‘Shrinking’ dinosaurs allowed birds to evolve as warm-blooded creatures – study

By Nilima Marshall, PA Science Reporter

Dinosaurs gradually “shrunk” and paved the way for birds to evolve as warm-blooded creatures, new research claims.

Scientists believe that as dinosaurs decreased in size over millions of years they developed to have higher metabolism.

This evolutionary process allowed the creatures to burn energy from food at a faster rate and regulate their own body temperature.

The researchers, from the University of Chile, say their findings may also explain how mammals became warm-blooded, giving them the capability to generate heat by metabolic means.

Warm blooded animals, or endotherms, need a lot of energy compared to cold blooded creatures – such reptiles, fish and amphibians – who rely on environmental heat sources.

But the researchers say there are many benefits to being warm-blooded, which include being able to live in a wider geographic range as well as having greater mobility, brain power, stamina and tolerance to harsher conditions.

Over the years, scientists have struggled to understand the origins of endothermy, mainly due to the lack of fossil evidence.

To find out more about the evolution of warm-bloodedness, lead author Enrico Rezende and his team used a popular biologists’ model for heat regulation known as the Scholander-Irving model.

Dinosaurs evolved higher metabolism as they decreased in size to give rise to warm-blooded birds (Mauricio Alvarez/Science Advances)

According to this model, warm-blooded birds and mammals must balance their metabolic heat production with their heat loss to the environment in order to stay comfortable.

The team combined the Scholander-Irving model with different body sizes of theropods – a group of two-legged, three-toed dinosaurs from which all birds evolved.

Their findings suggest metabolic rates rose steadily throughout most of the Early to Middle Jurassic period, around 180 to 170 million years ago.

Simulations indicate a warm-blooded creature would need to shrink nine-fold to attain the same energy requirements as a cold-blooded creature of the same original body size.

For instance, a 43kg modern-day bird would have the same energy requirements as its 370kg dinosaur ancestor.

The researchers conclude that their smaller sizes and higher energy turnover rates ultimately favoured the theropod lineage, with simulations showing a 30-fold increase in population as endothermy evolved.

Writing in the journal Science Advances, the authors noted: “Results suggest that a reduction in size constitutes the path of least resistance for endothermy to evolve, maximising thermal niche expansion while obviating the costs of elevated energy requirements.”