Repeated discoveries of well-preserved dinosaurs in breeding postures atop their nests have long fascinated palaeontologists. These fossils wield strong evidence that dinosaurs incubated their eggs in a bird-like manner. However, a recent study lends evidence to an even more intriguing suggestion—that the egg-sitters were males.
David Varricchio of Montana State University and researchers from three other institutions reached this conclusion through information gathered from the creatures’ egg clutch volumes and bone tissue. Their results have been published in the journal Science.
To deduce what dinosaur parental behaviour would have been like, scientists examined modern birds, their closest living relatives and descendents of small, feathered dinosaurs. They also looked at dinosaurs’ next-of-kin, the crocodilians. The team noted that the ratio of adult body mass to total egg volume in these animals can be related with statistical significance to which sex was the primary caregiver.
The scientists studied Troodon, Oviraptor, and Citipati, three fleet-footed, bipedal, and bird-like dinosaur species that made nests. These dinosaurs laid batches of 22 to 30 big eggs, a similar number as birds that display predominantly paternal care, as opposed to those that have maternal care, or where both parents share the job. The species that engaged in full-time fatherhood had more voluminous egg clutches than the other two parenting styles.
Although statistics support the paternal care idea, they do not determine whether the dinosaurs sitting on their eggs were actually male. However, bone histology offers a clue. Laying eggs is a heavy drain on calcium and phosphorus resources. Female reptiles, such as crocodiles, extract these minerals from their bones. This leads to the formation of re-absorption cavities in the bones when their eggs are developing. In the case of mother birds, calcium and phosphorus is stored by depositing a complex of irregular bone tissue along the inside surface of long bones, called medullary bone. Although mostly reabsorbed for use during the egg-formation and laying, some of this bone tissue can linger for days to weeks afterward.
Medullary bone has been found in the bones of female dinosaurs as well, including species such as Tyrannosaurus and Allosaurus, which are less related to birds than the dinosaurs focused on in the study. When the team analyzed long bones from eight nest-brooding adult Troodon and Citipati specimens, and no medullary bone or reabsorption cavities were found, the egg-sitters were concluded to have been either male or non-reproducing females. This is consistent with male care and at the least, does not falsify the fatherhood hypothesis.
The idea that dinosaurs were dutiful dads supplies some answers to questions about the evolutionary history of bird parenting. It has been observed that birds seem exceptional among vertebrate animal groups in their degree of male care. In only five per cent of mammal species—even fewer in non-avian reptiles—do fathers help raise offspring at all. For 90 per cent of birds, both parents contribute.
A particular group of predominantly flightless birds belonging to the superorder Palaeognathae, which includes ostriches and kiwis, is unique for a behavior system in which males mate with many females and then take responsibility for the eggs the females leave behind. This set of birds is the most primitive living branch of the avian family tree. Consequently, scientists wonder if their reproductive behaviour was present in the original ancestor of modern birds. This recent study of dinosaurs may suggest a paternal role in egg-rearing was not only an ancestral state, but one that evolved early in the line of dinosaurian ancestors leading to birds.
