Males emus and rheas essentially raise their hatchlings alone.
Bone to Be Dad
Compared to the sizes of the adult skeletons, the clutches were very large, suggesting that multiple females contributed eggs to each clutch, which would have been guarded by a single male.
This led the researchers to question which theropod parents were doing what. To test the idea that these dinosaurs had male-only care, the team took a closer look at the brooding adults' fossils.
Varricchio and his colleagues microscopically studied the bones of eight brooding adults to identify gender.
(Related: "He Rex or She Rex? Experts Find Way to Tell Dino Gender" [June 2, 2005].)
Female birds in preparation for egg-laying generate extra bone tissue inside long bones like femurs. The mothers draw on these stored minerals during egg production.
Traces of this extra bone can often be seen when avian leg bones are studied carefully. Past research on Tyrannosaurus and Allosaurus leg bones has proven that such traces can also be seen in dinosaur females.
Yet, of the brooding theropods Varricchio and his colleagues analyzed, none possessed this special bone.
"The absence of this bone does not definitively prove that they are males, but it certainly suggests that these were males caring for the eggs," Varricchio said.
"It's fantastic to see that the wealth of data on egg and clutch size in living birds and crocodiles is being used to so effectively shed light on parental behaviors in dinosaurs," said Darla Zelenitsky, a paleontologist at the University of Calgary who was not involved in the study.
"This represents another significant piece of the puzzle for understanding the evolution of birds from their theropod dinosaur ancestors," she added.
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