When paleontologists Jack Horner and Bob Makela named a large hadrosaur that had been found among eggshells and nests in 1979, they called it Maisaura, the “good mother reptile.” The name suggested that the young of this genus were raised with motherly love. Producing eggs would be energetically expensive, and caring for the nest would have helped mothers get a better “return” on their reproductive investment. But what about the fathers?
Egg-laying vertebrates have a variety of reproductive strategies. In some species, males mate with many females and provide almost no parental care, while in others females reverse the roles, leaving their eggs with the male to raise the young alone. In still others, males and females both contribute to raising their young. Different dinosaurs probably exhibited a variety of reproductive behaviors just like living vertebrates.
In a paper
How could they tell? Eggs are nutritionally expensive to produce, and like birds, female dinosaurs required calcium and phosphorus to produce egg shells. The source for these minerals came from medullary bone, a spongy looking type of bone lining the inside of the shafts of long bones like the femur. Medullary bone has been found in other dinosaur species like Tyrannosaurus, Allosaurus, and Tenontosaurus. If the dinosaurs on the nests showed evidence of this kind of bone, then they could be identified as females.
When the researchers looked at the cross-sections of femurs from the nesting dinosaurs, they found no medullary bone and little sign of the bone remodeling that goes along with egg formation. What could this mean? There are several possibilities.
One is that the dinosaurs on the nests were females, but they had a different pattern of bone transformation that obliterated the evidence as to their sex. Another is that the dinosaurs on the nests were non-reproductive females — individuals past their prime or not yet laying eggs. This might mean that females cared for the eggs of a mother or sister.
This would have raised what is known as their inclusive fitness, for even if they can’t reproduce themselves, some of the genes they share with their relative can be passed on by helping their kin. It is presently impossible to confirm this. The conclusion of the authors, however, is that the individuals on the nests were male. Given that the results were consistent across individuals from several different genera, making it seem more likely that the same phenomenon was occurring over and over, and given what is seen in birds, the simplest explanation is that the brooding dinosaurs were males.
Oviraptor, Citipati, and Troodon are all maniraptorans, closely related to the dinosaurs that give rise to birds. If the dinosaurs sitting on the nests are really males, then male parental care may have been another trait that appeared in dinosaurs first and was then passed on to birds, where it was modified in many different ways. This hypothesis raises further questions, though.
Did females sit on the nests, too? Did they contribute to raising the offspring, or did they leave males to raise the young alone? What sort of mating system did these dinosaurs have? Further discoveries will be needed, but the present study is a wonderful meld of behavioral observations with breakthroughs in dinosaur bone analysis.