Origin and Development of Endothermy: From the Standpoint of Ecological Energetics of Birds

The study is based on the results of the integrated measurement of the energy expenditure at rest
and common activity in birds belonging to various systematic groups. Endothermy has formed in birds
and mammals independently and in different geological ages. However, in both groups, it originated
as a side effect of selection for aerobic metabolism improvement that provided a higher level of
activity. The advantages of having high and stable body temperature, which are inevitably related to
metabolism intensification, led to the development of thermoregulatory adaptations such as fur and
feathers. This made it possible to retain the metabolically generated heat and reduce heat absorption
in hot environments. The emergence of endothermy with an aerobic supply of motion activity,
possibilities to regulate the level of metabolism, and thermal conductance, has opened a lot of
opportunities for homoeothermic animals. Achieving such a level of energy utilization allowed them to
maintain activity for a longer time, while development of sensory systems due to stable temperature
and high level of oxygen support led to complications and diversification of endoterms behavioral
repertoire facilitating the conquest of an almost the entire part of the biosphere that is suitable for
living. This process was favored by the development of nurturing and passing on the information,
collected throughout life, to new generations. The formation of high levels of aerobic metabolism in
birds and mammals was proceeding in parallel among different groups of reptilian ancestors. The
level of endothermy, at which aerobic metabolism is able to maintain prolonged activity, developed in
birds and mammals in different ways: they have got dissimilar partitioning of venous and arterial
networks, erythrocytes with or without a cell nucleus, different lungs design – but, at that, similar
minimum metabolic power and rather close body temperatures which correspond well to the
environmental conditions on the Earth. Natural selection allowed animals with high energetic
metabolism to increase their diversity and abundance, but only when endothermic animals could
satisfy their demands for food resources, that has risen manifold. That happened in the middle of
Cretaceous, in time with the appearance of angiosperms and expansion of related fauna of
invertebrates. The catalyst of explosive radiation of endothermic animals was the emergence of
angiosperms and the associated invertebrate fauna, which provided the necessary energy channel for
the development of homoeothermic endothermic animals. Apparently, all important adaptations
develop this way. The driving force for these adaptations was developments in the environment that
triggered these processes and caused major evolutionary breakthroughs.