New Evidence For Warm Blooded Dinosaurs

The cold blooded versus warm-blooded debate is one of the most contentious surrounding the dinosaurs. As reptiles we would expect them to be cold-blooded or ectotherms to use the proper term, reliant on an external heat source such as the sun. Yet their lifestyle, size and close affinities with warm-blooded birds presents a strong case they they may in fact have also been warm-blooded or endotherms, creatures which generate their own body heat through metabolism. The debate continues.

A study conducted in 2014 suggested that the dinosaurs may have occupied a middle ground. However, data re-analysis suggests a shift towards the warm blooded end of the spectrum. 'The study that I re-analysed was remarkable for its breadth -- the authors compiled an unprecedented dataset on growth and metabolism from studies of hundreds of living animals,' said Dr Michael D'Emic from Department of Anatomical Sciences at Stony Brook. 'Upon re-analysis, it was apparent that dinosaurs weren't just somewhat like living mammals in their physiology. They fit right within our understanding of what it means to be a warm-blooded mammal'.

A diagrammatic representation of the disparity between the growth
rate calculated by the 2014 study and Dr D'Emic reanalysis

The original study scaled yearly to daily growth rates so that comparisons could be standardised. The issue with this is that mammals do not grow continuously throughout the year. Colder, drier and more stressful conditions will slow or even halt growth until more favourable conditions are encountered. The study did not take account of the uneven growth of dinosaurs.

The growth values themselves were based on the analysis of transverse sections of dinosaur bones. Like the trunk of a tree, bones also contain rings indicative of periods of fast and slow growth.

A specialist in microanatomy, Dr D'Emic, demonstrated that the growth rates represented by the rings had been underestimated.

By spreading the size of the growth ring over the course of a year rather than into a few months when seasonal conditions were optimal, the growth rates for dinosaurs calculated in the original study were low, suggesting that they were ectotherms. D'Emic's re-analysis shows that the growth rates were faster and are more concordant with endotherms.

Additionally, the data was statistically re-analysed with that of birds, the dinosaurs closest living relatives. The results again suggested that dinosaurs were warm blooded. When they were first discovered, comparative anatomy of complete bones was about the most sophisticated form of analysis available. Even 100 years ago, insights beyond those of an anatomical nature, were limited. Our current ability to probe the physiological and biomechanical constraints of long extinct creatures is remarkable. Doubtless, the dinosaurs will continue to yield surprises.