Male Dinosaurs May Not Have Looked after the Nest After All
Back in December 2008, Everything Dinosaur published details of an academic paper that concluded that male theropod dinosaurs undertook the majority of the brooding of eggs in nests. Dr David Varricchio (Montana State University) and his colleagues assessed the known nest fossil material of three theropod dinosaurs from Upper Cretaceous strata. The dinosaurs studied were Citipati osmolskae, Oviraptor philoceratops (both oviraptorids) and Troodon formosus (troodontid). The team used a number of methodologies to conclude that, just like 90% of the birds species alive today, it was the male theropods who did most of the brooding on the nests. The conclusions were reached after examining fossils for evidence of bone cavities associated with the loss of calcium from the body in order to produce egg shell. Such cavities would be expected to be seen only in females. Since none of the fossil bones included in the study showed such cavities, it was suggested that the individual dinosaurs found in close proximity to a nest of their own species were probably male.
A Close up of a Dinosaur Nest (Theropoda)
The number of eggs laid per nest compared to the body size of the adult dinosaur supported this view that the brooding dinosaurs were male. The study proposed that theropod dinosaurs produced unusually large numbers of eggs per nest for their body size. This pattern is often seen in extant Aves (bird species alive today), when the male alone takes on the parental duties. The female can afford to lay more eggs, as she will not be looking after them so she can be away from her maternal duties and get back to feeding herself up to replenish lost reserves in her body. So it was concluded that the theropod males brooded the nest and probably played a significant role in looking after the hatch-lings.
To view an article on the original research: Doting Fathers – A Dinosaur Trait Passed Onto Birds.
However, scientists from the University of Lincoln (England), have reviewed the data used in the 2008 study and they have come up with a different interpretation of the fossil evidence.
It is very difficult to infer behaviour, especially something as complicated as parental behaviour with just the fragmentary fossil record to go on. In the review of the 2008 research, a number of factors known to affect egg and clutch sizes in living bird species were not taken into account. A new scaling analysis of the clutch masses of birds suggests that the type of parental care may not be inferred. The evolutionary relationship between the theropods and Aves may be widely accepted but this does not necessarily mean that these two types of creature raised their young in the same or similar ways. The maturity of the hatch-lings precocial (independent young) or altricial (dependent young) would have a significant bearing on the behaviour of the parents. Intriguingly, the new study from the University of Lincoln team suggests that most theropods seem to exhibit precociality (hatch-lings born relatively mature and independent to a degree from their parents).
The male theropod dinosaurs may not have been such dedicated dads after all.
Dr Charles Deeming (School of Life Sciences) and his Lincoln University colleagues point out a number of factors that need to be considered before attempting to work out how adult theropods behaved around the nest. Dr Deeming specialises in the study of avian and reptilian reproduction, he is very well placed to compare extinct fauna with extant descendants. For example, a number of bird species today, deliberately lay their eggs in another bird’s nest so as to avoid any form of parental responsibility at all. This will distort the size of some nests, theropod dinosaurs may have behaved in the same way. There are evolutionary advantages when it comes to getting others to do work for you.
Dr Deeming and his team took a different approach to the statistical analysis. They counted the eggs in all known fossil nests for the theropod species included in the original study and then worked out an average clutch size for each species. Dr Varricchio and the Montana State University researchers had based their calculations on the largest clutch size for each dinosaur species.
When the British-based team compared their average figures with the adult dinosaur body mass, they found that the theropod dinosaurs were not included in the group of male-only brooders.
Commenting on the earlier research, Dr Deeming stated:
“The Varricchio analysis is now being used by other palaeontologists working on other dinosaur species. It’s time to stand up and say it doesn’t quite work.”
For Dr Varricchio, the new insight is most welcome, but he warns:
“Regardless of what this paper or our paper says, we are really operating with only a few pieces of the puzzle. To address the [parental] care in these dinosaurs, one needs to consider their other relatives and not just birds. For instance, crocodiles, which share a common ancestor with all dinosaurs, might be one source of clues to dinosaur brooding behaviour.”
The two research teams are agreed on certain points, there is a need for further study and looking at the nests of crocodiles may yield useful data.
Dr Deeming added:
“If you look at the eggs in those dinosaur nests, they’re structure is similar to crocodile eggs.”
Crocodilians bury their eggs and the temperature at which the nest is kept is essential for healthy hatch-lings, the temperature of the nest also determines the sex of the offspring. A buried nest hypothesis would add an additional and very significant factor to any parental behaviour study.
Dr Deeming concluded by saying:
“Crocodiles don’t incubate their eggs, they just sit on the buried eggs to protect them from predators. I think that’s probably what was going on in the dinosaurs too.”
It is difficult to assign behaviour to fossil material, in the absence of a time machine and an ability to travel back to the Late Cretaceous to observe oviraptorids and dromaeosaurs in the breeding season, this debate is likely to rumble on.