News release

From: Frontiers

Researchers show dinos hatched eggs less efficiently than modern birds

Research using dinosaur body model suggests that – unlike modern birds – bird-like dinosaurs may have used the sun’s warmth to help hatch eggs, shedding light on the evolution of avian-style incubation.

Researchers reconstructed clutches of oviraptors, bird-like but flightless dinosaurs that lived between 70 and 66 million years ago, to simulate heat transfer and infer hatching patterns in their clutches. They found that these dinosaurs didn’t hatch their eggs in the same way modern birds do. In addition to sitting on them, they likely used the sun as a co-incubator. The team pointed out that this method – although less efficient than the brooding of modern birds – wasn’t necessarily better or worse, but may have been a unique adaptation to the environment these dinosaurs lived in and could represent a step between semi-buried and fully-exposed styles of incubation.

What do we really know about how oviraptors – bird-like but flightless dinosaurs – hatched their eggs? Did they use environmental heat, like crocodiles, or body heat from an adult, like birds? In a new Frontiers in Ecology and Evolution study, researchers in Taiwan examined the brooding behavior and hatching patterns of oviraptors. They also modelled heat transfer simulations of oviraptor clutches and compared hatching efficiency to modern birds. To do so, they experimented with a life-sized oviraptor incubator and eggs.

“We show the difference in oviraptor hatching patterns was induced by the relative position of the incubating adult to the eggs,” said senior author Dr Tzu-Ruei Yang, an associate curator of vertebrate paleontology at Taiwan’s National Museum of Natural Science.

“Moreover, we obtained an estimate of the incubation efficiency of oviraptors, which is much lower than that of modern birds,” added first author Chun-Yu Su, who attended Washington High School in Taichung when the research was conducted.

Building a dinosaur

The reconstructed oviraptor Heyuannia huangi lived between 70 and 66 million years ago in what today is China. Estimated to be around 1.5 meters long and weighing around 20kg, it built semi-open nests made up of several rings of eggs.

The incubating oviraptor’s trunk was made from polystyrene foam and wood for the skeletal frame and cotton, bubble paper, and cloth for the soft tissue. Eggs were molded from casting resin. In the two clutches used in the experiments, eggs were arranged in double-rings based on real oviraptor clutches.

“Part of the difficulty lies in reconstructing oviraptor incubation realistically,” said Su. “For example, their eggs are unlike those of any living species, so we invented the resin eggs to approximate real oviraptor eggs as best as we could.”

When the team ran experiments to find out if clutch attendance of a brooding adult or different environmental circumstances may have impacted hatching patterns, they found that in colder temperatures, where a brooding adult attended the clutch, the eggs’ temperatures in the outer ring differed by up to 6°C, which could have resulted in asynchronous hatching, a pattern where eggs in the same nest hatch at different times. In warmer conditions, the difference in egg temperatures in the outer ring was just 0.6°C, suggesting that oviraptors living in warmer conditions may have exhibited a different pattern of asynchronous hatching because they could use the sun as an additional, powerful heat source.

“It’s unlikely that large dinosaurs sat atop their clutches. Supposedly they used the heat of the sun or soil to hatch their eggs, like turtles. Since oviraptor clutches are open to the air, heat from the sun likely mattered much more than heat from the soil,” Yang explained.

Better hatchers?

The team also investigated how oviraptor incubation efficiency compares to that of modern birds. Most birds use thermoregulatory contact incubation (TCI), where adults sit directly on the eggs to transfer heat. TCI requires three prerequisites – the adult bird must be in contact with every egg, be the main heat source, and maintain all eggs within a constrained temperature range – which oviraptors didn’t fulfil. For example, their egg arrangement prevented the adult from making full contact with all eggs in the clutch.

“Oviraptors may not have been able to conduct TCI as modern birds do,” said Su. Instead, these dinosaurs and the sun may have been co-incubators – a less efficient incubation behavior than that displayed by modern birds. Yet, the combination of adult incubation and an ambient heat source – perhaps a behavioral adaptation associated with the evolution from buried to semi-open nests – isn’t necessarily worse.

“Modern birds aren’t ‘better’ at hatching eggs. Instead, birds living today and oviraptors have a very different way of incubation or, more specifically, brooding,” Yang pointed out. “Nothing is better or worse. It just depends on the environment.”

The team pointed out that their findings are specific to the reconstructed nest and are limited by the fact that today’s climate does not resemble the Late Cretaceous climate, which may have impacted the results. Oviraptors also exhibited a longer incubation period than modern birds.

Yet, the study advances our understanding of oviraptor brooding strategies through innovative approaches. It represents an important bridge between physics-based simulations and paleontological interpretations, potentially enabling paleontologists to investigate topics for which approaches were limited until now.

“It also truly is an encouragement for all students, especially in Taiwan,” concluded Yang. “There are no dinosaur fossils in Taiwan but that does not mean that we cannot do dinosaur studies.”