Evolution Of Endothermy In Mammals Research Articles

The thermal performance curve for aerobic metabolism of a flying endotherm.

Performance benefits of stable, warm muscles are believed to be important for the evolution of endothermy in mammals, birds and flying insects. However, thermal performance curves have never been measured for a free-flying endotherm, as it is challenging to vary body temperatures of these animals, and maximal flight performance is difficult to elicit. We varied air temperatures and gas densities to manipulate thoracic temperatures of flying honeybees from 29°C to 44°C, with low air densities used to increase flight metabolic rates to maximal values. Honeybees showed a clear thermal performance curve with an optimal temperature of 39°C. Maximal flight metabolic rates increased by approximately 2% per 1°C increase in thoracic temperature at suboptimal thoracic temperatures, but decreased approximately 5% per 1°C increase as the bees continued to heat up. This study provides the first quantification of the maximal metabolic performance benefit of thermoregulation in an endotherm. These data directly support aerobic capacity models for benefits of thermoregulation in honeybees, and suggest that improved aerobic capacity probably contributes to the multiple origins of endothermic heterothermy in bees and other insects.

Impacts of Climate Variability and Change on (Marine) Animals: Physiological Underpinnings and Evolutionary Consequences.

Understanding thermal ranges and limits of organisms becomes important in light of climate change and observed effects on ecosystems as reported by the IPCC (2014). Evolutionary adaptation to temperature is presently unable to keep animals and other organisms in place; if they can these rather follow the moving isotherms. These effects of climate change on aquatic and terrestrial ecosystems have brought into focus the mechanisms by which temperature and its oscillations shape the biogeography and survival of species. For animals, the integrative concept of oxygen and capacity limited thermal tolerance (OCLTT) has successfully characterized the sublethal limits to performance and the consequences of such limits for ecosystems. Recent models illustrate how routine energy demand defines the realized niche. Steady state temperature-dependent performance profiles thus trace the thermal window and indicate a key role for aerobic metabolism, and the resulting budget of available energy (power), in defining performance under routine conditions, from growth to exercise and reproduction. Differences in the performance and productivity of marine species across latitudes relate to changes in mitochondrial density, capacity, and other features of cellular design. Comparative studies indicate how and why such mechanisms underpinning OCLTT may have developed on evolutionary timescales in different climatic zones and contributed to shaping the functional characteristics and species richness of the respective fauna. A cause-and-effect understanding emerges from considering the relationships between fluctuations in body temperature, cellular design, and performance. Such principles may also have been involved in shaping the functional characteristics of survivors in mass extinction events during earth's history; furthermore, they may provide access to understanding the evolution of endothermy in mammals and birds. Accordingly, an understanding is emerging how climate changes and variability throughout earth's history have influenced animal evolution and co-defined their success or failure from a bio-energetic point of view. Deepening such understanding may further reduce uncertainty about projected impacts of anthropogenic climate variability and change on the distribution, productivity and last not least, survival of aquatic and terrestrial species.

The evolution of endothermy in Cenozoic mammals: a plesiomorphic‐apomorphic continuum

The evolution of endothermy in birds and mammals was one of the most important events in the evolution of the vertebrates. Past tests of hypotheses on the evolution of endothermy in mammals have relied largely on analyses of the relationship between basal and maximum metabolic rate, and artificial selection experiments. I argue that components of existing hypotheses, as well as new hypotheses, can be tested using an alternative macrophysiological modeling approach by examining the development of endothermy during the Cenozoic. Recent mammals display a 10°C range in body temperature which is sufficiently large to identify the selective forces that have driven the development of endothermy from a plesiomorphic (ancestral) Cretaceous or Jurassic condition. A model is presented (the Plesiomorphic-Apomorphic Endothermy Model, PAE Model) which proposes that heterothermy, i.e. bouts of normothermy (constant body temperature) interspersed with adaptive heterothermy (e.g. daily torpor and/or hibernation), was the ancestral condition from which apomorphic (derived), rigid homeothermy evolved. All terrestrial mammal lineages are examined for existing data to test the model, as well as for missing data that could be used to test the model. With the exception of Scandentia and Dermoptera, about which little is known, all mammalian orders that include small-sized mammals (<500 g), have species which are heterothermic and display characteristics of endothermy which fall somewhere along a plesiomorphic-apomorphic continuum. Orders which do not have heterothermic representatives (Cetartiodactyla, Perissodactyla, Pholidota, and Lagomorpha) are comprised of medium- to large-sized mammals that have either lost the capacity for heterothermy, or in which heterothermy has yet to be measured. Mammalian heterothermy seems to be plesiomorphic and probably evolved once in the mammalian lineage. Several categories of endothermy are identified (protoendothermy, plesioendothermy, apoendothermy, basoendothermy, mesoendothermy, supraendothermy, and reversed mesoendothermy) to describe the evolution of endothermy during the Cenozoic. The PAE Model should facilitate the testing of hypotheses using a range of macrophysiological methods (e.g. the comparative method and the reconstruction of ancestral states).

The Evolution of Endothermy: Role for Membranes and Molecular Activity

On the basis of the comparative approach and three models of metabolism (endothermic and ectothermic vertebrates, body mass, and mammalian development), we suggest that a few common cellular processes, linked either directly or indirectly to membranes, consume the majority of energy used by most organisms; that membranes act as pacemakers of metabolism through changes in lipid composition, altering membrane characteristics and the working environment of membrane proteins--specifically, that changes in the membrane environment similarly affect the molecular activities (specific rates of activity) of membrane-bound proteins; and that polyunsaturation of membranes increases whereas monounsaturation decreases the activity of membrane proteins. Experiments designed to test this theory using the sodium pump support this supposition. Potential mechanisms considered include fluidity, electrical fields, and related surface area requirements of lipids. In considering the evolution of endothermy in mammals, for example, if the first mammals were small, possibly nocturnal and active organisms, all these factors would favour increased polyunsaturation of membranes. Such changes (from monounsaturated to polyunsaturated membranes) would allow membranes to set the pace of metabolism in the evolution of endothermy.

The evolution of endothermy in mammals and birds: from physiology to fossils.

Continuous, metabolically based maintenance of high and relatively stable body temperature in the face of greatly fluctuating ambient temperature is among the most remarkable attributes of mammals and birds. Such warm­ bloodedness or, more correctly, endothermic homeothermy, generally results from a combination of high resting, aerobically supported heat production rates (about fiveto tenfold that of reptiles) in virtually all soft tissues, and insulation sufficient to retard excessive heat loss. As a consequence, many temperature­ sensitive physiological processes in birds and mammals proceed at relatively stable rates over a wide range of ambient temperatures. In contrast, most organisms cannot thermoregulate in this way because endogenous heat production is insufficient to alter body temperature, and internal heat is necessarily derived primarily from the environment (ectother­ mic poikilothermy). These organisms tend to be thermoconformers whose body temperature frequently corresponds passively with environmental temperature. Usually such thermoconformity in thermally homogeneous environments (e.g. many marine or freshwater ecosystems) does not result in severe physiological disruption: genetic adaptation and/or phenotypic acclimation of numerous biochemical traits to ecologically relevant temperatures often ameliorate ef­ fects of temperature (e.g. enzyme function; G Somero, this volume) and mem­ brane structure (J Hazel, this volume; see also 63).

The Evolution of Endothermy in Mammals and Birds: From Physiology to Fossils

The Evolution of Endothermy in Mammals and Birds: From Physiology to Fossils