Physiology, thermoregulation and bipedalism

It has long been recognized that the bipedal posture reduces the surface area of the body exposed to the sun. In recent years, a theory has been developed by Wheeler that bipedalism evolved in the ancestor of the Hominidae in order to help relieve thermal stress on the animals in open equatorial environments. Bipedalism was said to afford a distinct adaptive advantage over quadrupedalism by permitting hominids to remain active in the open throughout the day. The heat load of the hypothetical hominid comprises the external environment as modelled by Wheeler and the animal's internal environment (i.e., the internal heat generated by its metabolic and locomotor activities, and its evaporative and respirative cooling capacities). When these factors are integrated in the calculation of the animal's thermal budget, the putative advantage of the bipedal over the quadrupedal posture is considerably reduced. The simulations conducted in this study suggest that the increased time afforded to early hominids in the open by bipedalism was relatively short and, therefore, of little or no adaptive significance. These results suggest that thermoregulatory considerations cannot be implicated as a first cause in the evolution of bipedalism in the hominid ancestor.     

Thermophysiology of Streptococcus mutans and related lactic-acid bacteria

The temperature ranges for growth of Streptococcus mutans GS-5 and S. sobrinus 6715 were found to be very narrow, from about 30 to 47 °C, with optimal growth around 37 °C. Thus, the organisms showed little potential to grow in the environment outside of the animal host. In contrast wider ranges were found for Enterococcus hirae, S. rattus and S. sanguis. Detailed study of S. mutans GS-5 showed that energetic coupling, reflected in yields of biomass per mol of glucose utilized, were not greatly affected by changes in temperature within the growth range. However, since glycolysis occurred over a wider temperature range (about 10 to 52 °C) than growth, yield values dropped to zero at temperatures above or below the growth range. The temperature range for glycolysis could be related to temperature sensitivity of the phosphoenolpyruvate: sugar phosphotransferase system for sugar uptake. F-ATPases were active over a similar range of temperatures, but with a broad optimal range from about 30 to 50 °C. Proton permeability of S. mutans increased steadily with temperature in a manner similar to that of other mesophilic bacteria, such as Escherichia coli. Growth of the bacteria in media supplemented with various fatty acids had major effects on proton permeabilities but the effects were not well reflected by changes in growth or glycolysis of the bacteria. The overall conclusions were that S. mutans is a typical mesophile in relation to membrane and catabolic functions but its narrow temperature range for growth is related to temperature sensitivities of anabolic systems.     

Evolution of the third eye: a phylogenetic comparative study of parietal‐eye size as an ecophysiological adaptation in Liolaemus lizards

The parietal, or third, eye is a photosensory organ situated in the middle of the skull of many lizards. Despite many hypotheses, its exact ecological functions are still unclear. Studies have compared the presence and absence of a functioning parietal eye, although there are no quantitative studies of parietal‐eye traits in relation to ecology, physiology or behaviour. In the present study, we report the first comparative study of relative parietal‐eye size in relation to climatic and thermophysiological variables. We studied thirty species of Liolaemus, a genus of South‐American lizards inhabiting a range of climatic conditions, but found little evidence for adaptation to thermal environment, in that parietal‐eye size did not vary meaningfully with latitude, altitude or any measures of environmental temperature. Neither did it relate to thermophysiology; there was a weak relation to thermal tolerance, although this was partially confounded with body size, which explained 23% of the among‐species variance after controlling for within‐species variation. The negative results obtained could not be explained by phylogenetic constraints because we found no evidence of phylogenetic inertia. We also observed high intraspecific variation indicating that parietal‐eye size may not be under strong selection for accuracy. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 870–883.     

Climatic constraints on the biogeographic history of Mesozoic dinosaurs

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Thermophysiology,microclimates, and species distributions of lizards in the mountains of the Brazilian Atlantic Forest

Thermophysiological traits, particularly thermal tolerances and sensitivity, are key to understanding how organisms are affected by environmental conditions. In the face of ongoing climate change, determining how physiological traits structure species’ ranges is especially important in tropical montane systems. In this study, we ask whether thermal sensitivity in physiological performance restricts montane lizards to high elevations and excludes them from the warmer environments reported at low elevations. For three montane lizard species in the Brazilian Atlantic Forest, we collect thermophysiological data from lizards in the highest elevation site of each species’ distribution, and ask how well the individuals exhibiting those traits would perform across the Atlantic Forest. We use microclimatic and organism‐specific models to directly relate environmental conditions to an organism's body temperature and physiological traits, and estimate measures of thermophysiological performance. Our findings demonstrate that thermophysiological constraints do not restrict montane lizards to high elevations in this system, and thus likely do not determine the warm boundaries of these montane species’ distributions. Results also suggest that competition may be important in limiting the warm boundaries of the species’ ranges for two of the focal species. These experimental results suggest that caution should be used when claiming that physiology drives patterns of diversity and endemism within montane environments. They also highlight the importance of interdisciplinary experimental studies that bridge the fields of evolution and ecology to improve predictions of biological responses to future environmental shifts.     

The evolution of gigantism in active marine predators

A novel hypothesis to better understand the evolution of gigantism in active marine predators and the diversity of body sizes, feeding strategies and thermophysiologies of extinct and living aquatic vertebrates is proposed. Recent works suggest that some aspects of animal energetics can act as constraining factors for body size. Given that mass-specific metabolic rate decreases with body mass, the body size of active predators should be limited by the high metabolic demand of this feeding strategy. In this context, we propose that shifts towards higher metabolic levels can enable the same activity and feeding strategy to be maintained at bigger body sizes, offering a satisfactory explanation for the evolution of gigantism in active predators, including a vast quantity of fossil taxa. Therefore, assessing the metabolic ceilings of living aquatic vertebrates and the thermoregulatory strategies of certain key extinct groups is now crucial to define the energetic limits of predation and provide quantitative support for this model.