Climate Variability and the Energetic Pathways of Evolution: The Origin of Endothermy in Mammals and Birds

Abstract

Large-scale climate oscillations in earth's history have influenced the directions of evolution, last but not least, through mass extinction events. This analysis tries to identify some unifying forces behind the course of evolution that favored an increase in organismic complexity and performance, paralleled by an increase in energy turnover, and finally led to endothermy. The analysis builds on the recent concept of oxygen-limited thermal tolerance and on the hypothesis that unifying principles exist in the temperature-dependent biochemical design of the eukaryotic cell in animals. The comparison of extant waterbreathing and air-breathing animal species from various climates provides a cause-and-effect understanding of the tradeoffs and constraints in thermal adaptation and their energetic consequences. It is hypothesized that the high costs of functional adaptation to fluctuating temperatures, especially in the cold (cold eurythermy), cause an increase in energy turnover and, at the same time, mobility and agility. These costs are associated with elevated mitochondrial capacities at minimized levels of activation enthalpies for proton leakage. Cold eurythermy is seen as a precondition for the survival of evolutionary crises elicited by repeated cooling events during extreme climate fluctuations. The costs of cold eurythermy appear as the single most important reason why metazoan evolution led to life forms with high energy turnover. They also explain why dinosaurs were able to live in subpolar climates. Finally, they give insight into the pathways, benefits, and trade-offs involved in the evolution of constant, elevated body temperature maintained by endothermy. Eurythermy, which encompasses cold tolerance, is thus hypothesized to be the "missing link" between ectothermy and endothermy. Body temperatures between 32Њ and 42ЊC in mammals and birds then result from trade-offs *

Biological Impact of Elevated Ocean CO2 Concentrations: Lessons from Animal Physiology and Earth History

“…However, although CO 2 may have been one of the effective parameters, the relevant analysis by Knoll et al neglects the role of temperature oscillations, which most probably contributed to mass extinction events through the long term effects of repeated exposure to extreme climate oscillations and associated cooling events (Pörtner, 2001(Pörtner, , 2004. These considerations indicate that current trends of warming and CO 2 accumulation in marine surface waters may exert synergistic effects on marine fauna.…”

Oxygen limited thermal tolerance in fish?

“…This is likely true for many hibernating amphibians and includes mammalian hibernators, where at least the brain remains fully aerobic, despite extremely cold body temperatures (Bock et al, 2002c). Passive hibernation is thus interpreted to be a strategy which allows the animal to survive at minimal cost and thereby escape from the costly mechanisms of eurythermal cold adaptation outlined above (Pörtner, 2004). The factors and mechanisms eliciting metabolic depression in excess of the one elicited by cold temperature itself are currently unknown.…”

Physiological capacity of Cancer setosus larvae — Adaptation to El Niño Southern Oscillation conditions

“…A mismatch between the oxygen demand of tissues and the supply by the circulatory and ventilatory system is the first mechanism restricting survival at unfavorable temperatures (Heilmayer et al, 2004;Pörtner, 2001;Pörtner, 2004;Storch et al, 2009). The thermal tolerance thresholds (pejus and critical limits) in the concept of oxygen and capacity limited thermal tolerance (OCLT) presented by Pörtner et al (2005) describe the shortage of aerobic capacity due to unfavorable temperatures.…”