Shrinking dinosaurs and the evolution of endothermy in birds

The evolution of endothermy represents a major transition in vertebrate
history and a major factor underlying the diversity of birds and mammals.
Despite the several advantages of an endothermic lifestyle, the tempo and
mode of the evolution of endothermy in these lineages remains one of the
most controversial subjects in paleontology and evolutionary physiology.
Here, we combine a heat transfer model with body size estimates in the
theropod phylogeny to reconstruct the evolution of metabolic rates along
the bird stem lineage. This model suggests that the continuous reduction
in size from ancestral theropod dinosaurs to birds constitutes the
evolutionary path of least resistance for endothermy to evolve, because it
maximizes thermal niche expansion while obviating the im- pact of elevated
energy requirements on population size. In this scenario, metabolic rates
would have increased steadily with the accelerated miniaturization
observed primarily in the Early-Middle Jurassic (~180 to 170 million years
ago), resulting in a rise in metabolic levels along those clades that
diverged from the bird stem lineage during this period. Whereas basal
theropods would generally exhibit lower metabolic rates, more recent
groups such as nonavian maniraptorans were likely decent thermoregulators
with higher metabolic rates than similar- sized extant ectotherms. We
postulate that the evolution of smaller sizes concomitantly with increased
aerobic capacity has shaped the thermal physiology of theropod dinosaurs
and driven the emergence of true endothermy within this clade, and we
propose a temporal sequence of key evolutionary transitions that might
have ultimately resulted in the emergence of small, endothermic, feathered
flying dinosaurs. The detailed hypotheses stemming from our model,
regarding the variation in metabolic levels across theropod lineages as
well as the sequence of events that have led to the radiation of birds,
can be readily put to a test and should be subject to further scrutiny.