­saur kin.

Er­ick­son and col­leagues stud­ied fos­sils of some of the di­no­saurs be­lieved to be most closely re­lat­ed to liv­ing birds. The dino­saurs in­cluded groups known as tro­odon­tid and ovi­rap­tor­ids, from the Cre­ta­ceous per­iod that ended 65 mil­lion years ago.

In di­no­saur fos­sils where adult di­no­saurs have been found on top of nests, Er­ick­son said his group found that skele­tal ev­i­dence in­di­cat­ed only males were sit­ting on the nests.

More­o­ver, the num­ber of eggs in the nest is “very large rel­a­tive to the size of the nest­ing an­i­mals,” he added. 

“This sug­gests mul­ti­ple fe­males con­trib­uted the eggs and the male guard­ed them. No­ta­bly, the ra­tio of egg vol­umes to the nest­ing an­i­mal’s size is con­sist­ent with those in liv­ing birds where the male is the sole or pri­ma­ry nest at­ten­dant.”

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They’re devoted fathers—and polygamists. The males of several flightless birds, including emus, serve as the sole incubators and caregivers for oversized broods from multiple mothers. It’s a rare set of behaviors, but research described in the Dec. 19 issue of the research journal Science found that it runs in this avian family, all the way back to its dinosaur ancestors. Scientists had long wondered about the origins of polygamy and paternal care patterns among modern-day Paleognathes, an ancient branch of the bird family believed to have originated soon after birds evolved from dinosaurs. The group includes ostriches, emus and tinamous. Males contribute to parental care in less than five percent of mammal and non-avian reptile species, the researchers said. And while more than 90 percent of bird species co-parent to some degree, it is only among the Paleognathes that both polygamy and paternal care rule. In the new study, paleobiologist Gregory M. Erickson of Florida State University and colleagues connected the evolutionary dots linking the reproductive patterns of living birds to the behavior of their extinct dinosaur kin. Erickson and colleagues studied fossils of theropods, the dinosaurs believed to be most closely related to living birds. Theropods, which walked on two hind legs with bird-like feet, include the infamous Tyrannosaurus Rex. In dinosaur fossils where adult dinosaurs have been found on top of nests, Erickson said his group found that skeletal evidence indicated only males were sitting on the dinosaur nests. Moreover, the number of eggs in the nest is “very large relative to the size of the nesting animals,” he added. “This suggests multiple females contributed the eggs and the male guarded them. Notably, the ratio of egg volumes to the nesting animal’s size is consistent with those in living birds where the male is the sole or primary nest attendant.”