Endothermy

The origin of mammalian endothermy has long been held to reside within the early therapsid groups. However, shared histological characteristics have been observed in the
bone matrix and vascularity between Ophiacodontidae and the later therapsids (Synapsida). Historically, this coincidence has been explained as simply a reflection of the presumed aquatic lifestyle of Ophiacodon or even a sign of immaturity. Here we show, by histologically sampling an ontogenetic series of Ophiacodon humeri, as well as additional material, the existence of true fibrolamellar bone in the postcranial bones of a member of ‘Pelycosauria’. Our findings have reaffirmed what previous studies first described as fast
growing tissue, and by proxy, have disproven that the highly vascularized cortex is simply a reflection of young age. This tissue demonstrates the classic histological characteristics
of true fibrolamellar bone (FLB). The cortex consists of primary osteons in a woven bone matrix and remains highly vascularized throughout ontogeny providing evidence to fast
skeletal growth. Overall, the FLB tissue we have described in Ophiacodon is more derived or “mammal-like” in terms of the osteonal development, bone matrix, and skeletal growth
then what has been described thus far for any other pelycosaur taxa. Ophiacodon bone histology does not show well-developed Haversian tissue. With regards to the histological record, our results remain inconclusive as to the preferred ecology of Ophiacodon, but support the growing evidence for an aquatic lifestyle. Our findings have set the evolutionary origins of modern mammalian endothermy and high skeletal growth rates back approximately 20 M.Y. to the Early Permian, and by phylogenetic extension perhaps the Late Carboniferous.

A 1972 model for the evolutionary origin of hair suggested a primary mechanoreceptor role improving behavioral thermoregulation contributed to the success of late Paleozoic mammal-like reptiles. An insulatory role appeared secondarily subsequent to protohair multiplication. That model is updated in light of new data on (a) palaeoecology of mammalian ancestors; (b) involvement of HRPs in keratinization; (c) lipogenic lamellar bodies that form the barrier to cutaneous water loss; and (d) growth factors involved in hair follicle embryogenesis and turnover. It is now proposed that multiplication of sensory protohairs caused by mutations in patterning genes initially protected the delicate barrier tissues and eventually produced the minimal morphology necessary for an insulatory pelage. The latter permitted Mesozoic mammals to occupy the nocturnal niche 'in the shadow of dinosaurs'. When the giant reptiles became extinct, mammals underwent rapid radiation and reemerged as the dominant terrestrial vertebrates.

The evolution of endothermic thermoregulation is rooted in the processes involving high metabolism, which allows the maintenance of high and stable body temperatures (Tb ). In turn, selection for high endothermic metabolism correlates with increased sizes of metabolically active organs, and thus with high basal metabolic rate (BMR). Endothermic animals are characterized by MR several times higher than similarly-sized ectotherms. However, many small mammals are temporally heterothermic and are able to temporally decrease Tb and MR by entering daily torpor or hibernation. Both BMR and minimum metabolic rate during torpor (TMRmin) likely result from oxidative respiration in mitochondria of the same tissues. It should be expected that these two MRs are positively correlated, suggesting that the evolution of endothermy and higher BMR set the limit for the ability to reduce MR while entering torpor. Using published data for 96 mammal species, we tested the hypothesis that among heterothermic mammals, the processes leading to the evolution of higher BMR limit the ability to down-regulate metabolism during torpor. We found that body mass (mb) adjusted BMR was positively correlated with mb- and Tb-adjusted TMRmin, in a phylogenetically corrected analysis. Phylogenetic path modeling indicated that the mechanisms underlying the evolutionary increase of BMR in endotherms most likely constrain their ability to reduce MR during torpor. Given that heterothermy is considered an ancestral state in mammals, these results suggest an increase of BMR during the evolution of endothermy in homeothermic animals, which leads to the loss of their ability to enter torpor.

A major theme in evolutionary and ecological physiology of terrestrial vertebrates encompasses the factors underlying the evolution of endothermy in birds and mammals and interspecific variation of basal metabolic rate (BMR). Here, we applied the experimental evolution approach and compared BMR in lines of a wild rodent, the bank vole (Myodes glareolus), selected for 11 generations for: high swim-induced aerobic metabolism (A), ability to maintain body mass on a low-quality herbivorous diet (H) and intensity of predatory behaviour towards crickets (P). Four replicate lines were maintained for each of the selection directions and an unselected control (C). In comparison to C lines, A lines achieved a 49% higher maximum rate of oxygen consumption during swimming, H lines lost 1.3 g less mass in the test with low-quality diet and P lines attacked crickets five times more frequently. BMR was significantly higher in A lines than in C or H lines (60.8, 56.6 and 54.4 ml O2 h(-1), respectiv...

Lamnid sharks are regionally endothermic fishes that maintain visceral temperatures elevated above the ambient water temperature. Visceral en-dothermy is thought to increase rates of digestion and food processing and allow thermal niche expansion. We tested the hypothesis that, at in vivo temperatures, the endothermic shortfin mako shark, Isurus oxy-rinchus, has higher specific activities of three digestive enzymes—gastric pepsin and pancreatic trypsin and lipase—than the thresher shark, Alopias vulpinus, and the blue shark, Prionace glauca, neither of which can maintain elevated visceral temperatures. Ho-mogenized stomach or pancreas tissue obtained from sharks collected by pelagic longline was incubated at both 15 and 25 °C, at saturating substrate concentrations, to quantify tissue enzymatic activity. The mako had significantly higher enzyme activities at 25 °C than did the thresher and blue sharks at 15 °C. This difference was not a simple temperature effect, because at 25 °C the mako had higher trypsin activity than the blue shark and higher activities for all enzymes than the thresher shark. We also hypothesized that the thermal coefficient, or Q 10 value, would be higher for the mako shark than for the thresher and blue sharks because of its more stable visceral temperature. However, the mako and thresher sharks had similar Q 10 values for all enzymes, perhaps because of their closer phylogenetic relationship. The higher in vivo digestive enzyme activities in the mako shark should result in higher rates of food processing and may represent a selective advantage of regional visceral endothermy.

Marsupial joeys are born ectothermic and develop endothermy within their mother's thermally stable pouch. We hypothesised that tammar wallaby joeys would switch from α-stat to pH-stat regulation during the transition from ectothermy to endothermy. To address this, we compared ventilation (VE), metabolic rate (VO2) and variables relevant to blood gas and acid-base regulation and oxygen transport including the ventilatory requirements (VE⁄VO2)and VE⁄VCO2), partial pressures of oxygen (PaO2), carbon dioxide (PaCO2), pHa and oxygen content (CaO2) during progressive hypothermia in ecto- and endothermic tammar wallabies. We also measured the same variables in the well-studied endotherm, the Sprague-Dawley rat. Hypothermia was induced in unrestrained, unanesthetized joeys and rats by progressively dropping the ambient temperature (Ta). Rats were additionally exposed to helox (80% helium, 20% oxygen) to facilitate heat loss. Respiratory, metabolic and blood-gas variables were measured over a large body temperature (Tb) range (~15-16°C in both species). Ectothermic joeys displayed limited thermogenic ability during cooling: after an initial plateau, VO2 decreased with the progressive drop in Tb. The Tb of endothermic joeys and rats fell despite VO2 nearly doubling with the initiation of cold stress. In all three groups the changes in VO2 were met by changes in VE, resulting in constant VE⁄VO2 and VE⁄VCO2, blood gases and pHa. Thus, although thermogenic capability was nearly absent in ectothermic joeys, blood acid-base regulation was similar to endothermic joeys and rats. This suggests that unlike some reptiles, unanesthetized mammals protect arterial blood pH with changing Tb, irrespective of their thermogenic ability and/or stage of development.

Metabolic rate is a key aspect of organismal biology and the identification of selective factors that have led to species differences is a major goal of evolutionary physiology. We tested whether environmental characteristics and/or diet were significant predictors of interspecific variation in rodent metabolic rates. Mass-specific basal metabolic rates (BMR) and maximum metabolic rates (MMR, measured during cold exposure in